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Temporal range: Late Triassic – Recent; 225 or 167–0 MaSee discussion of dates in text
Mammal Diversity 2011.png
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Amniota
Clade: Synapsida
Clade: Mammaliaformes
Class: Mammalia
Linnaeus, 1758
Living subgroups

A mammal (from Latin mamma  'breast') [1] is a vertebrate animal of the class Mammalia ( /məˈmli.ə/ ). Mammals are characterized by the presence of milk-producing mammary glands for feeding their young, a neocortex region of the brain, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Around 6,400 extant species of mammals have been described and divided into 29 orders.


The largest orders of mammals, by number of species, are the rodents, bats, and Eulipotyphla (including hedgehogs, moles and shrews). The next three are the Primates (including humans, monkeys and lemurs), the even-toed ungulates (including pigs, camels, and whales), and the Carnivora (including cats, dogs, and seals).

Mammals are the only living members of Synapsida; this clade, together with Sauropsida (reptiles and birds), constitutes the larger Amniota clade. Early synapsids are referred to as "pelycosaurs". The more advanced therapsids became dominant during the Middle Permian. Mammals originated from cynodonts, an advanced group of therapsids, during the Late Triassic  – Early Jurassic. Modern mammalian acheived their modern diversity in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, and have been the dominant terrestrial animal group from 66 million years ago to the present.

The basic mammalian body type is quadruped, and most mammals use their four extremities for terrestrial locomotion; but in some, the extremities are adapted for life at sea, in the air, in trees, underground, or on two legs. Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 30 m (98 ft) blue whale—possibly the largest animal to have ever lived. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals. The most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation.

Most mammals are intelligent, with some possessing large brains, self-awareness, and tool use. Mammals can communicate and vocalize in several ways, including the production of ultrasound, scent-marking, alarm signals, singing, echolocation; and, in the case of humans, complex language. Mammals can organize themselves into fission-fusion societies, harems, and hierarchies—but can also be solitary and territorial. Most mammals are polygynous, but some can be monogamous or polyandrous.

Domestication of many types of mammals by humans played a major role in the Neolithic Revolution, and resulted in farming replacing hunting and gathering as the primary source of food for humans. This led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, and ultimately the development of the first civilizations. Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food (meat and dairy products), fur, and leather. Mammals are also hunted and raced for sport, and are used as model organisms in science. Mammals have been depicted in art since Paleolithic times, and appear in literature, film, mythology, and religion. Decline in numbers and extinction of many mammals is primarily driven by human poaching and habitat destruction, primarily deforestation.


Over 70% of mammal species come from the orders Rodentia, rodents (blue); Chiroptera, bats (red); and Soricomorpha, shrews (yellow).
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Proboscidea Mammal species pie chart.svg
Over 70% of mammal species come from the orders Rodentia, rodents (blue); Chiroptera, bats (red); and Soricomorpha, shrews (yellow).

Mammal classification has been through several revisions since Carl Linnaeus initially defined the class, and at present, no classification system is universally accepted. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums. [2] Simpson (1945) [3] provides systematics of mammal origins and relationships that had been taught universally until the end of the 20th century. However, since 1945, a large amount of new and more detailed information has gradually been found: The paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though fieldwork and lab work progressively outdated Simpson's classification, it remains the closest thing to an official classification of mammals, despite its known issues. [4]

Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers of species are Rodentia: mice, rats, porcupines, beavers, capybaras, and other gnawing mammals; Chiroptera: bats; and Soricomorpha: shrews, moles, and solenodons. The next three biggest orders, depending on the biological classification scheme used, are the Primates: apes, monkeys, and lemurs; the Cetartiodactyla: whales and even-toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears, seals, and allies. [5] According to Mammal Species of the World , 5,416 species were identified in 2006. These were grouped into 1,229  genera, 153  families and 29 orders. [5] In 2008, the International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. [6] According to research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495, including 96 recently extinct. [7]


The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and Therian mammals (marsupials and placentals) and all descendants of that ancestor. [8] Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century. [9] If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group. [10]

T. S. Kemp has provided a more traditional definition: "Synapsids that possess a dentarysquamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals. [11] The earliest known synapsid satisfying Kemp's definitions is Tikitherium , dated 225 Ma, so the appearance of mammals in this broader sense can be given this Late Triassic date. [12] [13]

McKenna/Bell classification

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. The authors worked together as paleontologists at the American Museum of Natural History. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa, that reflects the historical genealogy of Mammalia. [4] Their 1997 book, Classification of Mammals above the Species Level, [14] is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the groupings.

In the following list, extinct groups are labelled with a dagger (†).

Class Mammalia

Molecular classification of placentals

Genus-level molecular phylogeny of 116 extant mammals inferred from the gene tree information of 14,509 coding DNA sequences. The major clades are colored: Marsupials (magenta), Xenarthrans (orange), afrotherians (red), laurasiatherians (green), and euarchontoglires (blue). OrthoMaM v10b 2019 116genera circular tree.svg
Genus-level molecular phylogeny of 116 extant mammals inferred from the gene tree information of 14,509 coding DNA sequences. The major clades are colored: Marsupials (magenta), Xenarthrans (orange), afrotherians (red), laurasiatherians (green), and euarchontoglires (blue).

As of the early 21st century, molecular studies based on DNA analysis have suggested new relationships among mammal families. Most of these findings have been independently validated by retrotransposon presence/absence data. [17] Classification systems based on molecular studies reveal three major groups or lineages of placental mammals—Afrotheria, Xenarthra and Boreoeutheria—which diverged in the Cretaceous. The relationships between these three lineages is contentious, and all three possible hypotheses have been proposed with respect to which group is basal. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra) and Exafroplacentalia (basal Afrotheria). [18] Boreoeutheria in turn contains two major lineages—Euarchontoglires and Laurasiatheria.

Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used (such as nuclear or mitochondrial) [19] and varying interpretations of paleogeographic data. [18]

Tarver et al. 2016 [20] Sandra Álvarez-Carretero et al. 2022 [21] [22]








Euarchonta Bechuana of Distinction-1841 (white background).jpg







Artiodactyla Eubalaena glacialis NOAA.jpg

Perissodactyla Rhino white background.jpg



Carnivora Zalophus californianus J. Smit (white background).jpg


Monotremata Genera mammalium Ornithorhynchus anatinus.jpg


Paucituberculata Phylogenetic tree of marsupials derived from retroposon data (Paucituberculata).png

Didelphimorphia A hand-book to the marsupialia and monotremata (Plate XXXII) (white background).jpg



Diprotodontia A monograph of the Macropodidae, or family of kangaroos (9398404841) white background.jpg


Notoryctemorphia Phylogenetic tree of marsupials derived from retroposon data (Notoryctemorphia).png

Peramelemorphia Phylogenetic tree of marsupials derived from retroposon data (Paramelemorphia).png

Dasyuromorphia Phylogenetic tree of marsupials derived from retroposon data (Dasyuromorphia).png


Pilosa Natural history of the animal kingdom for the use of young people (Plate XV) (Myrmecophaga tridactyla).jpg

Cingulata Nine-banded-Armadillo white background.jpg


Sirenia Manatee white background.jpg

Proboscidea Indian elephant white background.jpg

Hyracoidea DendrohyraxEminiSmit white background.jpg


Tubulidentata Aardvark2 (PSF) colourised.png


Macroscelidea Rhynchocyon chrysopygus-J Smit white background.jpg

Afrosoricida Potamogale velox illustration.jpg


Eulipotyphla Mole white background.jpg


Pholidota FMIB 46859 Pangolin a grosse queue white background.jpeg

Carnivora Cynailurus guttata - 1818-1842 - Print - Iconographia Zoologica - Special Collections University of Amsterdam - (white background).jpg


Perissodactyla Equus quagga (white background).jpg

Artiodactyla Walia ibex illustration white background.png

Chiroptera Vampire bat white background.jpg


Dermoptera Cynocephalus volans Brehm1883 (white background).jpg

Primates Die Saugthiere in Abbildungen nach der Natur, mit Beschreibungen (Plate 8) (white background).jpg

Scandentia Die Saugthiere in Abbildungen nach der Natur, mit Beschreibungen (Plate 34) (white background).jpg


Lagomorpha Bruno Liljefors - Hare studies 1885 white background.jpg

Rodentia Ruskea rotta.png



Synapsida, a clade that contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod (~323 million to ~300 million years ago), when they split from the reptile lineage. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic. The cladogram takes Mammalia to be the crown group. [23]


Morganucodontidae Morganucodon.jpg

Docodonta Docofossor NT flipped.jpg



Australosphenida (incl. Monotremata) Steropodon BW.jpg



Multituberculata Sunnyodon.jpg


Eutriconodonta (incl. Gobiconodonta) Repenomamus BW.jpg

Trechnotheria (incl. Theria) Juramaia NT.jpg

Evolution from older amniotes

The original synapsid skull structure contains one temporal opening behind the orbitals, in a fairly low position on the skull (lower right in this image). This opening might have assisted in containing the jaw muscles of these organisms which could have increased their biting strength. Skull synapsida 1.png
The original synapsid skull structure contains one temporal opening behind the orbitals, in a fairly low position on the skull (lower right in this image). This opening might have assisted in containing the jaw muscles of these organisms which could have increased their biting strength.

The first fully terrestrial vertebrates were amniotes. Like their amphibious early tetrapod predecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that allow the developing embryo to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.

The first amniotes apparently arose in the Pennsylvanian subperiod of the Carboniferous. They descended from earlier reptiliomorph amphibious tetrapods, [24] which lived on land that was already inhabited by insects and other invertebrates as well as ferns, mosses and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which would later include the common ancestor of the mammals; and the sauropsids, which now include turtles, lizards, snakes, crocodilians and dinosaurs (including birds). [25] Synapsids have a single hole (temporal fenestra) low on each side of the skull. Primitive synapsids included the largest and fiercest animals of the early Permian such as Dimetrodon. [26] Nonmammalian synapsids were traditionally—and incorrectly—called "mammal-like reptiles" or pelycosaurs; we now know they were neither reptiles nor part of reptile lineage. [27] [28]

Therapsids, a group of synapsids, evolved in the Middle Permian, about 265 million years ago, and became the dominant land vertebrates. [27] They differ from basal eupelycosaurs in several features of the skull and jaws, including: larger skulls and incisors which are equal in size in therapsids, but not for eupelycosaurs. [27] The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their early synapsid ancestors and ending with probainognathian cynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by: [29]

First mammals

The Permian–Triassic extinction event about 252 million years ago, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivorous therapsids. [31] In the early Triassic, most medium to large land carnivore niches were taken over by archosaurs [32] which, over an extended period (35 million years), came to include the crocodylomorphs, [33] the pterosaurs and the dinosaurs; [34] however, large cynodonts like Trucidocynodon and traversodontids still occupied large sized carnivorous and herbivorous niches respectively. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well. [35]

The first mammals (in Kemp's sense) appeared in the Late Triassic epoch (about 225 million years ago), 40 million years after the first therapsids. They expanded out of their nocturnal insectivore niche from the mid-Jurassic onwards; [36] The Jurassic Castorocauda , for example, was a close relative of true mammals that had adaptations for swimming, digging and catching fish. [37] Most, if not all, are thought to have remained nocturnal (the nocturnal bottleneck), accounting for much of the typical mammalian traits. [38] The majority of the mammal species that existed in the Mesozoic Era were multituberculates, eutriconodonts and spalacotheriids. [39] The earliest known metatherian is Sinodelphys , found in 125 million-year-old Early Cretaceous shale in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues. [40]

Restoration of Juramaia sinensis, the oldest known Eutherian (160 M.Y.A.) Juramaia NT.jpg
Restoration of Juramaia sinensis , the oldest known Eutherian (160 M.Y.A.)

The oldest known fossil among the Eutheria ("true beasts") is the small shrewlike Juramaia sinensis , or "Jurassic mother from China", dated to 160 million years ago in the late Jurassic. [41] A later eutherian relative, Eomaia , dated to 125 million years ago in the early Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage. [42] In particular, the epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, other nontherian mammals and Ukhaatherium , an early Cretaceous animal in the eutherian order Asioryctitheria. This also applies to the multituberculates. [43] They are apparently an ancestral feature, which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus during gestation periods. A narrow pelvic outlet indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development. [44]

One of the earliest known monotremes was Teinolophos , which lived about 120 million years ago in Australia. [45] Monotremes have some features which may be inherited from the original amniotes such as the same orifice to urinate, defecate and reproduce (cloaca)—as lizards and birds also do— [46] and they lay eggs which are leathery and uncalcified. [47]

Earliest appearances of features

Hadrocodium , whose fossils date from approximately 195 million years ago, in the early Jurassic, provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids. [48]

Fossil of Thrinaxodon at the National Museum of Natural History Thrinaxodon Lionhinus.jpg
Fossil of Thrinaxodon at the National Museum of Natural History

The earliest clear evidence of hair or fur is in fossils of Castorocauda and Megaconus , from 164 million years ago in the mid-Jurassic. In the 1950s, it was suggested that the foramina (passages) in the maxillae and premaxillae (bones in the front of the upper jaw) of cynodonts were channels which supplied blood vessels and nerves to vibrissae (whiskers) and so were evidence of hair or fur; [49] [50] it was soon pointed out, however, that foramina do not necessarily show that an animal had vibrissae, as the modern lizard Tupinambis has foramina that are almost identical to those found in the nonmammalian cynodont Thrinaxodon . [28] [51] Popular sources, nevertheless, continue to attribute whiskers to Thrinaxodon. [52] Studies on Permian coprolites suggest that non-mammalian synapsids of the epoch already had fur, setting the evolution of hairs possibly as far back as dicynodonts. [53]

When endothermy first appeared in the evolution of mammals is uncertain, though it is generally agreed to have first evolved in non-mammalian therapsids. [53] [54] Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals, [55] but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians. [56] Likewise, some modern therians like afrotheres and xenarthrans have secondarily developed lower body temperatures. [57]

The evolution of erect limbs in mammals is incomplete—living and fossil monotremes have sprawling limbs. The parasagittal (nonsprawling) limb posture appeared sometime in the late Jurassic or early Cretaceous; it is found in the eutherian Eomaia and the metatherian Sinodelphys, both dated to 125 million years ago. [58] Epipubic bones, a feature that strongly influenced the reproduction of most mammal clades, are first found in Tritylodontidae, suggesting that it is a synapomorphy between them and mammaliformes. They are omnipresent in non-placental mammaliformes, though Megazostrodon and Erythrotherium appear to have lacked them. [59]

It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals. [60] [61] In human females, mammary glands become fully developed during puberty, regardless of pregnancy. [62]

Rise of the mammals

Hyaenodon horridus, a North American species of hypercarnivore within the now-extinct order Hyaenodonta, at the Royal Ontario Museum. The genus Hyaenodon was amongst the most successful mammals of the late Eocene-early Miocene epochs spanning for most of the Paleogene and some of the Neogene periods, undergoing many endemic radiations in North America, Europe, and Asia. Hyaenodon horridus, Niobrara County, Wyoming, USA, Late Oligocene - Royal Ontario Museum - DSC00114.JPG
Hyaenodon horridus, a North American species of hypercarnivore within the now-extinct order Hyaenodonta, at the Royal Ontario Museum. The genus Hyaenodon was amongst the most successful mammals of the late Eocene-early Miocene epochs spanning for most of the Paleogene and some of the Neogene periods, undergoing many endemic radiations in North America, Europe, and Asia.

Therian mammals took over the medium- to large-sized ecological niches in the Cenozoic, after the Cretaceous–Paleogene extinction event approximately 66 million years ago emptied ecological space once filled by non-avian dinosaurs and other groups of reptiles, as well as various other mammal groups, [64] and underwent an exponential increase in body size (megafauna). [65] Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity. [64] For example, the earliest known bat dates from about 50 million years ago, only 16 million years after the extinction of the non-avian dinosaurs. [66]

Molecular phylogenetic studies initially suggested that most placental orders diverged about 100 to 85 million years ago and that modern families appeared in the period from the late Eocene through the Miocene. [67] However, no placental fossils have been found from before the end of the Cretaceous. [68] The earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of the non-avian dinosaurs. [68] (Scientists identified an early Paleocene animal named Protungulatum donnae as one of the first placental mammals, [69] but it has since been reclassified as a non-placental eutherian.) [70] Recalibrations of genetic and morphological diversity rates have suggested a Late Cretaceous origin for placentals, and a Paleocene origin for most modern clades. [71]

The earliest known ancestor of primates is Archicebus achilles [72] from around 55 million years ago. [72] This tiny primate weighed 20–30 grams (0.7–1.1 ounce) and could fit within a human palm. [72]


Distinguishing features

Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk to nourish their young. [73] In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils. [74]

Many traits shared by all living mammals appeared among the earliest members of the group:

For the most part, these characteristics were not present in the Triassic ancestors of the mammals. [80] Nearly all mammaliaforms possess an epipubic bone, the exception being modern placentals. [81]

Sexual dimorphism

Sexual dimorphism in aurochs, the extinct wild ancestor of cattle Aurochsfeatures.jpg
Sexual dimorphism in aurochs, the extinct wild ancestor of cattle

On average, male mammals are larger than females, with males being at least 10% larger than females in over 45% of investigated species. Most mammalian orders also exhibit male-biased sexual dimorphism, although some orders do not show any bias or are significantly female-biased (Lagomorpha). Sexual size dimorphism increases with body size across mammals (Rensch's rule), suggesting that there are parallel selection pressures on both male and female size. Male-biased dimorphism relates to sexual selection on males through male–male competition for females, as there is a positive correlation between the degree of sexual selection, as indicated by mating systems, and the degree of male-biased size dimorphism. The degree of sexual selection is also positively correlated with male and female size across mammals. Further, parallel selection pressure on female mass is identified in that age at weaning is significantly higher in more polygynous species, even when correcting for body mass. Also, the reproductive rate is lower for larger females, indicating that fecundity selection selects for smaller females in mammals. Although these patterns hold across mammals as a whole, there is considerable variation across orders. [82]

Biological systems

The majority of mammals have seven cervical vertebrae (bones in the neck). The exceptions are the manatee and the two-toed sloth, which have six, and the three-toed sloth which has nine. [83] All mammalian brains possess a neocortex, a brain region unique to mammals. [84] Placental brains have a corpus callosum, unlike monotremes and marsupials. [85]

Didactic model of a mammal heart 01-FMVZ USP-07.jpeg
Didactic model of a mammal heart 02-FMVZ USP-08.jpeg
Didactic model of a mammal heart 03--FMVZ USP-10.jpeg
Didactic model of a mammal heart 04--FMVZ USP-11.jpeg
Didactic models of a mammalian heart

Circulatory systems

The mammalian heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. [86] The heart has four valves, which separate its chambers and ensures blood flows in the correct direction through the heart (preventing backflow). After gas exchange in the pulmonary capillaries (blood vessels in the lungs), oxygen-rich blood returns to the left atrium via one of the four pulmonary veins. Blood flows nearly continuously back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. The heart also requires nutrients and oxygen found in blood like other muscles, and is supplied via coronary arteries. [87]

Respiratory systems

Raccoon lungs being inflated manually Lung expansion simulation with Raccoon.gif
Raccoon lungs being inflated manually

The lungs of mammals are spongy and honeycombed. Breathing is mainly achieved with the diaphragm, which divides the thorax from the abdominal cavity, forming a dome convex to the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the lung cavity. Air enters through the oral and nasal cavities, and travels through the larynx, trachea and bronchi, and expands the alveoli. Relaxing the diaphragm has the opposite effect, decreasing the volume of the lung cavity, causing air to be pushed out of the lungs. During exercise, the abdominal wall contracts, increasing pressure on the diaphragm, which forces air out quicker and more forcefully. The rib cage is able to expand and contract the chest cavity through the action of other respiratory muscles. Consequently, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. [88] [89] This type of lung is known as a bellows lung due to its resemblance to blacksmith bellows. [89]

Integumentary systems

Mammal skin: (1) hair, (2) epidermis, (3) sebaceous gland, (4) Arrector pili muscle, (5) dermis, (6) hair follicle, (7) sweat gland. Not labeled, the bottom layer: hypodermis, showing round adipocytes The skin of mammals.jpg
Mammal skin: (1) hair, (2) epidermis, (3) sebaceous gland, (4) Arrector pili muscle, (5) dermis, (6) hair follicle, (7) sweat gland. Not labeled, the bottom layer: hypodermis, showing round adipocytes

The integumentary system (skin) is made up of three layers: the outermost epidermis, the dermis and the hypodermis. The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is 15 to 40 times thicker than the epidermis. The dermis is made up of many components, such as bony structures and blood vessels. The hypodermis is made up of adipose tissue, which stores lipids and provides cushioning and insulation. The thickness of this layer varies widely from species to species; [90] :97 marine mammals require a thick hypodermis (blubber) for insulation, and right whales have the thickest blubber at 20 inches (51 cm). [91] Although other animals have features such as whiskers, feathers, setae, or cilia that superficially resemble it, no animals other than mammals have hair. It is a definitive characteristic of the class, though some mammals have very little. [90] :61

Digestive systems

Canis lupus 02 MWNH 358.jpg
The carnassials (teeth in the very back of the mouth) of the insectivorous aardwolf (left) vs. that of a gray wolf (right) which consumes large vertebrates

Herbivores have developed a diverse range of physical structures to facilitate the consumption of plant material. To break up intact plant tissues, mammals have developed teeth structures that reflect their feeding preferences. For instance, frugivores (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialized for grinding foliage and seeds. Grazing animals that tend to eat hard, silica-rich grasses, have high-crowned teeth, which are capable of grinding tough plant tissues and do not wear down as quickly as low-crowned teeth. [92] Most carnivorous mammals have carnassialiforme teeth (of varying length depending on diet), long canines and similar tooth replacement patterns. [93]

The stomach of even-toed ungulates (Artiodactyla) is divided into four sections: the rumen, the reticulum, the omasum and the abomasum (only ruminants have a rumen). After the plant material is consumed, it is mixed with saliva in the rumen and reticulum and separates into solid and liquid material. The solids lump together to form a bolus (or cud), and is regurgitated. When the bolus enters the mouth, the fluid is squeezed out with the tongue and swallowed again. Ingested food passes to the rumen and reticulum where cellulolytic microbes (bacteria, protozoa and fungi) produce cellulase, which is needed to break down the cellulose in plants. [94] Perissodactyls, in contrast to the ruminants, store digested food that has left the stomach in an enlarged cecum, where it is fermented by bacteria. [95] Carnivora have a simple stomach adapted to digest primarily meat, as compared to the elaborate digestive systems of herbivorous animals, which are necessary to break down tough, complex plant fibers. The caecum is either absent or short and simple, and the large intestine is not sacculated or much wider than the small intestine. [96]

Excretory and genitourinary systems

Bovine kidney Glycerination of Bovine kidney.jpg
Bovine kidney
Genitourinary system of a male and female rabbit Image from page 702 of "Outlines of zoology" (1895) (20732795545).jpg
Genitourinary system of a male and female rabbit

The mammalian excretory system involves many components. Like most other land animals, mammals are ureotelic, and convert ammonia into urea, which is done by the liver as part of the urea cycle. [97] Bilirubin, a waste product derived from blood cells, is passed through bile and urine with the help of enzymes excreted by the liver. [98] The passing of bilirubin via bile through the intestinal tract gives mammalian feces a distinctive brown coloration. [99] Distinctive features of the mammalian kidney include the presence of the renal pelvis and renal pyramids, and of a clearly distinguishable cortex and medulla, which is due to the presence of elongated loops of Henle. Only the mammalian kidney has a bean shape, although there are some exceptions, such as the multilobed reniculate kidneys of pinnipeds, cetaceans and bears. [100] [101] Most adult placental mammals have no remaining trace of the cloaca. In the embryo, the embryonic cloaca divides into a posterior region that becomes part of the anus, and an anterior region that has different fates depending on the sex of the individual: in females, it develops into the vestibule that receives the urethra and vagina, while in males it forms the entirety of the penile urethra. [101] However, the tenrecs, golden moles, and some shrews retain a cloaca as adults. [102] In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally. [101] Monotremes, which translates from Greek into "single hole", have a true cloaca. [103]

Sound production

A diagram of ultrasonic signals emitted by a bat, and the echo from a nearby object Animal echolocation.svg
A diagram of ultrasonic signals emitted by a bat, and the echo from a nearby object

As in all other tetrapods, mammals have a larynx that can quickly open and close to produce sounds, and a supralaryngeal vocal tract which filters this sound. The lungs and surrounding musculature provide the air stream and pressure required to phonate. The larynx controls the pitch and volume of sound, but the strength the lungs exert to exhale also contributes to volume. More primitive mammals, such as the echidna, can only hiss, as sound is achieved solely through exhaling through a partially closed larynx. Other mammals phonate using vocal folds. The movement or tenseness of the vocal folds can result in many sounds such as purring and screaming. Mammals can change the position of the larynx, allowing them to breathe through the nose while swallowing through the mouth, and to form both oral and nasal sounds; nasal sounds, such as a dog whine, are generally soft sounds, and oral sounds, such as a dog bark, are generally loud. [104]

Beluga whale echolocation sounds

Some mammals have a large larynx and thus a low-pitched voice, namely the hammer-headed bat (Hypsignathus monstrosus) where the larynx can take up the entirety of the thoracic cavity while pushing the lungs, heart, and trachea into the abdomen. [105] Large vocal pads can also lower the pitch, as in the low-pitched roars of big cats. [106] The production of infrasound is possible in some mammals such as the African elephant (Loxodonta spp.) and baleen whales. [107] [108] Small mammals with small larynxes have the ability to produce ultrasound, which can be detected by modifications to the middle ear and cochlea. Ultrasound is inaudible to birds and reptiles, which might have been important during the Mesozoic, when birds and reptiles were the dominant predators. This private channel is used by some rodents in, for example, mother-to-pup communication, and by bats when echolocating. Toothed whales also use echolocation, but, as opposed to the vocal membrane that extends upward from the vocal folds, they have a melon to manipulate sounds. Some mammals, namely the primates, have air sacs attached to the larynx, which may function to lower the resonances or increase the volume of sound. [104]

The vocal production system is controlled by the cranial nerve nuclei in the brain, and supplied by the recurrent laryngeal nerve and the superior laryngeal nerve, branches of the vagus nerve. The vocal tract is supplied by the hypoglossal nerve and facial nerves. Electrical stimulation of the periaqueductal gray (PEG) region of the mammalian midbrain elicit vocalizations. The ability to learn new vocalizations is only exemplified in humans, seals, cetaceans, elephants and possibly bats; in humans, this is the result of a direct connection between the motor cortex, which controls movement, and the motor neurons in the spinal cord. [104]


Porcupines use their spines for defense. Stekelvarken Aiguilles Porc-epic.jpg
Porcupines use their spines for defense.

The primary function of the fur of mammals is thermoregulation. Others include protection, sensory purposes, waterproofing, and camouflage. [109] Different types of fur serve different purposes: [90] :99


Hair length is not a factor in thermoregulation: for example, some tropical mammals such as sloths have the same length of fur length as some arctic mammals but with less insulation; and, conversely, other tropical mammals with short hair have the same insulating value as arctic mammals. The denseness of fur can increase an animal's insulation value, and arctic mammals especially have dense fur; for example, the musk ox has guard hairs measuring 30 cm (12 in) as well as a dense underfur, which forms an airtight coat, allowing them to survive in temperatures of −40 °C (−40 °F). [90] :162–163 Some desert mammals, such as camels, use dense fur to prevent solar heat from reaching their skin, allowing the animal to stay cool; a camel's fur may reach 70 °C (158 °F) in the summer, but the skin stays at 40 °C (104 °F). [90] :188 Aquatic mammals, conversely, trap air in their fur to conserve heat by keeping the skin dry. [90] :162–163

A leopard's disruptively colored coat provides camouflage for this ambush predator. Great male Leopard in South Afrika-JD.JPG
A leopard's disruptively colored coat provides camouflage for this ambush predator.


Mammalian coats are colored for a variety of reasons, the major selective pressures including camouflage, sexual selection, communication, and thermoregulation. Coloration in both the hair and skin of mammals is mainly determined by the type and amount of melanin; eumelanins for brown and black colors and pheomelanin for a range of yellowish to reddish colors, giving mammals an earth tone. [110] [111] Some mammals have more vibrant colors; certain monkeys such mandrills and vervet monkeys, and opossums such as the Mexican mouse opossums and Derby's woolly opossums, have blue skin due to light diffraction in collagen fibers. [112] Many sloths appear green because their fur hosts green algae; this may be a symbiotic relation that affords camouflage to the sloths. [113]

Camouflage is a powerful influence in a large number of mammals, as it helps to conceal individuals from predators or prey. [114] In arctic and subarctic mammals such as the arctic fox (Alopex lagopus), collared lemming (Dicrostonyx groenlandicus), stoat (Mustela erminea), and snowshoe hare (Lepus americanus), seasonal color change between brown in summer and white in winter is driven largely by camouflage. [115] Some arboreal mammals, notably primates and marsupials, have shades of violet, green, or blue skin on parts of their bodies, indicating some distinct advantage in their largely arboreal habitat due to convergent evolution. [112]

Aposematism, warning off possible predators, is the most likely explanation of the black-and-white pelage of many mammals which are able to defend themselves, such as in the foul-smelling skunk and the powerful and aggressive honey badger. [116] Coat color is sometimes sexually dimorphic, as in many primate species. [117] Differences in female and male coat color may indicate nutrition and hormone levels, important in mate selection. [118] Coat color may influence the ability to retain heat, depending on how much light is reflected. Mammals with a darker colored coat can absorb more heat from solar radiation, and stay warmer, and some smaller mammals, such as voles, have darker fur in the winter. The white, pigmentless fur of arctic mammals, such as the polar bear, may reflect more solar radiation directly onto the skin. [90] :166–167 [109] The dazzling black-and-white striping of zebras appear to provide some protection from biting flies. [119]

Reproductive system

Goat kids stay with their mother until they are weaned. Goat family.jpg
Goat kids stay with their mother until they are weaned.

Mammals are solely gonochoric (an animal is born with either male or female genitalia, as opposed to hermaphrodites where there is no such schism). [120] In male placentals, the penis is used both for urination and copulation. Depending on the species, an erection may be fueled by blood flow into vascular, spongy tissue or by muscular action. A penis may be contained in a prepuce when not erect, and some placentals also have a penis bone (baculum). [121] Marsupials typically have forked penises, [122] while the echidna penis generally has four heads with only two functioning. [123] The testicles of most mammals descend into the scrotum which is typically posterior to the penis but is often anterior in marsupials. Female mammals generally have a vulva (clitoris and labia) on the outside, while the internal system contains paired oviducts, 1–2 uteri, 1–2 cervices and a vagina. [124] [125] Marsupials have two lateral vaginas and a medial vagina. The "vagina" of monotremes is better understood as a "urogenital sinus". The uterine systems of placental mammals can vary between a duplex, where there are two uteri and cervices which open into the vagina, a bipartite, where two uterine horns have a single cervix that connects to the vagina, a bicornuate, which consists where two uterine horns that are connected distally but separate medially creating a Y-shape, and a simplex, which has a single uterus. [126] [127] [90] :220–221,247

Matschie's tree-kangaroo with young in pouch Dendrolagus matschiei 1.jpg
Matschie's tree-kangaroo with young in pouch

The ancestral condition for mammal reproduction is the birthing of relatively undeveloped, either through direct vivipary or a short period as soft-shelled eggs. This is likely due to the fact that the torso could not expand due to the presence of epipubic bones. The oldest demonstration of this reproductive style is with Kayentatherium , which produced undeveloped perinates, but at much higher litter sizes than any modern mammal, 38 specimens. [128] Most modern mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypus and the four species of echidna, lay eggs. The monotremes have a sex-determination system different from that of most other mammals. [129] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal. [130]

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a short gestation period, typically shorter than its estrous cycle and generally giving birth to a number of undeveloped newborns that then undergo further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. This is the plesiomorphic condition among viviparous mammals; the presence of epipubic bones in all non-placental mammals prevents the expansion of the torso needed for full pregnancy. [81] Even non-placental eutherians probably reproduced this way. [43] The placentals give birth to relatively complete and developed young, usually after long gestation periods. [131] They get their name from the placenta, which connects the developing fetus to the uterine wall to allow nutrient uptake. [132] In placental mammals, the epipubic is either completely lost or converted into the baculum; allowing the torso to be able to expand and thus birth developed offspring. [128]

The mammary glands of mammals are specialized to produce milk, the primary source of nutrition for newborns. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly. [133] Compared to placental mammals, the milk of marsupials changes greatly in both production rate and in nutrient composition, due to the underdeveloped young. In addition, the mammary glands have more autonomy allowing them to supply separate milks to young at different development stages. [134] Lactose is the main sugar in placental mammal milk while monotreme and marsupial milk is dominated by oligosaccharides. [135] Weaning is the process in which a mammal becomes less dependent on their mother's milk and more on solid food. [136]


Nearly all mammals are endothermic ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for ectothermic ("cold-blooded") reptiles and insects. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles. [137] Small insectivorous mammals eat prodigious amounts for their size. A rare exception, the naked mole-rat produces little metabolic heat, so it is considered an operational poikilotherm. [138] Birds are also endothermic, so endothermy is not unique to mammals. [139]

Species lifespan

Among mammals, species maximum lifespan varies significantly (for example the shrew has a lifespan of two years, whereas the oldest bowhead whale is recorded to be 211 years). [140] Although the underlying basis for these lifespan differences is still uncertain, numerous studies indicate that the ability to repair DNA damage is an important determinant of mammalian lifespan. In a 1974 study by Hart and Setlow, [141] it was found that DNA excision repair capability increased systematically with species lifespan among seven mammalian species. Species lifespan was observed to be robustly correlated with the capacity to recognize DNA double-strand breaks as well as the level of the DNA repair protein Ku80. [140] In a study of the cells from sixteen mammalian species, genes employed in DNA repair were found to be up-regulated in the longer-lived species. [142] The cellular level of the DNA repair enzyme poly ADP ribose polymerase was found to correlate with species lifespan in a study of 13 mammalian species. [143] Three additional studies of a variety of mammalian species also reported a correlation between species lifespan and DNA repair capability. [144] [145] [146]



Running gait. Photographs by Eadweard Muybridge, 1887 Muybridge race horse animated.gif
Running gait. Photographs by Eadweard Muybridge, 1887

Most vertebrates—the amphibians, the reptiles and some mammals such as humans and bears—are plantigrade, walking on the whole of the underside of the foot. Many mammals, such as cats and dogs, are digitigrade, walking on their toes, the greater stride length allowing more speed. Some animals such as horses are unguligrade, walking on the tips of their toes. This even further increases their stride length and thus their speed. [147] A few mammals, namely the great apes, are also known to walk on their knuckles, at least for their front legs. Giant anteaters [148] and platypuses [149] are also knuckle-walkers. Some mammals are bipeds, using only two limbs for locomotion, which can be seen in, for example, humans and the great apes. Bipedal species have a larger field of vision than quadrupeds, conserve more energy and have the ability to manipulate objects with their hands, which aids in foraging. Instead of walking, some bipeds hop, such as kangaroos and kangaroo rats. [150] [151]

Animals will use different gaits for different speeds, terrain and situations. For example, horses show four natural gaits, the slowest horse gait is the walk, then there are three faster gaits which, from slowest to fastest, are the trot, the canter and the gallop. Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the main human gaits are bipedal walking and running, but they employ many other gaits occasionally, including a four-legged crawl in tight spaces. [152] Mammals show a vast range of gaits, the order that they place and lift their appendages in locomotion. Gaits can be grouped into categories according to their patterns of support sequence. For quadrupeds, there are three main categories: walking gaits, running gaits and leaping gaits. [153] Walking is the most common gait, where some feet are on the ground at any given time, and found in almost all legged animals. Running is considered to occur when at some points in the stride all feet are off the ground in a moment of suspension. [152]


Gibbons are very good brachiators because their elongated limbs enable them to easily swing and grasp on to branches. Brachiating Gibbon (Some rights reserved).jpg
Gibbons are very good brachiators because their elongated limbs enable them to easily swing and grasp on to branches.

Arboreal animals frequently have elongated limbs that help them cross gaps, reach fruit or other resources, test the firmness of support ahead and, in some cases, to brachiate (swing between trees). [154] Many arboreal species, such as tree porcupines, silky anteaters, spider monkeys, and possums, use prehensile tails to grasp branches. In the spider monkey, the tip of the tail has either a bare patch or adhesive pad, which provides increased friction. Claws can be used to interact with rough substrates and reorient the direction of forces the animal applies. This is what allows squirrels to climb tree trunks that are so large to be essentially flat from the perspective of such a small animal. However, claws can interfere with an animal's ability to grasp very small branches, as they may wrap too far around and prick the animal's own paw. Frictional gripping is used by primates, relying upon hairless fingertips. Squeezing the branch between the fingertips generates frictional force that holds the animal's hand to the branch. However, this type of grip depends upon the angle of the frictional force, thus upon the diameter of the branch, with larger branches resulting in reduced gripping ability. To control descent, especially down large diameter branches, some arboreal animals such as squirrels have evolved highly mobile ankle joints that permit rotating the foot into a 'reversed' posture. This allows the claws to hook into the rough surface of the bark, opposing the force of gravity. Small size provides many advantages to arboreal species: such as increasing the relative size of branches to the animal, lower center of mass, increased stability, lower mass (allowing movement on smaller branches) and the ability to move through more cluttered habitat. [154] Size relating to weight affects gliding animals such as the sugar glider. [155] Some species of primate, bat and all species of sloth achieve passive stability by hanging beneath the branch. Both pitching and tipping become irrelevant, as the only method of failure would be losing their grip. [154]


Slow-motion and normal speed of Egyptian fruit bats flying

Bats are the only mammals that can truly fly. They fly through the air at a constant speed by moving their wings up and down (usually with some fore-aft movement as well). Because the animal is in motion, there is some airflow relative to its body which, combined with the velocity of the wings, generates a faster airflow moving over the wing. This generates a lift force vector pointing forwards and upwards, and a drag force vector pointing rearwards and upwards. The upwards components of these counteract gravity, keeping the body in the air, while the forward component provides thrust to counteract both the drag from the wing and from the body as a whole. [156]

The wings of bats are much thinner and consist of more bones than those of birds, allowing bats to maneuver more accurately and fly with more lift and less drag. [157] [158] By folding the wings inwards towards their body on the upstroke, they use 35% less energy during flight than birds. [159] The membranes are delicate, ripping easily; however, the tissue of the bat's membrane is able to regrow, such that small tears can heal quickly. [160] The surface of their wings is equipped with touch-sensitive receptors on small bumps called Merkel cells, also found on human fingertips. These sensitive areas are different in bats, as each bump has a tiny hair in the center, making it even more sensitive and allowing the bat to detect and collect information about the air flowing over its wings, and to fly more efficiently by changing the shape of its wings in response. [161]

Fossorial and subterranean

Semi-fossorial wombat (left) vs. fully fossorial eastern mole (right)

A fossorial (from Latin fossor, meaning "digger") is an animal adapted to digging which lives primarily, but not solely, underground. Some examples are badgers, and naked mole-rats. Many rodent species are also considered fossorial because they live in burrows for most but not all of the day. Species that live exclusively underground are subterranean, and those with limited adaptations to a fossorial lifestyle sub-fossorial. Some organisms are fossorial to aid in temperature regulation while others use the underground habitat for protection from predators or for food storage. [162]

Fossorial mammals have a fusiform body, thickest at the shoulders and tapering off at the tail and nose. Unable to see in the dark burrows, most have degenerated eyes, but degeneration varies between species; pocket gophers, for example, are only semi-fossorial and have very small yet functional eyes, in the fully fossorial marsupial mole the eyes are degenerated and useless, talpa moles have vestigial eyes and the cape golden mole has a layer of skin covering the eyes. External ears flaps are also very small or absent. Truly fossorial mammals have short, stout legs as strength is more important than speed to a burrowing mammal, but semi-fossorial mammals have cursorial legs. The front paws are broad and have strong claws to help in loosening dirt while excavating burrows, and the back paws have webbing, as well as claws, which aids in throwing loosened dirt backwards. Most have large incisors to prevent dirt from flying into their mouth. [163]

Many fossorial mammals such as shrews, hedgehogs, and moles were classified under the now obsolete order Insectivora. [164]


A pod of short-beaked common dolphins swimming

Fully aquatic mammals, the cetaceans and sirenians, have lost their legs and have a tail fin to propel themselves through the water. Flipper movement is continuous. Whales swim by moving their tail fin and lower body up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. Their skeletal anatomy allows them to be fast swimmers. Most species have a dorsal fin to prevent themselves from turning upside-down in the water. [165] [166] The flukes of sirenians are raised up and down in long strokes to move the animal forward, and can be twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing. [167]

Semi-aquatic mammals, like pinnipeds, have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. The elbows and ankles are enclosed within the body. [168] [169] Pinnipeds have several adaptions for reducing drag. In addition to their streamlined bodies, they have smooth networks of muscle bundles in their skin that may increase laminar flow and make it easier for them to slip through water. They also lack arrector pili, so their fur can be streamlined as they swim. [170] They rely on their fore-flippers for locomotion in a wing-like manner similar to penguins and sea turtles. [171] Fore-flipper movement is not continuous, and the animal glides between each stroke. [169] Compared to terrestrial carnivorans, the fore-limbs are reduced in length, which gives the locomotor muscles at the shoulder and elbow joints greater mechanical advantage; [168] the hind-flippers serve as stabilizers. [170] Other semi-aquatic mammals include beavers, hippopotamuses, otters and platypuses. [172] Hippos are very large semi-aquatic mammals, and their barrel-shaped bodies have graviportal skeletal structures, [173] adapted to carrying their enormous weight, and their specific gravity allows them to sink and move along the bottom of a river. [174]


Communication and vocalization

Vervet monkeys use at least four distinct alarm calls for different predators. Monkey & Baby.JPG
Vervet monkeys use at least four distinct alarm calls for different predators.

Many mammals communicate by vocalizing. Vocal communication serves many purposes, including in mating rituals, as warning calls, [176] to indicate food sources, and for social purposes. Males often call during mating rituals to ward off other males and to attract females, as in the roaring of lions and red deer. [177] The songs of the humpback whale may be signals to females; [178] they have different dialects in different regions of the ocean. [179] Social vocalizations include the territorial calls of gibbons, and the use of frequency in greater spear-nosed bats to distinguish between groups. [180] The vervet monkey gives a distinct alarm call for each of at least four different predators, and the reactions of other monkeys vary according to the call. For example, if an alarm call signals a python, the monkeys climb into the trees, whereas the eagle alarm causes monkeys to seek a hiding place on the ground. [175] Prairie dogs similarly have complex calls that signal the type, size, and speed of an approaching predator. [181] Elephants communicate socially with a variety of sounds including snorting, screaming, trumpeting, roaring and rumbling. Some of the rumbling calls are infrasonic, below the hearing range of humans, and can be heard by other elephants up to 6 miles (9.7 km) away at still times near sunrise and sunset. [182]

Orca calling including occasional echolocation clicks

Mammals signal by a variety of means. Many give visual anti-predator signals, as when deer and gazelle stot, honestly indicating their fit condition and their ability to escape, [183] [184] or when white-tailed deer and other prey mammals flag with conspicuous tail markings when alarmed, informing the predator that it has been detected. [185] Many mammals make use of scent-marking, sometimes possibly to help defend territory, but probably with a range of functions both within and between species. [186] [187] [188] Microbats and toothed whales including oceanic dolphins vocalize both socially and in echolocation. [189] [190] [191]


A short-beaked echidna foraging for insects

To maintain a high constant body temperature is energy expensive—mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals—this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants, which contain complex carbohydrates such as cellulose. An herbivorous diet includes subtypes such as granivory (seed eating), folivory (leaf eating), frugivory (fruit eating), nectarivory (nectar eating), gummivory (gum eating) and mycophagy (fungus eating). The digestive tract of an herbivore is host to bacteria that ferment these complex substances, and make them available for digestion, which are either housed in the multichambered stomach or in a large cecum. [94] Some mammals are coprophagous, consuming feces to absorb the nutrients not digested when the food was first ingested. [90] :131–137 An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive tract because the proteins, lipids and minerals found in meat require little in the way of specialized digestion. Exceptions to this include baleen whales who also house gut flora in a multi-chambered stomach, like terrestrial herbivores. [192]

The size of an animal is also a factor in determining diet type (Allen's rule). Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high energy requirements and a high metabolic rate. Mammals that weigh less than about 18 ounces (510 g; 1.1 lb) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of an herbivore. Larger animals, on the other hand, generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (carnivores that feed on larger vertebrates) or a slower digestive process (herbivores). [193] Furthermore, mammals that weigh more than 18 ounces (510 g; 1.1 lb) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites). [194]

A polar bear (Ursus maritimus) scavenging a narwhal whale (Monodon monoceros) carcass - journal.pone.0060797.g001-A.png
The hypocarnivorous American black bear (Ursus americanus) vs. the hypercarnivorous polar bear (Ursus maritimus) [195]

Some mammals are omnivores and display varying degrees of carnivory and herbivory, generally leaning in favor of one more than the other. Since plants and meat are digested differently, there is a preference for one over the other, as in bears where some species may be mostly carnivorous and others mostly herbivorous. [196] They are grouped into three categories: mesocarnivory (50–70% meat), hypercarnivory (70% and greater of meat), and hypocarnivory (50% or less of meat). The dentition of hypocarnivores consists of dull, triangular carnassial teeth meant for grinding food. Hypercarnivores, however, have conical teeth and sharp carnassials meant for slashing, and in some cases strong jaws for bone-crushing, as in the case of hyenas, allowing them to consume bones; some extinct groups, notably the Machairodontinae, had saber-shaped canines. [195]

Some physiological carnivores consume plant matter and some physiological herbivores consume meat. From a behavioral aspect, this would make them omnivores, but from the physiological standpoint, this may be due to zoopharmacognosy. Physiologically, animals must be able to obtain both energy and nutrients from plant and animal materials to be considered omnivorous. Thus, such animals are still able to be classified as carnivores and herbivores when they are just obtaining nutrients from materials originating from sources that do not seemingly complement their classification. [197] For example, it is well documented that some ungulates such as giraffes, camels, and cattle, will gnaw on bones to consume particular minerals and nutrients. [198] Also, cats, which are generally regarded as obligate carnivores, occasionally eat grass to regurgitate indigestible material (such as hairballs), aid with hemoglobin production, and as a laxative. [199]

Many mammals, in the absence of sufficient food requirements in an environment, suppress their metabolism and conserve energy in a process known as hibernation. [200] In the period preceding hibernation, larger mammals, such as bears, become polyphagic to increase fat stores, whereas smaller mammals prefer to collect and stash food. [201] The slowing of the metabolism is accompanied by a decreased heart and respiratory rate, as well as a drop in internal temperatures, which can be around ambient temperature in some cases. For example, the internal temperatures of hibernating arctic ground squirrels can drop to −2.9 °C (26.8 °F), however the head and neck always stay above 0 °C (32 °F). [202] A few mammals in hot environments aestivate in times of drought or extreme heat, for example the fat-tailed dwarf lemur (Cheirogaleus medius). [203]


In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats, for example, are considered to be highly intelligent, as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey. [194]

A bonobo fishing for termites with a stick A Bonobo at the San Diego Zoo "fishing" for termites.jpg
A bonobo fishing for termites with a stick

Tool use by animals may indicate different levels of learning and cognition. The sea otter uses rocks as essential and regular parts of its foraging behaviour (smashing abalone from rocks or breaking open shells), with some populations spending 21% of their time making tools. [204] Other tool use, such as chimpanzees using twigs to "fish" for termites, may be developed by watching others use tools and may even be a true example of animal teaching. [205] Tools may even be used in solving puzzles in which the animal appears to experience a "Eureka moment". [206] Other mammals that do not use tools, such as dogs, can also experience a Eureka moment. [207]

Brain size was previously considered a major indicator of the intelligence of an animal. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for more complex cognitive tasks. Allometric analysis indicates that mammalian brain size scales at approximately the 23 or 34 exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalisation quotient that can be used as another indication of animal intelligence. [208] Sperm whales have the largest brain mass of any animal on earth, averaging 8,000 cubic centimetres (490 in3) and 7.8 kilograms (17 lb) in mature males. [209]

Self-awareness appears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such as metacognitive reasoning. The traditional method for measuring this is the mirror test, which determines if an animal possesses the ability of self-recognition. [210] Mammals that have passed the mirror test include Asian elephants (some pass, some do not); [211] chimpanzees; [212] bonobos; [213] orangutans; [214] humans, from 18 months (mirror stage); [215] bottlenose dolphins [lower-alpha 1] [216] killer whales; [217] and false killer whales. [217]

Social structure

Female elephants live in stable groups, along with their offspring. Borneo elephants.png
Female elephants live in stable groups, along with their offspring.

Eusociality is the highest level of social organization. These societies have an overlap of adult generations, the division of reproductive labor and cooperative caring of young. Usually insects, such as bees, ants and termites, have eusocial behavior, but it is demonstrated in two rodent species: the naked mole-rat [218] and the Damaraland mole-rat. [219]

Presociality is when animals exhibit more than just sexual interactions with members of the same species, but fall short of qualifying as eusocial. That is, presocial animals can display communal living, cooperative care of young, or primitive division of reproductive labor, but they do not display all of the three essential traits of eusocial animals. Humans and some species of Callitrichidae (marmosets and tamarins) are unique among primates in their degree of cooperative care of young. [220] Harry Harlow set up an experiment with rhesus monkeys, presocial primates, in 1958; the results from this study showed that social encounters are necessary in order for the young monkeys to develop both mentally and sexually. [221]

A fission-fusion society is a society that changes frequently in its size and composition, making up a permanent social group called the "parent group". Permanent social networks consist of all individual members of a community and often varies to track changes in their environment. In a fission–fusion society, the main parent group can fracture (fission) into smaller stable subgroups or individuals to adapt to environmental or social circumstances. For example, a number of males may break off from the main group in order to hunt or forage for food during the day, but at night they may return to join (fusion) the primary group to share food and partake in other activities. Many mammals exhibit this, such as primates (for example orangutans and spider monkeys), [222] elephants, [223] spotted hyenas, [224] lions, [225] and dolphins. [226]

Solitary animals defend a territory and avoid social interactions with the members of its species, except during breeding season. This is to avoid resource competition, as two individuals of the same species would occupy the same niche, and to prevent depletion of food. [227] A solitary animal, while foraging, can also be less conspicuous to predators or prey. [228]

Red kangaroos "boxing" for dominance Fighting red kangaroos 2.jpg
Red kangaroos "boxing" for dominance

In a hierarchy, individuals are either dominant or submissive. A despotic hierarchy is where one individual is dominant while the others are submissive, as in wolves and lemurs, [229] and a pecking order is a linear ranking of individuals where there is a top individual and a bottom individual. Pecking orders may also be ranked by sex, where the lowest individual of a sex has a higher ranking than the top individual of the other sex, as in hyenas. [230] Dominant individuals, or alphas, have a high chance of reproductive success, especially in harems where one or a few males (resident males) have exclusive breeding rights to females in a group. [231] Non-resident males can also be accepted in harems, but some species, such as the common vampire bat (Desmodus rotundus), may be more strict. [232]

Some mammals are perfectly monogamous, meaning that they mate for life and take no other partners (even after the original mate's death), as with wolves, Eurasian beavers, and otters. [233] [234] There are three types of polygamy: either one or multiple dominant males have breeding rights (polygyny), multiple males that females mate with (polyandry), or multiple males have exclusive relations with multiple females (polygynandry). It is much more common for polygynous mating to happen, which, excluding leks, are estimated to occur in up to 90% of mammals. [235] Lek mating occurs when males congregate around females and try to attract them with various courtship displays and vocalizations, as in harbor seals. [236]

All higher mammals (excluding monotremes) share two major adaptations for care of the young: live birth and lactation. These imply a group-wide choice of a degree of parental care. They may build nests and dig burrows to raise their young in, or feed and guard them often for a prolonged period of time. Many mammals are K-selected, and invest more time and energy into their young than do r-selected animals. When two animals mate, they both share an interest in the success of the offspring, though often to different extremes. Mammalian females exhibit some degree of maternal aggression, another example of parental care, which may be targeted against other females of the species or the young of other females; however, some mammals may "aunt" the infants of other females, and care for them. Mammalian males may play a role in child rearing, as with tenrecs, however this varies species to species, even within the same genus. For example, the males of the southern pig-tailed macaque (Macaca nemestrina) do not participate in child care, whereas the males of the Japanese macaque (M. fuscata) do. [237]

Humans and other mammals

In human culture

Upper Paleolithic cave painting of a variety of large mammals, Lascaux, c. 17,300 years old. Lascaux painting.jpg
Upper Paleolithic cave painting of a variety of large mammals, Lascaux, c.17,300 years old.

Non-human mammals play a wide variety of roles in human culture. They are the most popular of pets, with tens of millions of dogs, cats and other animals including rabbits and mice kept by families around the world. [238] [239] [240] Mammals such as mammoths, horses and deer are among the earliest subjects of art, being found in Upper Paleolithic cave paintings such as at Lascaux. [241] Major artists such as Albrecht Dürer, George Stubbs and Edwin Landseer are known for their portraits of mammals. [242] Many species of mammals have been hunted for sport and for food; deer and wild boar are especially popular as game animals. [243] [244] [245] Mammals such as horses and dogs are widely raced for sport, often combined with betting on the outcome. [246] [247] There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own. [248] Mammals further play a wide variety of roles in literature, [249] [250] [251] film, [252] mythology, and religion. [253] [254] [255]

Uses and importance

Cattle have been kept for milk for thousands of years. Hand milking a cow at Cobbes Farm Museum.jpg
Cattle have been kept for milk for thousands of years.

The domestication of mammals was instrumental in the Neolithic development of agriculture and of civilization, causing farmers to replace hunter-gatherers around the world. [lower-alpha 2] [257] This transition from hunting and gathering to herding flocks and growing crops was a major step in human history. The new agricultural economies, based on domesticated mammals, caused "radical restructuring of human societies, worldwide alterations in biodiversity, and significant changes in the Earth's landforms and its atmosphere... momentous outcomes". [258]

Domestic mammals form a large part of the livestock raised for meat across the world. They include (2009) around 1.4 billion cattle, 1 billion sheep, 1 billion domestic pigs, [259] [260] and (1985) over 700 million rabbits. [261] Working domestic animals including cattle and horses have been used for work and transport from the origins of agriculture, their numbers declining with the arrival of mechanised transport and agricultural machinery. In 2004 they still provided some 80% of the power for the mainly small farms in the third world, and some 20% of the world's transport, again mainly in rural areas. In mountainous regions unsuitable for wheeled vehicles, pack animals continue to transport goods. [262] Mammal skins provide leather for shoes, clothing and upholstery. Wool from mammals including sheep, goats and alpacas has been used for centuries for clothing. [263] [264]

Livestock make up 62% of the world's mammal biomass; humans account for 34%; and wild mammals are just 4% Distribution of Mammals on Earth.png
Livestock make up 62% of the world's mammal biomass; humans account for 34%; and wild mammals are just 4%

Mammals serve a major role in science as experimental animals, both in fundamental biological research, such as in genetics, [266] and in the development of new medicines, which must be tested exhaustively to demonstrate their safety. [267] Millions of mammals, especially mice and rats, are used in experiments each year. [268] A knockout mouse is a genetically modified mouse with an inactivated gene, replaced or disrupted with an artificial piece of DNA. They enable the study of sequenced genes whose functions are unknown. [269] A small percentage of the mammals are non-human primates, used in research for their similarity to humans. [270] [271] [272]

Despite the benefits domesticated mammals had for human development, humans have an increasingly detrimental effect on wild mammals across the world. It has been estimated that the mass of all wild mammals has declined to only 4% of all mammals, with 96% of mammals being humans and their livestock now (see figure). In fact, terrestrial wild mammals make up only 2% of all mammals. [273] [274]


Equus quagga quagga, coloured.jpg
Rau Quagga on Devils Peak.jpg
A true quagga, 1870 (left) vs. a bred-back quagga, 2014 (right)

Hybrids are offspring resulting from the breeding of two genetically distinct individuals, which usually will result in a high degree of heterozygosity, though hybrid and heterozygous are not synonymous. The deliberate or accidental hybridizing of two or more species of closely related animals through captive breeding is a human activity which has been in existence for millennia and has grown for economic purposes. [275] Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids. [276] Natural hybrids will occur in hybrid zones, where two populations of species within the same genera or species living in the same or adjacent areas will interbreed with each other. Some hybrids have been recognized as species, such as the red wolf (though this is controversial). [277]

Artificial selection, the deliberate selective breeding of domestic animals, is being used to breed back recently extinct animals in an attempt to achieve an animal breed with a phenotype that resembles that extinct wildtype ancestor. A breeding-back (intraspecific) hybrid may be very similar to the extinct wildtype in appearance, ecological niche and to some extent genetics, but the initial gene pool of that wild type is lost forever with its extinction. As a result, bred-back breeds are at best vague look-alikes of extinct wildtypes, as Heck cattle are of the aurochs. [278]

Purebred wild species evolved to a specific ecology can be threatened with extinction [279] through the process of genetic pollution, the uncontrolled hybridization, introgression genetic swamping which leads to homogenization or out-competition from the heterosic hybrid species. [280] When new populations are imported or selectively bred by people, or when habitat modification brings previously isolated species into contact, extinction in some species, especially rare varieties, is possible. [281] Interbreeding can swamp the rarer gene pool and create hybrids, depleting the purebred gene pool. For example, the endangered wild water buffalo is most threatened with extinction by genetic pollution from the domestic water buffalo. Such extinctions are not always apparent from a morphological standpoint. Some degree of gene flow is a normal evolutionary process, nevertheless, hybridization threatens the existence of rare species. [282] [283]


Biodiversity of large mammal species per continent before and after humans arrived there Extinctions Africa Austrailia NAmerica Madagascar.gif
Biodiversity of large mammal species per continent before and after humans arrived there

The loss of species from ecological communities, defaunation, is primarily driven by human activity. [284] This has resulted in empty forests, ecological communities depleted of large vertebrates. [285] [286] In the Quaternary extinction event, the mass die-off of megafaunal variety coincided with the appearance of humans, suggesting a human influence. One hypothesis is that humans hunted large mammals, such as the woolly mammoth, into extinction. [287] [288] The 2019 Global Assessment Report on Biodiversity and Ecosystem Services by IPBES states that the total biomass of wild mammals has declined by 82 percent since the beginning of human civilization. [289] [290] Wild animals make up just 4% of mammalian biomass on earth, while humans and their domesticated animals make up 96%. [274]

Various species are predicted to become extinct in the near future, [291] among them the rhinoceros, [292] giraffes, [293] and species of primates [294] and pangolins. [295] According to the WWF's 2020 Living Planet Report , vertebrate wildlife populations have declined by 68% since 1970 as a result of human activities, particularly overconsumption, population growth and intensive farming, which is evidence that humans have triggered a sixth mass extinction event. [296] [297] Hunting alone threatens hundreds of mammalian species around the world. [298] [299] Scientists claim that the growing demand for meat is contributing to biodiversity loss as this is a significant driver of deforestation and habitat destruction; species-rich habitats, such as significant portions of the Amazon rainforest, are being converted to agricultural land for meat production. [300] [301] [302] Another influence is over-hunting and poaching, which can reduce the overall population of game animals, [303] especially those located near villages, [304] as in the case of peccaries. [305] The effects of poaching can especially be seen in the ivory trade with African elephants. [306] Marine mammals are at risk from entanglement from fishing gear, notably cetaceans, with discard mortalities ranging from 65,000 to 86,000 individuals annually. [307]

Attention is being given to endangered species globally, notably through the Convention on Biological Diversity, otherwise known as the Rio Accord, which includes 189 signatory countries that are focused on identifying endangered species and habitats. [308] Another notable conservation organization is the IUCN, which has a membership of over 1,200 governmental and non-governmental organizations. [309]

Recent extinctions can be directly attributed to human influences. [310] [284] The IUCN characterizes 'recent' extinction as those that have occurred past the cut-off point of 1500, [311] and around 80 mammal species have gone extinct since that time and 2015. [312] Some species, such as the Père David's deer [313] are extinct in the wild, and survive solely in captive populations. Other species, such as the Florida panther, are ecologically extinct, surviving in such low numbers that they essentially have no impact on the ecosystem. [314] :318 Other populations are only locally extinct (extirpated), still existing elsewhere, but reduced in distribution, [314] :75–77 as with the extinction of gray whales in the Atlantic. [315]


  1. Decreased latency to approach the mirror, repetitious head circling and close viewing of the marked areas were considered signs of self-recognition since they do not have arms and cannot touch the marked areas. [216]
  2. Diamond discussed this matter further in his 1997 book Guns, Germs, and Steel . [256]

See also

External resources

Related Research Articles

<span class="mw-page-title-main">Cenozoic</span> Third era of the Phanerozoic Eon (66 million years ago to present)

The Cenozoic is Earth's current geological era, representing the last 66 million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants. It is the latest of three geological eras, preceded by the Mesozoic and Paleozoic. The Cenozoic started with the Cretaceous–Paleogene extinction event, when many species, including the non-avian dinosaurs, became extinct in an event attributed by most experts to the impact of a large asteroid or other celestial body, the Chicxulub impactor.

<span class="mw-page-title-main">Marsupial</span> Infraclass of mammals in the clade Metatheria

Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a pouch. Living marsupials include kangaroos, koalas, opossums, Tasmanian devils, wombats, wallabies, and bandicoots among others, while many extinct species, such as the thylacine, Thylacoleo, and Diprotodon, are also known.

<span class="mw-page-title-main">Multituberculata</span> Extinct order of mammals

Multituberculata is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, and reached a peak diversity during the Late Cretaceous and Paleocene. They eventually declined from the mid-Paleocene onwards, disappearing from the known fossil record in the late Eocene. They are the most diverse order of Mesozoic mammals with more than 200 species known, ranging from mouse-sized to beaver-sized. These species occupied a diversity of ecological niches, ranging from burrow-dwelling to squirrel-like arborealism to jerboa-like hoppers. Multituberculates are usually placed as crown mammals outside either of the two main groups of living mammals—Theria, including placentals and marsupials, and Monotremata—but usually as closer to Theria than to monotremes. They are considered to be closely related to Euharamiyida and Gondwanatheria as part of Allotheria.

<span class="mw-page-title-main">Placentalia</span> Infraclass of mammals in the clade Eutheria

Placental mammals are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that the fetus is carried in the uterus of its mother to a relatively late stage of development. The name is something of a misnomer considering that marsupials also nourish their fetuses via a placenta, though for a relatively briefer period, giving birth to less developed young which are then nurtured for a period inside the mother's pouch.

<span class="mw-page-title-main">Molar (tooth)</span> Large tooth at the back of the mouth

The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name molar derives from Latin, molaris dens, meaning "millstone tooth", from mola, millstone and dens, tooth. Molars show a great deal of diversity in size and shape across mammal groups. The third molar of humans is sometimes vestigial.

<span class="mw-page-title-main">Megafauna</span> Large animals

In zoology, megafauna are large animals. The most common thresholds to be a megafauna are weighing over 46 kilograms (100 lb) or weighing over a tonne, 1,000 kilograms (2,205 lb). The first of these include many species not popularly thought of as overly large, and being the only few large animals left in a given range/area, such as white-tailed deer, Thomson's gazelle, and red kangaroo.

<i>Eomaia</i> Extinct genus of mammals

Eomaia is a genus of extinct fossil mammals containing the single species Eomaia scansoria, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about 125 million years ago. The single fossil specimen of this species is 10 centimetres (3.9 in) in length and virtually complete. An estimate of the body weight is 20–25 grams (0.71–0.88 oz). It is exceptionally well-preserved for a 125-million-year-old specimen. Although the fossil's skull is squashed flat, its teeth, tiny foot bones, cartilages and even its fur are visible.

<span class="mw-page-title-main">Eutheria</span> Clade of mammals in the subclass Theria

Eutheria, also called Placentalia sensu lato or Pan-Placentalia, is the clade consisting of placental mammals and all therian mammals that are more closely related to placentals than to marsupials.

<span class="mw-page-title-main">Metatheria</span> Clade of marsupials and close relatives

Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives.

<span class="mw-page-title-main">Theria</span> Subclass of mammals in the clade Theriiformes

Theria is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians and the metatherians but excludes the egg-laying monotremes and various extinct mammals evolving prior to the common ancestor of placentals and marsupials.

<span class="mw-page-title-main">Cimolesta</span> Extinct order of mammals

Cimolesta is an extinct order of non-placental eutherian mammals. Cimolestans had a wide variety of body shapes, dentition and lifestyles, though the majority of them were small to medium-sized general mammals that bore superficial resemblances to rodents, lagomorphs, mustelids, and marsupials.

<span class="mw-page-title-main">Mammaliaformes</span> Clade of mammals and extinct relatives

Mammaliaformes is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent common ancestor of Morganucodonta and the crown group mammals; the latter is the clade originating with the most recent common ancestor of extant Monotremata, Marsupialia, and Placentalia. Besides Morganucodonta and the crown group mammals, Mammaliaformes includes Docodonta and Hadrocodium as well as the Triassic Tikitherium, the earliest known member of the group.

<span class="mw-page-title-main">Boreoeutheria</span> Magnorder of mammals containing Laurasiatheria and Euarchontoglires

Boreoeutheria is a magnorder of placental mammals that groups together superorders Euarchontoglires and Laurasiatheria. With a few exceptions male animals in the clade have a scrotum, an ancestral feature of the clade. The sub-clade Scrotifera was named after this feature.

The mammals of Australia have a rich fossil history, as well as a variety of extant mammalian species, dominated by the marsupials, but also including monotremes and placentals. The marsupials evolved to fill specific ecological niches, and in many cases they are physically similar to the placental mammals in Eurasia and North America that occupy similar niches, a phenomenon known as convergent evolution. For example, the top mammalian predators in Australia, the Tasmanian tiger and the marsupial lion, bore a striking resemblance to large canids such as the gray wolf and large cats respectively; gliding possums and flying squirrels have similar adaptations enabling their arboreal lifestyle; and the numbat and anteaters are both digging insectivores. Most of Australia's mammals are herbivores or omnivores.

<span class="mw-page-title-main">Evolution of mammals</span> Derivation of mammals from a synapsid precursor, and the adaptive radiation of mammal species

The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes. Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.

<span class="mw-page-title-main">Epipubic bone</span>

Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials, monotremes and fossil mammals like multituberculates, and even basal eutherians . They first occur in non-mammalian cynodonts such as tritylodontids, suggesting that they are a synapomorphy between them and Mammaliformes.

Color vision, a proximate adaptation of the vision sensory modality, allows for the discrimination of light based on its wavelength components.

<span class="mw-page-title-main">Monotreme</span> Order of egg-laying mammals

Monotremes are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified by structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts, compared to the more common mammalian types. Although they are different from almost all mammals in that they lay eggs, like all mammals, the female monotremes nurse their young with milk.

<span class="mw-page-title-main">Mammalian reproduction</span> Most mammals are viviparous, giving birth to live young

Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.

Cochlea is Latin for “snail, shell or screw” and originates from the Greek word κοχλίας kokhlias. The modern definition, the auditory portion of the inner ear, originated in the late 17th century. Within the mammalian cochlea exists the organ of Corti, which contains hair cells that are responsible for translating the vibrations it receives from surrounding fluid-filled ducts into electrical impulses that are sent to the brain to process sound.


  1. Lewis, Charlton T.; Short, Charles (1879). "mamma". A Latin Dictionary. Perseus Digital Library.
  2. Vaughan TA, Ryan JM, Czaplewski NJ (2013). "Classification of Mammals". Mammalogy (6th ed.). Jones and Bartlett Learning. ISBN   978-1-284-03209-3.
  3. Simpson GG (1945). "Principles of classification, and a classification of mammals". American Museum of Natural History . 85.
  4. 1 2 Szalay FS (1999). "Classification of mammals above the species level: Review". Journal of Vertebrate Paleontology. 19 (1): 191–195. doi:10.1080/02724634.1999.10011133. JSTOR   4523980.
  5. 1 2 Wilson DE, Reeder DM, eds. (2005). "Preface and introductory material". Mammal Species of the World: A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press. p. xxvi. ISBN   978-0-8018-8221-0. OCLC   62265494.
  6. "Mammals". The IUCN Red List of Threatened Species. International Union for Conservation of Nature (IUCN). April 2010. Retrieved 23 August 2016.
  7. Burgin CJ, Colella JP, Kahn PL, Upham NS (1 February 2018). "How many species of mammals are there?". Journal of Mammalogy . 99 (1): 1–14. doi: 10.1093/jmammal/gyx147 .
  8. Rowe T (1988). "Definition, diagnosis, and origin of Mammalia" (PDF). Journal of Vertebrate Paleontology. 8 (3): 241–264. Bibcode:1988JVPal...8..241R. doi:10.1080/02724634.1988.10011708.
  9. Lyell C (1871). The Student's Elements of Geology. London: John Murray. p. 347. ISBN   978-1-345-18248-4.
  10. Cifelli RL, Davis BM (December 2003). "Paleontology. Marsupial origins". Science. 302 (5652): 1899–1900. doi:10.1126/science.1092272. PMID   14671280. S2CID   83973542.
  11. Kemp TS (2005). The Origin and Evolution of Mammals (PDF). United Kingdom: Oxford University Press. p. 3. ISBN   978-0-19-850760-4. OCLC   232311794.
  12. Datta PM (2005). "Earliest mammal with transversely expanded upper molar from the Late Triassic (Carnian) Tiki Formation, South Rewa Gondwana Basin, India". Journal of Vertebrate Paleontology. 25 (1): 200–207. doi:10.1671/0272-4634(2005)025[0200:EMWTEU]2.0.CO;2. S2CID   131236175.
  13. Luo ZX, Martin T (2007). "Analysis of Molar Structure and Phylogeny of Docodont Genera" (PDF). Bulletin of Carnegie Museum of Natural History. 39: 27–47. doi:10.2992/0145-9058(2007)39[27:AOMSAP]2.0.CO;2. S2CID   29846648. Archived from the original (PDF) on 3 March 2016. Retrieved 8 April 2013.
  14. McKenna MC, Bell SG (1997). Classification of Mammals above the Species Level. New York: Columbia University Press. ISBN   978-0-231-11013-6. OCLC   37345734.
  15. Nilsson MA, Churakov G, Sommer M, Tran NV, Zemann A, Brosius J, Schmitz J (July 2010). "Tracking marsupial evolution using archaic genomic retroposon insertions". PLOS Biology. 8 (7): e1000436. doi: 10.1371/journal.pbio.1000436 . PMC   2910653 . PMID   20668664.
  16. Scornavacca C, Belkhir K, Lopez J, Dernat R, Delsuc F, Douzery EJ, Ranwez V (April 2019). "OrthoMaM v10: Scaling-up orthologous coding sequence and exon alignments with more than one hundred mammalian genomes". Molecular Biology and Evolution. 36 (4): 861–862. doi:10.1093/molbev/msz015. PMC   6445298 . PMID   30698751.
  17. Kriegs JO, Churakov G, Kiefmann M, Jordan U, Brosius J, Schmitz J (April 2006). "Retroposed elements as archives for the evolutionary history of placental mammals". PLOS Biology. 4 (4): e91. doi: 10.1371/journal.pbio.0040091 . PMC   1395351 . PMID   16515367.
  18. 1 2 Nishihara H, Maruyama S, Okada N (March 2009). "Retroposon analysis and recent geological data suggest near-simultaneous divergence of the three superorders of mammals". Proceedings of the National Academy of Sciences of the United States of America. 106 (13): 5235–5240. Bibcode:2009PNAS..106.5235N. doi: 10.1073/pnas.0809297106 . PMC   2655268 . PMID   19286970.
  19. Springer MS, Murphy WJ, Eizirik E, O'Brien SJ (February 2003). "Placental mammal diversification and the Cretaceous-Tertiary boundary". Proceedings of the National Academy of Sciences of the United States of America. 100 (3): 1056–1061. Bibcode:2003PNAS..100.1056S. doi: 10.1073/pnas.0334222100 . PMC   298725 . PMID   12552136.
  20. Tarver JE, Dos Reis M, Mirarab S, Moran RJ, Parker S, O'Reilly JE, et al. (January 2016). "The Interrelationships of Placental Mammals and the Limits of Phylogenetic Inference". Genome Biology and Evolution. 8 (2): 330–344. doi:10.1093/gbe/evv261. hdl:1983/64d6e437-3320-480d-a16c-2e5b2e6b61d4. PMC   4779606 . PMID   26733575.
  21. Álvarez-Carretero S, Tamuri AU, Battini M, Nascimento FF, Carlisle E, Asher RJ, Yang Z, Donoghue PC, et al. (2022). "A species-level timeline of mammal evolution integrating phylogenomic data". Nature (602): 263–267. doi:10.1038/s41586-021-04341-1.
  22. "Data for A Species-Level Timeline of Mammal Evolution Integrating Phylogenomic Data". Figshare. Retrieved 11 November 2023.
  23. Meng J, Wang Y, Li C (April 2011). "Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont". Nature. 472 (7342): 181–185. Bibcode:2011Natur.472..181M. doi:10.1038/nature09921. PMID   21490668. S2CID   4428972.
  24. Ahlberg PE, Milner AR (April 1994). "The Origin and Early Diversification of Tetrapods". Nature. 368 (6471): 507–514. Bibcode:1994Natur.368..507A. doi:10.1038/368507a0. S2CID   4369342.
  25. "Amniota – Palaeos". Archived from the original on 20 December 2010.
  26. "Synapsida overview – Palaeos". Archived from the original on 20 December 2010.
  27. 1 2 3 Kemp TS (July 2006). "The origin and early radiation of the therapsid mammal-like reptiles: a palaeobiological hypothesis" (PDF). Journal of Evolutionary Biology. 19 (4): 1231–1247. doi:10.1111/j.1420-9101.2005.01076.x. PMID   16780524. S2CID   3184629. Archived from the original (PDF) on 8 March 2021. Retrieved 14 January 2012.
  28. 1 2 Bennett AF, Ruben JA (1986). "The metabolic and thermoregulatory status of therapsids". In Hotton III N, MacLean JJ, Roth J, Roth EC (eds.). The ecology and biology of mammal-like reptiles. Washington, DC: Smithsonian Institution Press. pp. 207–218. ISBN   978-0-87474-524-5.
  29. Kermack DM, Kermack KA (1984). The evolution of mammalian characters. Washington, DC: Croom Helm. ISBN   978-0-7099-1534-8. OCLC   10710687.
  30. Araújo; et al. (28 July 2022). "Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy". Nature. 607 (7920): 726–731. Bibcode:2022Natur.607..726A. doi:10.1038/s41586-022-04963-z. PMID   35859179. S2CID   236245230.
  31. Tanner LH, Lucas SG, Chapman MG (2004). "Assessing the record and causes of Late Triassic extinctions" (PDF). Earth-Science Reviews. 65 (1–2): 103–139. Bibcode:2004ESRv...65..103T. doi:10.1016/S0012-8252(03)00082-5. Archived from the original (PDF) on 25 October 2007.
  32. Brusatte SL, Benton MJ, Ruta M, Lloyd GT (September 2008). "Superiority, competition, and opportunism in the evolutionary radiation of dinosaurs" (PDF). Science. 321 (5895): 1485–1488. Bibcode:2008Sci...321.1485B. doi:10.1126/science.1161833. hdl:20.500.11820/00556baf-6575-44d9-af39-bdd0b072ad2b. PMID   18787166. S2CID   13393888.
  33. Gauthier JA (1986). "Saurischian monophyly and the origin of birds". In Padian K (ed.). The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Sciences. Vol. 8. San Francisco: California Academy of Sciences. pp. 1–55.
  34. Sereno PC (1991). "Basal archosaurs: phylogenetic relationships and functional implications". Memoirs of the Society of Vertebrate Paleontology. 2: 1–53. doi:10.2307/3889336. JSTOR   3889336.
  35. MacLeod N, Rawson PF, Forey PL, Banner FT, Boudagher-Fadel MK, Bown PR, et al. (1997). "The Cretaceous–Tertiary biotic transition". Journal of the Geological Society. 154 (2): 265–292. Bibcode:1997JGSoc.154..265M. doi:10.1144/gsjgs.154.2.0265. S2CID   129654916.
  36. Hunt DM, Hankins MW, Collin SP, Marshall NJ (2014). Evolution of Visual and Non-visual Pigments. London: Springer. p. 73. ISBN   978-1-4614-4354-4. OCLC   892735337.
  37. Bakalar N (2006). "Jurassic "Beaver" Found; Rewrites History of Mammals". National Geographic News. Archived from the original on 3 March 2006. Retrieved 28 May 2016.
  38. Hall MI, Kamilar JM, Kirk EC (December 2012). "Eye shape and the nocturnal bottleneck of mammals". Proceedings of the Royal Society B: Biological Sciences. 279 (1749): 4962–4968. doi:10.1098/rspb.2012.2258. PMC   3497252 . PMID   23097513.
  39. Luo ZX (December 2007). "Transformation and diversification in early mammal evolution". Nature. 450 (7172): 1011–1019. Bibcode:2007Natur.450.1011L. doi:10.1038/nature06277. PMID   18075580. S2CID   4317817.
  40. Pickrell J (2003). "Oldest Marsupial Fossil Found in China". National Geographic News. Archived from the original on 17 December 2003. Retrieved 28 May 2016.
  41. 1 2 Luo ZX, Yuan CX, Meng QJ, Ji Q (August 2011). "A Jurassic eutherian mammal and divergence of marsupials and placentals". Nature. 476 (7361): 442–5. Bibcode:2011Natur.476..442L. doi:10.1038/nature10291. PMID   21866158. S2CID   205225806.
  42. Ji Q, Luo ZX, Yuan CX, Wible JR, Zhang JP, Georgi JA (April 2002). "The earliest known eutherian mammal". Nature. 416 (6883): 816–822. Bibcode:2002Natur.416..816J. doi:10.1038/416816a. PMID   11976675. S2CID   4330626.
  43. 1 2 Novacek MJ, Rougier GW, Wible JR, McKenna MC, Dashzeveg D, Horovitz I (October 1997). "Epipubic bones in eutherian mammals from the late Cretaceous of Mongolia". Nature. 389 (6650): 483–486. Bibcode:1997Natur.389..483N. doi:10.1038/39020. PMID   9333234. S2CID   205026882.
  44. Power ML, Schulkin J (2012). "Evolution of Live Birth in Mammals". Evolution of the Human Placenta. Baltimore: Johns Hopkins University Press. p. 68. ISBN   978-1-4214-0643-5.
  45. Rowe T, Rich TH, Vickers-Rich P, Springer M, Woodburne MO (January 2008). "The oldest platypus and its bearing on divergence timing of the platypus and echidna clades". Proceedings of the National Academy of Sciences of the United States of America. 105 (4): 1238–1242. Bibcode:2008PNAS..105.1238R. doi: 10.1073/pnas.0706385105 . PMC   2234122 . PMID   18216270.
  46. Grant T (1995). "Reproduction". The Platypus: A Unique Mammal. Sydney: University of New South Wales. p. 55. ISBN   978-0-86840-143-0. OCLC   33842474.
  47. Goldman AS (June 2012). "Evolution of immune functions of the mammary gland and protection of the infant". Breastfeeding Medicine. 7 (3): 132–142. doi:10.1089/bfm.2012.0025. PMID   22577734.
  48. 1 2 Rose KD (2006). The Beginning of the Age of Mammals. Baltimore: Johns Hopkins University Press. pp. 82–83. ISBN   978-0-8018-8472-6. OCLC   646769601.
  49. Brink AS (1955). "A study on the skeleton of Diademodon". Palaeontologia Africana. 3: 3–39.
  50. Kemp TS (1982). Mammal-like reptiles and the origin of mammals. London: Academic Press. p. 363. ISBN   978-0-12-404120-2. OCLC   8613180.
  51. Estes R (1961). "Cranial anatomy of the cynodont reptile Thrinaxodon liorhinus". Bulletin of the Museum of Comparative Zoology (1253): 165–180.
  52. "Thrinaxodon: The Emerging Mammal". National Geographic Daily News. 11 February 2009. Archived from the original on 14 February 2009. Retrieved 26 August 2012.
  53. 1 2 Bajdek P, Qvarnström M, Owocki K, Sulej T, Sennikov AG, Golubev VK, Niedźwiedzki G (2015). "Microbiota and food residues including possible evidence of pre-mammalian hair in Upper Permian coprolites from Russia". Lethaia. 49 (4): 455–477. doi:10.1111/let.12156.
  54. Botha-Brink J, Angielczyk KD (2010). "Do extraordinarily high growth rates in Permo-Triassic dicynodonts (Therapsida, Anomodontia) explain their success before and after the end-Permian extinction?". Zoological Journal of the Linnean Society. 160 (2): 341–365. doi: 10.1111/j.1096-3642.2009.00601.x .
  55. Paul GS (1988). Predatory Dinosaurs of the World. New York: Simon and Schuster. p.  464. ISBN   978-0-671-61946-6. OCLC   18350868.
  56. Watson JM, Graves JA (1988). "Monotreme Cell-Cycles and the Evolution of Homeothermy". Australian Journal of Zoology. 36 (5): 573–584. doi:10.1071/ZO9880573.
  57. McNab BK (1980). "Energetics and the limits to the temperate distribution in armadillos". Journal of Mammalogy. 61 (4): 606–627. doi:10.2307/1380307. JSTOR   1380307.
  58. Kielan-Jaworowska Z, Hurum JH (2006). "Limb posture in early mammals: Sprawling or parasagittal" (PDF). Acta Palaeontologica Polonica. 51 (3): 10237–10239.
  59. Lillegraven JA, Kielan-Jaworowska Z, Clemens WA (1979). Mesozoic Mammals: The First Two-Thirds of Mammalian History. University of California Press. p. 321. ISBN   978-0-520-03951-3. OCLC   5910695.
  60. Oftedal OT (July 2002). "The mammary gland and its origin during synapsid evolution". Journal of Mammary Gland Biology and Neoplasia. 7 (3): 225–252. doi:10.1023/A:1022896515287. PMID   12751889. S2CID   25806501.
  61. Oftedal OT (July 2002). "The origin of lactation as a water source for parchment-shelled eggs". Journal of Mammary Gland Biology and Neoplasia. 7 (3): 253–266. doi:10.1023/A:1022848632125. PMID   12751890. S2CID   8319185.
  62. "Breast Development". Texas Children's Hospital. Archived from the original on 13 January 2021. Retrieved 13 January 2021.
  63. Pfaff, Cathrin; Nagel, Doris; Gunnell, Gregg; Weber, Gerhard W.; Kriwet, Jürgen; Morlo, Michael; Bastl, Katharina (2017). "Palaeobiology of Hyaenodon exiguus (Hyaenodonta, Mammalia) based on morphometric analysis of the bony labyrinth". Journal of Anatomy. 230 (2): 282–289. doi:10.1111/joa.12545. PMC   5244453 . PMID   27666133.
  64. 1 2 Sahney S, Benton MJ, Ferry PA (August 2010). "Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land". Biology Letters. 6 (4): 544–547. doi:10.1098/rsbl.2009.1024. PMC   2936204 . PMID   20106856.
  65. Smith FA, Boyer AG, Brown JH, Costa DP, Dayan T, Ernest SK, et al. (November 2010). "The evolution of maximum body size of terrestrial mammals". Science. 330 (6008): 1216–1219. Bibcode:2010Sci...330.1216S. CiteSeerX . doi:10.1126/science.1194830. PMID   21109666. S2CID   17272200.
  66. Simmons NB, Seymour KL, Habersetzer J, Gunnell GF (February 2008). "Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation". Nature. 451 (7180): 818–821. Bibcode:2008Natur.451..818S. doi:10.1038/nature06549. hdl: 2027.42/62816 . PMID   18270539. S2CID   4356708.
  67. Bininda-Emonds OR, Cardillo M, Jones KE, MacPhee RD, Beck RM, Grenyer R, et al. (March 2007). "The delayed rise of present-day mammals" (PDF). Nature. 446 (7135): 507–512. Bibcode:2007Natur.446..507B. doi:10.1038/nature05634. PMID   17392779. S2CID   4314965.
  68. 1 2 Wible JR, Rougier GW, Novacek MJ, Asher RJ (June 2007). "Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary". Nature. 447 (7147): 1003–1006. Bibcode:2007Natur.447.1003W. doi:10.1038/nature05854. PMID   17581585. S2CID   4334424.
  69. O'Leary MA, Bloch JI, Flynn JJ, Gaudin TJ, Giallombardo A, Giannini NP, et al. (February 2013). "The placental mammal ancestor and the post-K-Pg radiation of placentals". Science. 339 (6120): 662–667. Bibcode:2013Sci...339..662O. doi:10.1126/science.1229237. hdl: 11336/7302 . PMID   23393258. S2CID   206544776.
  70. Halliday TJ, Upchurch P, Goswami A (February 2017). "Resolving the relationships of Paleocene placental mammals". Biological Reviews of the Cambridge Philosophical Society. 92 (1): 521–550. doi:10.1111/brv.12242. PMC   6849585 . PMID   28075073.
  71. Halliday TJ, Upchurch P, Goswami A (June 2016). "Eutherians experienced elevated evolutionary rates in the immediate aftermath of the Cretaceous-Palaeogene mass extinction". Proceedings. Biological Sciences. 283 (1833): 20153026. doi:10.1098/rspb.2015.3026. PMC   4936024 . PMID   27358361.
  72. 1 2 3 Ni X, Gebo DL, Dagosto M, Meng J, Tafforeau P, Flynn JJ, Beard KC (June 2013). "The oldest known primate skeleton and early haplorhine evolution". Nature. 498 (7452): 60–64. Bibcode:2013Natur.498...60N. doi:10.1038/nature12200. PMID   23739424. S2CID   4321956.
  73. Romer SA, Parsons TS (1977). The Vertebrate Body. Philadelphia: Holt-Saunders International. pp. 129–145. ISBN   978-0-03-910284-5. OCLC   60007175.
  74. Purves WK, Sadava DE, Orians GH, Helle HC (2001). Life: The Science of Biology (6th ed.). New York: Sinauer Associates, Inc. p. 593. ISBN   978-0-7167-3873-2. OCLC   874883911.
  75. Anthwal N, Joshi L, Tucker AS (January 2013). "Evolution of the mammalian middle ear and jaw: adaptations and novel structures". Journal of Anatomy. 222 (1): 147–160. doi:10.1111/j.1469-7580.2012.01526.x. PMC   3552421 . PMID   22686855.
  76. van Nievelt AF, Smith KK (2005). "To replace or not to replace: the significance of reduced functional tooth replacement in marsupial and placental mammals". Paleobiology. 31 (2): 324–346. doi:10.1666/0094-8373(2005)031[0324:trontr];2. S2CID   37750062.
  77. Libertini G, Ferrara N (April 2016). "Aging of perennial cells and organ parts according to the programmed aging paradigm". Age. 38 (2): 35. doi:10.1007/s11357-016-9895-0. PMC   5005898 . PMID   26957493.
  78. Mao F, Wang Y, Meng J (2015). "A Systematic Study on Tooth Enamel Microstructures of Lambdopsalis bulla (Multituberculate, Mammalia) – Implications for Multituberculate Biology and Phylogeny". PLOS ONE. 10 (5): e0128243. Bibcode:2015PLoSO..1028243M. doi: 10.1371/journal.pone.0128243 . PMC   4447277 . PMID   26020958.
  79. Osborn HF (1900). "Origin of the Mammalia, III. Occipital Condyles of Reptilian Tripartite Type". The American Naturalist. 34 (408): 943–947. doi: 10.1086/277821 . JSTOR   2453526.
  80. Crompton AW, Jenkins Jr FA (1973). "Mammals from Reptiles: A Review of Mammalian Origins". Annual Review of Earth and Planetary Sciences. 1: 131–155. Bibcode:1973AREPS...1..131C. doi:10.1146/annurev.ea.01.050173.001023.
  81. 1 2 Power ML, Schulkin J (2013). The Evolution Of The Human Placenta. Baltimore: Johns Hopkins University Press. pp. 1890–1891. ISBN   978-1-4214-0643-5. OCLC   940749490.
  82. Lindenfors P, Gittleman JL, Jones KE (2007). "Sexual size dimorphism in mammals". Sex, Size and Gender Roles: Evolutionary Studies of Sexual Size Dimorphism. Oxford: Oxford University Press. pp. 16–26. ISBN   978-0-19-920878-4.
  83. Dierauf LA, Gulland FM (2001). CRC Handbook of Marine Mammal Medicine: Health, Disease, and Rehabilitation (2nd ed.). Boca Raton: CRC Press. p. 154. ISBN   978-1-4200-4163-7. OCLC   166505919.
  84. Lui JH, Hansen DV, Kriegstein AR (July 2011). "Development and evolution of the human neocortex". Cell. 146 (1): 18–36. doi:10.1016/j.cell.2011.06.030. PMC   3610574 . PMID   21729779.
  85. Keeler CE (June 1933). "Absence of the Corpus Callosum as a Mendelizing Character in the House Mouse". Proceedings of the National Academy of Sciences of the United States of America. 19 (6): 609–611. Bibcode:1933PNAS...19..609K. doi: 10.1073/pnas.19.6.609 . JSTOR   86284. PMC   1086100 . PMID   16587795.
  86. Standring S, Borley NR (2008). Gray's anatomy: the anatomical basis of clinical practice (40th ed.). London: Churchill Livingstone. pp. 960–962. ISBN   978-0-8089-2371-8. OCLC   213447727.
  87. Betts JF, Desaix P, Johnson E, Johnson JE, Korol O, Kruse D, et al. (2013). Anatomy & physiology. Houston: Rice University Press. pp. 787–846. ISBN   978-1-938168-13-0. OCLC   898069394.
  88. Levitzky MG (2013). "Mechanics of Breathing". Pulmonary physiology (8th ed.). New York: McGraw-Hill Medical. ISBN   978-0-07-179313-1. OCLC   940633137.
  89. 1 2 Umesh KB (2011). "Pulmonary Anatomy and Physiology". Handbook of Mechanical Ventilation. New Delhi: Jaypee Brothers Medical Publishing. p. 12. ISBN   978-93-80704-74-6. OCLC   945076700.
  90. 1 2 3 4 5 6 7 8 9 Feldhamer GA, Drickamer LC, Vessey SH, Merritt JF, Krajewski C (2007). Mammalogy: Adaptation, Diversity, Ecology (3rd ed.). Baltimore: Johns Hopkins University Press. ISBN   978-0-8018-8695-9. OCLC   124031907.
  91. Tinker SW (1988). Whales of the World. Brill Archive. p. 51. ISBN   978-0-935848-47-2.
  92. Romer AS (1959). The vertebrate story (4th ed.). Chicago: University of Chicago Press. ISBN   978-0-226-72490-4.
  93. de Muizon C, Lange-Badré B (1997). "Carnivorous dental adaptations in tribosphenic mammals and phylogenetic reconstruction". Lethaia. 30 (4): 353–366. doi:10.1111/j.1502-3931.1997.tb00481.x.
  94. 1 2 Langer P (July 1984). "Comparative anatomy of the stomach in mammalian herbivores". Quarterly Journal of Experimental Physiology. 69 (3): 615–625. doi:10.1113/expphysiol.1984.sp002848. PMID   6473699. S2CID   30816018.
  95. Vaughan TA, Ryan JM, Czaplewski NJ (2011). "Perissodactyla". Mammalogy (5th ed.). Jones and Bartlett. p. 322. ISBN   978-0-7637-6299-5. OCLC   437300511.
  96. Flower WH, Lydekker R (1946). An Introduction to the Study of Mammals Living and Extinct. London: Adam and Charles Black. p. 496. ISBN   978-1-110-76857-8.
  97. Sreekumar S (2010). Basic Physiology. PHI Learning Pvt. Ltd. pp. 180–181. ISBN   978-81-203-4107-4.
  98. Cheifetz AS (2010). Oxford American Handbook of Gastroenterology and Hepatology. Oxford: Oxford University Press, US. p. 165. ISBN   978-0-19-983012-1.
  99. Kuntz E (2008). Hepatology: Textbook and Atlas. Germany: Springer. p. 38. ISBN   978-3-540-76838-8.
  100. Ortiz RM (June 2001). "Osmoregulation in marine mammals". The Journal of Experimental Biology. 204 (Pt 11): 1831–1844. doi: 10.1242/jeb.204.11.1831 . PMID   11441026.
  101. 1 2 3 Roman AS, Parsons TS (1977). The Vertebrate Body. Philadelphia: Holt-Saunders International. pp. 396–399. ISBN   978-0-03-910284-5.
  102. Biological Reviews – Cambridge Journals
  103. Dawkins R, Wong Y (2016). The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution (2nd ed.). Boston: Mariner Books. p. 281. ISBN   978-0-544-85993-7.
  104. 1 2 3 Fitch WT (2006). "Production of Vocalizations in Mammals" (PDF). In Brown K (ed.). Encyclopedia of Language and Linguistics. Oxford: Elsevier. pp. 115–121.
  105. Langevin P, Barclay RM (1990). "Hypsignathus monstrosus". Mammalian Species (357): 1–4. doi: 10.2307/3504110 . JSTOR   3504110.
  106. Weissengruber GE, Forstenpointner G, Peters G, Kübber-Heiss A, Fitch WT (September 2002). "Hyoid apparatus and pharynx in the lion (Panthera leo), jaguar (Panthera onca), tiger (Panthera tigris), cheetah (Acinonyxjubatus) and domestic cat (Felis silvestris f. catus)". Journal of Anatomy. 201 (3): 195–209. doi:10.1046/j.1469-7580.2002.00088.x. PMC   1570911 . PMID   12363272.
  107. Stoeger AS, Heilmann G, Zeppelzauer M, Ganswindt A, Hensman S, Charlton BD (2012). "Visualizing sound emission of elephant vocalizations: evidence for two rumble production types". PLOS ONE. 7 (11): e48907. Bibcode:2012PLoSO...748907S. doi: 10.1371/journal.pone.0048907 . PMC   3498347 . PMID   23155427.
  108. Clark CW (2004). "Baleen whale infrasonic sounds: Natural variability and function". Journal of the Acoustical Society of America. 115 (5): 2554. Bibcode:2004ASAJ..115.2554C. doi:10.1121/1.4783845.
  109. 1 2 Dawson TJ, Webster KN, Maloney SK (February 2014). "The fur of mammals in exposed environments; do crypsis and thermal needs necessarily conflict? The polar bear and marsupial koala compared". Journal of Comparative Physiology B. 184 (2): 273–284. doi:10.1007/s00360-013-0794-8. PMID   24366474. S2CID   9481486.
  110. Slominski A, Tobin DJ, Shibahara S, Wortsman J (October 2004). "Melanin pigmentation in mammalian skin and its hormonal regulation". Physiological Reviews. 84 (4): 1155–1228. doi:10.1152/physrev.00044.2003. PMID   15383650. S2CID   21168932.
  111. Hilton Jr B (1996). "South Carolina Wildlife". Animal Colors. Hilton Pond Center. 43 (4): 10–15. Retrieved 26 November 2011.
  112. 1 2 Prum RO, Torres RH (May 2004). "Structural colouration of mammalian skin: convergent evolution of coherently scattering dermal collagen arrays" (PDF). The Journal of Experimental Biology. 207 (Pt 12): 2157–2172. doi:10.1242/jeb.00989. hdl:1808/1599. PMID   15143148. S2CID   8268610.
  113. Suutari M, Majaneva M, Fewer DP, Voirin B, Aiello A, Friedl T, et al. (March 2010). "Molecular evidence for a diverse green algal community growing in the hair of sloths and a specific association with Trichophilus welckeri (Chlorophyta, Ulvophyceae)". BMC Evolutionary Biology. 10 (86): 86. doi: 10.1186/1471-2148-10-86 . PMC   2858742 . PMID   20353556.
  114. Caro T (2005). "The Adaptive Significance of Coloration in Mammals". BioScience. 55 (2): 125–136. doi: 10.1641/0006-3568(2005)055[0125:tasoci];2 .
  115. Mills LS, Zimova M, Oyler J, Running S, Abatzoglou JT, Lukacs PM (April 2013). "Camouflage mismatch in seasonal coat color due to decreased snow duration". Proceedings of the National Academy of Sciences of the United States of America. 110 (18): 7360–7365. Bibcode:2013PNAS..110.7360M. doi: 10.1073/pnas.1222724110 . PMC   3645584 . PMID   23589881.
  116. Caro T (February 2009). "Contrasting coloration in terrestrial mammals". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 364 (1516): 537–548. doi:10.1098/rstb.2008.0221. PMC   2674080 . PMID   18990666.
  117. Plavcan JM (2001). "Sexual dimorphism in primate evolution". American Journal of Physical Anthropology. Suppl 33 (33): 25–53. doi: 10.1002/ajpa.10011 . PMID   11786990. S2CID   31722173.
  118. Bradley BJ, Gerald MS, Widdig A, Mundy NI (2012). "Coat Color Variation and Pigmentation Gene Expression in Rhesus Macaques (Macaca Mulatta)" (PDF). Journal of Mammalian Evolution. 20 (3): 263–270. doi:10.1007/s10914-012-9212-3. S2CID   13916535. Archived from the original (PDF) on 24 September 2015.
  119. Caro T, Izzo A, Reiner RC, Walker H, Stankowich T (April 2014). "The function of zebra stripes". Nature Communications. 5: 3535. Bibcode:2014NatCo...5.3535C. doi: 10.1038/ncomms4535 . PMID   24691390. S2CID   9849814.
  120. Kobayashi K, Kitano T, Iwao Y, Kondo M (2018). Reproductive and Developmental Strategies: The Continuity of Life. Springer. p. 290. ISBN   978-4-431-56609-0.
  121. Lombardi J (1998). Comparative Vertebrate Reproduction. Springer Science & Business Media. ISBN   978-0-7923-8336-9.
  122. Tyndale-Biscoe H, Renfree M (1987). Reproductive Physiology of Marsupials. Cambridge University Press. ISBN   978-0-521-33792-2.
  123. Johnston SD, Smith B, Pyne M, Stenzel D, Holt WV (2007). "One‐Sided Ejaculation of Echidna Sperm Bundles" (PDF). The American Naturalist. 170 (6): E162–E164. doi:10.1086/522847. PMID   18171162. S2CID   40632746.
  124. Bacha Jr., William J.; Bacha, Linda M. (2012). Color Atlas of Veterinary Histology. Wiley. p. 308. ISBN   978-1-11824-364-0 . Retrieved 28 November 2023.
  125. Cooke, Fred; Bruce, Jenni (2004). The Encyclopedia of Animals: A Complete Visual Guide. University of California Press. p. 79. ISBN   978-0-52024-406-1 . Retrieved 28 November 2023.
  126. Maxwell KE (2013). The Sex Imperative: An Evolutionary Tale of Sexual Survival. Springer. pp. 112–113. ISBN   978-1-4899-5988-1.
  127. Vaughan TA, Ryan JP, Czaplewski NJ (2011). Mammalogy. Jones & Bartlett Publishers. p. 387. ISBN   978-0-03-025034-7.
  128. 1 2 Hoffman EA, Rowe TB (September 2018). "Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth". Nature. 561 (7721): 104–108. Bibcode:2018Natur.561..104H. doi:10.1038/s41586-018-0441-3. PMID   30158701. S2CID   205570021.
  129. Wallis MC, Waters PD, Delbridge ML, Kirby PJ, Pask AJ, Grützner F, et al. (2007). "Sex determination in platypus and echidna: autosomal location of SOX3 confirms the absence of SRY from monotremes". Chromosome Research. 15 (8): 949–959. doi:10.1007/s10577-007-1185-3. PMID   18185981. S2CID   812974.
  130. Marshall Graves JA (2008). "Weird animal genomes and the evolution of vertebrate sex and sex chromosomes" (PDF). Annual Review of Genetics. 42: 565–586. doi:10.1146/annurev.genet.42.110807.091714. PMID   18983263. Archived from the original (PDF) on 4 September 2012.
  131. Sally M (2005). "Mammal Behavior and Lifestyle". Mammals. Chicago: Raintree. p. 6. ISBN   978-1-4109-1050-9. OCLC   53476660.
  132. Verma PS, Pandey BP (2013). ISC Biology Book I for Class XI. New Delhi: S. Chand and Company. p. 288. ISBN   978-81-219-2557-0.
  133. Oftedal OT (July 2002). "The mammary gland and its origin during synapsid evolution". Journal of Mammary Gland Biology and Neoplasia. 7 (3): 225–252. doi:10.1023/a:1022896515287. PMID   12751889. S2CID   25806501.
  134. Krockenberger A (2006). "Lactation". In Dickman CR, Armati PJ, Hume ID (eds.). Marsupials. Cambridge University Press. p. 109. ISBN   978-1-139-45742-2.
  135. Schulkin J, Power ML (2016). Milk: The Biology of Lactation. Johns Hopkins University Press. p. 66. ISBN   978-1-4214-2042-4.
  136. Thompson KV, Baker AJ, Baker AM (2010). "Paternal Care and Behavioral Development in Captive Mammals". In Kleiman DG, Thompson KV, Baer CK (eds.). Wild Mammals in Captivity Principles and Techniques for Zoo Management (2nd ed.). University of Chicago Press. p. 374. ISBN   978-0-226-44011-8.
  137. Campbell NA, Reece JB (2002). Biology (6th ed.). Benjamin Cummings. p.  845. ISBN   978-0-8053-6624-2. OCLC   47521441.
  138. Buffenstein R, Yahav S (1991). "Is the naked mole-rat Hererocephalus glaber an endothermic yet poikilothermic mammal?". Journal of Thermal Biology. 16 (4): 227–232. doi:10.1016/0306-4565(91)90030-6.
  139. Schmidt-Nielsen K, Duke JB (1997). "Temperature Effects". Animal Physiology: Adaptation and Environment (5th ed.). Cambridge: Cambridge University Press. p. 218. ISBN   978-0-521-57098-5. OCLC   35744403.
  140. 1 2 Lorenzini A, Johnson FB, Oliver A, Tresini M, Smith JS, Hdeib M, et al. (2009). "Significant correlation of species longevity with DNA double strand break recognition but not with telomere length". Mechanisms of Ageing and Development. 130 (11–12): 784–792. doi:10.1016/j.mad.2009.10.004. PMC   2799038 . PMID   19896964.
  141. Hart RW, Setlow RB (June 1974). "Correlation between deoxyribonucleic acid excision-repair and life-span in a number of mammalian species". Proceedings of the National Academy of Sciences of the United States of America. 71 (6): 2169–2173. Bibcode:1974PNAS...71.2169H. doi: 10.1073/pnas.71.6.2169 . PMC   388412 . PMID   4526202.
  142. Ma S, Upneja A, Galecki A, Tsai YM, Burant CF, Raskind S, et al. (November 2016). "Cell culture-based profiling across mammals reveals DNA repair and metabolism as determinants of species longevity". eLife. 5. doi: 10.7554/eLife.19130 . PMC   5148604 . PMID   27874830.
  143. Grube K, Bürkle A (December 1992). "Poly(ADP-ribose) polymerase activity in mononuclear leukocytes of 13 mammalian species correlates with species-specific life span". Proceedings of the National Academy of Sciences of the United States of America. 89 (24): 11759–11763. Bibcode:1992PNAS...8911759G. doi: 10.1073/pnas.89.24.11759 . PMC   50636 . PMID   1465394.
  144. Francis AA, Lee WH, Regan JD (June 1981). "The relationship of DNA excision repair of ultraviolet-induced lesions to the maximum life span of mammals". Mechanisms of Ageing and Development. 16 (2): 181–189. doi:10.1016/0047-6374(81)90094-4. PMID   7266079. S2CID   19830165.
  145. Treton JA, Courtois Y (March 1982). "Correlation between DNA excision repair and mammalian lifespan in lens epithelial cells". Cell Biology International Reports. 6 (3): 253–260. doi:10.1016/0309-1651(82)90077-7. PMID   7060140.
  146. Maslansky CJ, Williams GM (February 1985). "Ultraviolet light-induced DNA repair synthesis in hepatocytes from species of differing longevities". Mechanisms of Ageing and Development. 29 (2): 191–203. doi:10.1016/0047-6374(85)90018-1. PMID   3974310. S2CID   23988416.
  147. Walker WF, Homberger DG (1998). Anatomy and Dissection of the Fetal Pig (5th ed.). New York: W. H. Freeman and Company. p. 3. ISBN   978-0-7167-2637-1. OCLC   40576267.
  148. Orr CM (November 2005). "Knuckle-walking anteater: a convergence test of adaptation for purported knuckle-walking features of African Hominidae". American Journal of Physical Anthropology. 128 (3): 639–658. doi:10.1002/ajpa.20192. PMID   15861420.
  149. Fish FE, Frappell PB, Baudinette RV, MacFarlane PM (February 2001). "Energetics of terrestrial locomotion of the platypus Ornithorhynchus anatinus" (PDF). The Journal of Experimental Biology. 204 (Pt 4): 797–803. doi:10.1242/jeb.204.4.797. hdl:2440/12192. PMID   11171362.
  150. Dhingra P (2004). "Comparative Bipedalism – How the Rest of the Animal Kingdom Walks on two legs". Anthropological Science. 131 (231).
  151. Alexander RM (May 2004). "Bipedal animals, and their differences from humans". Journal of Anatomy. 204 (5): 321–330. doi:10.1111/j.0021-8782.2004.00289.x. PMC   1571302 . PMID   15198697.
  152. 1 2 Dagg AI (1973). "Gaits in Mammals". Mammal Review. 3 (4): 135–154. doi:10.1111/j.1365-2907.1973.tb00179.x.
  153. Roberts TD (1995). Understanding Balance: The Mechanics of Posture and Locomotion. San Diego: Nelson Thornes. p. 211. ISBN   978-1-56593-416-0. OCLC   33167785.
  154. 1 2 3 Cartmill M (1985). "Climbing". In Hildebrand M, Bramble DM, Liem KF, Wake DB (eds.). Functional Vertebrate Morphology. Cambridge: Belknap Press. pp. 73–88. ISBN   978-0-674-32775-7. OCLC   11114191.
  155. Vernes K (2001). "Gliding Performance of the Northern Flying Squirrel (Glaucomys sabrinus) in Mature Mixed Forest of Eastern Canada". Journal of Mammalogy. 82 (4): 1026–1033. doi: 10.1644/1545-1542(2001)082<1026:GPOTNF>2.0.CO;2 . S2CID   78090049.
  156. Barba LA (October 2011). "Bats – the only flying mammals". Bio-Aerial Locomotion. Retrieved 20 May 2016.
  157. "Bats In Flight Reveal Unexpected Aerodynamics". ScienceDaily. 2007. Retrieved 12 July 2016.
  158. Hedenström A, Johansson LC (March 2015). "Bat flight: aerodynamics, kinematics and flight morphology" (PDF). The Journal of Experimental Biology. 218 (Pt 5): 653–663. doi:10.1242/jeb.031203. PMID   25740899. S2CID   21295393.
  159. "Bats save energy by drawing in wings on upstroke". ScienceDaily. 2012. Retrieved 12 July 2016.
  160. Karen T (2008). Hanging with Bats: Ecobats, Vampires, and Movie Stars. Albuquerque: University of New Mexico Press. p. 14. ISBN   978-0-8263-4403-8. OCLC   191258477.
  161. Sterbing-D'Angelo S, Chadha M, Chiu C, Falk B, Xian W, Barcelo J, et al. (July 2011). "Bat wing sensors support flight control". Proceedings of the National Academy of Sciences of the United States of America. 108 (27): 11291–11296. Bibcode:2011PNAS..10811291S. doi: 10.1073/pnas.1018740108 . PMC   3131348 . PMID   21690408.
  162. Damiani, R, 2003, Earliest evidence of cynodont burrowing, The Royal Society Publishing, Volume 270, Issue 1525
  163. Shimer HW (1903). "Adaptations to Aquatic, Arboreal, Fossorial and Cursorial Habits in Mammals. III. Fossorial Adaptations". The American Naturalist. 37 (444): 819–825. doi:10.1086/278368. JSTOR   2455381. S2CID   83519668.