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A mammal is a vertebrate animal of the class Mammalia. 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.
Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are primarily found in Australasia, Wallacea, and the Americas. One of the defining features of marsupials is their unique reproductive strategy, where the young are born in a relatively undeveloped state and then nurtured within a pouch.
In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus, and ends once it leaves the uterus.
The uterus or womb is the organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic and fetal development of one or more embryos until birth. The uterus is a hormone-responsive sex organ that contains glands in its lining that secrete uterine milk for embryonic nourishment.
The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal physiology during pregnancy. The placenta connects to the fetus via the umbilical cord, and on the opposite aspect to the maternal uterus in a species-dependent manner. In humans, a thin layer of maternal decidual (endometrial) tissue comes away with the placenta when it is expelled from the uterus following birth. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development.
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. Placentalia represents the only living group within Eutheria, which contains all mammals more closely related to placentals than to marsupials.
Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the fetus at a developmental stage when it is ready to feed and breathe.
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.
Eutheria, also called Pan-Placentalia, is the clade consisting of placental mammals and all therian mammals that are more closely related to placentals than to marsupials.
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.
Prenatal development includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal development until birth.
Marsupium, pl. marsupia; meaning pouch or purse in Latin, encompasses a diverse array of specialized anatomical structures found in various organisms across different taxonomic groups, predominantly in marsupials. Analogous structures are also observed in lower vertebrates and some invertebrates. This article provides an overview of marsupia across different taxa, highlighting their evolutionary significance, ecological roles, and physiological functions.
Placentation refers to the formation, type and structure, or arrangement of the placenta. The function of placentation is to transfer nutrients, respiratory gases, and water from maternal tissue to a growing embryo, and in some instances to remove waste from the embryo. Placentation is best known in live-bearing mammals (theria), but also occurs in some fish, reptiles, amphibians, a diversity of invertebrates, and flowering plants. In vertebrates, placentas have evolved more than 100 times independently, with the majority of these instances occurring in squamate reptiles.
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.
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.
A fetus or foetus is the unborn offspring that develops from a mammal embryo. Following embryonic development, the fetal stage of development takes place. In human prenatal development, fetal development begins from the ninth week after fertilization and continues until the birth of a newborn. Prenatal development is a continuum, with no clear defining feature distinguishing an embryo from a fetus. However, a fetus is characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final anatomical location.
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.
Pregnancy has been traditionally defined as the period of time eggs are incubated in the body after the egg-sperm union. Although the term often refers to placental mammals, it has also been used in the titles of many international, peer-reviewed, scientific articles on fish, e.g. Consistent with this definition, there are several modes of reproduction in fish, providing different amounts of parental care. In ovoviviparity, there is internal fertilization and the young are born live but there is no placental connection or significant trophic (feeding) interaction; the mother's body maintains gas exchange but the unborn young are nourished by egg yolk. There are two types of viviparity in fish. In histotrophic viviparity, the zygotes develop in the female's oviducts, but she provides no direct nutrition; the embryos survive by eating her eggs or their unborn siblings. In hemotrophic viviparity, the zygotes are retained within the female and are provided with nutrients by her, often through some form of placenta.
Ukhaatherium is a now extinct species of mammal that lived during the upper Cretaceous about 84 to 72 million years ago in today's East Asia. It is known above all from the fossil locality Ukhaa Tolgod, Mongolia. An adult Ukhaatherium has an estimated weight of about 32g and bears several similarities to lipotyphlan insectivorans such as the tenrec.
In the biology of birds and mammals, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. These categories form a continuum, without distinct gaps between them.
References
Citations
- ↑ infoplease.com > Health and Science > Environment and Nature > Nature > Gestation, Incubation, and Longevity of Selected Animals By James G. Doherty, general curator, The Wildlife Conservation Society. Retrieved on April 9, 2010
- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 Langer, P. (2008). The phases of maternal investment in eutherian mammals. Zoology, 111(2), 148-162.
- ↑ Paris Hill Farm. "Alpacas 101" . Retrieved 2012-07-14.
- ↑ Northwest Alpacas. "Alpaca Q&A". Archived from the original on 2012-07-21. Retrieved 2012-07-14.
- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 Sacher, G. A., & Staffeldt, E. F. (1974). Relation of gestation time to brain weight for placental mammals: implications for the theory of vertebrate growth. The American Naturalist, 108(963), 593-615.
- ↑ Christian R. Abee; Keith Mansfield; Suzette D. Tardif; Timothy Morris, eds. (May 9, 2012). Nonhuman Primates in Biomedical Research: Biology and Management. Academic Press. p. 215. ISBN 978-0-12-397837-0.
- ↑ University of Michigan. "Ursus Americanus" . Retrieved 2013-01-16.
- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Ardito, G. (1976). Check-list of the data on the gestation length of primates. Journal of human evolution, 5(2), 213-222.
- ↑ "Pregnant cows, timing of pregnancy, open cows, pregnancy rate".
- ↑ A.Rijnberk, F.J (2005). Anamnese en lichamelijk onderzoek bij gezelschapsdieren, A.Rijnberk, F.J.van Sluis, 2nd print, Bohn Stafleu van Loghum, 2005, (Current Dutch veterinary examination study book for small domestic mammals)
- 1 2 3 4 5 Anamnese en lichamelijk onderzoek bij gezelschapsdieren, A.Rijnberk, F.J.van Sluis, 2nd print, Bohn Stafleu van Loghum, 2005, (Current Dutch veterinary examination study book for small domestic mammals)
- 1 2 Kiltie, R. A. (1982). Intraspecific variation in the mammalian gestation period. Journal of Mammalogy, 63(4), 646-652.
- ↑ Larry Vogelnest; Rupert Woods (August 18, 2008). Medicine of Australian Mammals. Csiro Publishing. p. 439. ISBN 978-0-643-09928-9.
- ↑ Frances M.D. Gulland; Leslie A. Dierauf; Karyl L. Whitman (20 March 2018). CRC Handbook of Marine Mammal Medicine. CRC Press. p. 408. ISBN 978-1-351-38416-2.
- ↑ The Laboratory Rat, 2nd Edition, Eds. Mark A. Sucklow, Steven H. Weisbroth, and Craig L. Franklin. Page 151.
- ↑ . A timetable of embryonic development, and ovarian and uterine changes during pregnancy, in the stripe-faced dunnart, Sminthopsis macroura (Marsupialia: Dasyuridae). Journal of reproduction and fertility 91 1 (1991): 213-27. DOI 10.1530/jrf.0.0910213
- ↑ Sacher, G. A., & Staffeldt, E. F. (1974).Relation of gestation time to brain weight for placental mammals: implications for the theory of vertebrate growth. The American Naturalist, 108(963), 593-615.
- ↑ Martin, R. D., Genoud, M., & Hemelrijk, C. K. (2005). Problems of allometric scaling analysis: examples from mammalian reproductive biology. Journal of Experimental Biology, 208(9), 1731-1747.
- ↑ Zullinger, E. M., Ricklefs, R. E., Redford, K. H., & Mace, G. M. (1984). Fitting sigmoidal equations to mammalian growth curves. Journal of Mammalogy, 65(4), 607-636.
- ↑ Martin, R. D., & MacLarnon, A. M. (1985). Gestation period, neonatal size and maternal investment in placental mammals. Nature, 313(5999), 220-223.
- ↑ Derrickson, E. M. (1992). Comparative reproductive strategies of altricial and precocial eutherian mammals. Functional Ecology, 57-65.
- ↑ Isler, K., & van Schaik, C. P. (2009). The expensive brain: a framework for explaining evolutionary changes in brain size. Journal of human evolution, 57(4), 392-400.
- ↑ Racey, P. A., & Swift, S. M. (1981). Variations in gestation length in a colony of pipistrelle bats (Pipistrellus pipistrellus) from year to year. Reproduction, 61(1), 123-129.
- ↑ Bradley, S. (2006). The ecology of bat reproduction.
- ↑ Scheffer, V. B., & York, A. E. (1997). Timing of implantation in the northern fur seal, Callorhinus ursinus. Journal of Mammalogy, 78(2), 675-683.
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gestation period (P3063) (see uses )
