Megaherbivore

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Hippopotamus is an extant megaherbivore. Hippo (Hippopotamus amphibius) (16485955207).jpg
Hippopotamus is an extant megaherbivore.

Megaherbivores (Greek μέγας megas "large" and Latin herbivora "herbivore" [1] ) are large herbivores that can exceed 1,000 kg (2,200 lb) in weight. The earliest herbivores to reach such sizes like the parieasaurs appeared in the Permian period. During most of the Mesozoic, the megaherbivore niche was largely dominated by dinosaurs up until their extinction during the Cretaceous–Paleogene extinction event. After this period, small mammalian species evolved into large herbivores in the Paleogene. As part of the Late Pleistocene megafauna extinctions, 80% of megaherbivore species became extinct, with megaherbivores becoming entirely extinct in Europe, Australia and the Americas. Recent megaherbivores include elephants, rhinos, hippos, and giraffes. There are nine extant species of terrestrial megaherbivores living in Africa and Asia. The African bush elephant is the largest extant species.

Contents

Extant megaherbivores are keystone species in their environment. They defoliate the landscape and spread a greater number of seeds than other frugivores. Extant megaherbivores, like most large mammals, are K-selected species. They are characterized by their large size, relative immunity to predation, their effect on plant species, and their dietary tolerance.

Definition

Megaherbivores are large herbivores that weigh more than 1 ton when fully grown. [2] They include both marine and terrestrial herbivores. [1] A type of megafauna (>45 kg), they are the largest animals on land. [3]

Evolution

Permian

Megaherbivores first evolved in the early Permian (300 mya). [4] The earliest megaherbivores were synapsids; they became somewhat rare after the Permian–Triassic extinction event. [5] [6] Taxa mainly consisted of dicynodonts, [7] and pareiasaurs. [8] The exact cause of the extinction remains unknown. It is thought that the main cause of extinction was the flood basalt volcanic eruptions that created the Siberian Traps, [9] which released sulfur dioxide and carbon dioxide, resulting in euxinia, [10] elevating global temperatures, [11] and acidifying the oceans. [12]

Triassic

Lisowicia was the last dicynodont that lived and became extinct in the Late Triassic. [5] Some scientists have proposed that there was never a Triassic–Jurassic extinction event, but others argue that the extinctions occurred earlier. Nevertheless, flood basalts are thought to be the primary driver of the extinctions towards the end of the Triassic. [13] [14]

Amniota
Synapsida

Dicynodontia (Permian, Triassic) Dicynodont from PolandDB.jpg

Mammals (small, until Paleogene)

Sauropsida

Pareiasauria (Permian) Scutosaurus BW flipped.jpg

Dinosauria (small, until Jurassic)

Jurassic

The taxonomic structure then switched to sauropodomorphs. Other taxa included stegosaurs and ankylosaurs. [15] The change in taxonomy approximately occurred at the same time with the divergence of predominant vegetation and with extinctions. New taxa may have caused competitive exclusion (i.e. predominating and removing another taxa), or they may have adopted the ecological niche of extinct groups. [4] [16]

Cretaceous

From the Triassic to the Cretaceous, a diverse assemblage of megaherbivorous dinosaurs, such as sauropods, [17] occupied different ecological niches. Based on their dentition, ankylosaurs may have mainly consumed succulent plants, as opposed to nodosaurs, which were mainly browsers. It is thought that ceratopsids fed on rugged vegetation, due to their jaw being designed for a crushing effect. Studies on hadrosaur dentition concluded that they primarily fed on fruits. [18]

Paleogene

Skeleton of Paraceratherium, a rhinocerotoid Paleontologicheskii muzei Orlova (20221008151051).jpg
Skeleton of Paraceratherium , a rhinocerotoid

Following the Cretaceous–Paleogene mass extinction, megaherbivore dinosaurs were extirpated from the face of the earth. One mechanism is thought to have played a major role: an extraterrestrial impact event in the Yucatán Peninsula. [19] For about 25 million years, the earth was void of large terrestrial herbivores that weighed more than 1 ton. After this period, small mammalian species evolved into large herbivores across every continent around 40 mya. [2] The largest of these animals were Paraceratheriidae and Proboscidea . [20] Other taxa included Brontotheriidae . [21] The Sirenia, aquatic megaherbivores, such as Dugongidae , Protosirenidae , and Prorastomidae were present in the Eocene. [22] Megaherbivores inhabited every major landmass in the Cenozoic and Pleistocene before the arrival of humans. [4]

Pleistocene

There were around 50 different species by the Late Pleistocene: [3]

Diprotodon , the largest marsupial to ever exist, was present across the entire Australian continent by the Late Pleistocene. [23] Elsewhere, megaherbivores like glyptodonts were grazing herbivores, that possessed no incisor or canine teeth, but had cheek teeth that would have been able to grind up tough vegetation. They inhabited habitats of South and North America. [24] Ground sloths were herbivores, with some being browsers, [25] others grazers, [26] and some intermediate between the two as mixed feeders. [27] Fossilized specimens were primarily found in South and North America, with one specimen being found as far north as Alaska. [28] Mammoths, like modern day elephants, had hypsodont molars. These features allowed mammoths to live an expansive life because of the availability of grasses and trees. [29] Today, nine of the 50 species persist. The Americas saw the worst decline in megaherbivores, with all 27 species going extinct. [3]

The Quaternary Extinction Event is an event where many species of megafauna (particularly mammals) went extinct. This event caused the disappearances of megaherbivores on most continents on Earth. [30] Climate change and the arrival of humans are considered likely causes of the extinctions. [31] It is thought that humans hunted megaherbivores to extinction, which then led to the extinction of the carnivores and scavengers which had preyed upon those animals. [32] [33] [34] Scientists have proposed that increasingly extreme weather—hotter summers and colder winters—referred to as "continentality", or related changes in rainfall caused the extinctions. [35]

Recent

There are nine extant species of megaherbivores, found in Africa and Asia. [36] [37] They include elephants, rhinos, hippos and giraffes. [36] [38] : 1  [39] Elephants belong to the order Proboscidea; an order that has been around since the late Paleocene. [40] Hippopotamuses are the closest living relatives to cetaceans; soon after the common ancestor of whales and hippos diverged from even-toed ungulates, the lineages of cetaceans and hippopotamuses split apart. [41] [42] Giraffidae are a sister taxon to Antilocapridae, with an estimated split of more than 20 million years ago, according to a 2019 genome study. [43] Rhinoceroses may originate from Hyrachyus , an animal whose remains date back to the late Eocene. [38] : 17 

Megaherbivores and other large herbivores are becoming less common throughout their natural distribution, which is having an impact on animal species within the ecosystem. This is mainly attributed to the destruction of their natural environment, agriculture, overhunting, and human invasion of their habitats. [44] [45] As a consequence of their slow reproductive rate and the preference for targeting larger species, overexploitation poses the greatest threat to megaherbivores. As time progresses, it is thought that the situation will only worsen. [44]

Ecology of recent megaherbivory

Browsers and grazers

Living species exhibit the following adaptations: they have dietary tolerance, a strong effect on vegetation and with the exception of calves, face little threat from predators. [46] [47] [48]

Elephants and Indian rhinoceroses exhibit both grazing and browsing feeding habits. The hippopotamus and white rhinoceros prefer grazing herbivory, while giraffes and the three other rhinoceros species most often select browsing herbivory. [49] Mammalian megaherbivores predominantly consume graminoids. They prefer eating the leaves and stem of the plant, as well as its fruits. They also exhibit both foregut and hindgut fermentation, with rhinos, hippos, and elephants displaying the former and giraffes displaying the latter. [38] : 16  Their metabolic rate is lethargic, and as a result, digestion is slowed. During this prolonged digestion period, high-fiber plant matter is disintegrated. [39]

Due to their size, megaherbivores can defoliate the landscape; because of this, they are considered keystone species in their environment. [50] Megaherbivores affect the composition of plant species, which alters the movement and exchange of inorganic and organic matter back into the production of matter. They can open up areas through feeding behavior, which over time clears vegetation, including invasive alien plants. The number of seeds that megaherbivores spread is greater than that of other frugivores. [4] [51] In addition, megaherbivore grazers, like the white rhino, have a profound impact on short grass. In one study, short grass became more infrequent after the elimination of white rhinos, which effected smaller grazers in the area. [49]

In a 2018 study, it was concluded that megaherbivores were not affected by the "landscape of fear," a landscape in which prey avoid certain hot-spot predation areas, thereby altering predator-frightened trophic cascades. Their feces were most apparent in closed, dense areas, indicating that they distribute resources to risky areas in this "landscape of fear". [52]

Interspecific interactions

Most megaherbivore species are too big and powerful for most predators to kill. [39] Calves are, however, targeted by several predator species. [38] : 158  Giraffes are also susceptible to predation, and it is not rare for lions to hunt adult giraffes in some places. The young are especially vulnerable, with a quarter to half of giraffe calves not reaching adulthood. [53] [54] In Chobe National Park, lions have been recorded hunting young and sub-adult elephants. [55] Tigers are another known predator of young elephants. [56] Hippo calves may sometimes be prey items for lions, spotted hyenas and Nile crocodiles. [57]

Giraffes may flee or act in a non-aggressive manner, while white rhinos typically do not react to the presence of predators. Black and Indian rhinoceroses, elephants, and hippopotamuses on the other hand, react strongly to predators. [38] : 124–131 

Adaptations of extant megaherbivores

Size

Elephants are the largest members, weighing between 2.5 and 6.0 tons. Indian rhinos, white rhinos and hippos usually weigh between 1.4 and 2.3 tons. The Javan and black rhino average 1–1.3 tons in weight. Giraffes are the smallest members, with a general weight range of 0.8–1.2 tons. [38] : 14  [58]

K-selection

Extant megaherbivores are K-selected species, meaning they have high life expectancies, slow population growth, large offspring, lengthy pregnancies, and low mortality rates. They have selected slow reproduction to enhance their survival chances, and as a result, increase their lifespan. [59] [60] Their large size offers protection from predators, but at the same, it decreases the degree at which they reproduce due to restricted food sources. [61] [62] This slow population growth (elephants, for example, grow at a rate of 6–7%), indicates that populations may be drastically reduced if predation pressures are too great. [38] : 293  In stable environments, K-selection predominates as the ability to compete successfully for limited resources, and populations of K-selected organisms typically are very constant in number and close to the maximum that the environment can bear. [61] [38] : 200 

Reproduction

Giraffe calves do not stay with their mothers, they sit down and hide for most of the day, and their mothers briefly visit to feed them. Giraffes (Giraffa camelopardalis) female and calf (14007572496).jpg
Giraffe calves do not stay with their mothers, they sit down and hide for most of the day, and their mothers briefly visit to feed them.
Black rhino calves are vulnerable to predators, and stay close to their mothers for safety for 26 to 40 months. Black Rhinos Kenya.jpg
Black rhino calves are vulnerable to predators, and stay close to their mothers for safety for 26 to 40 months.

When females enter estrus, males will attempt to attract them as a mating partner. These breeding opportunities may be influenced by the hierarchical system of males. Giraffes and elephants mate for a relatively short time, while rhinos and hippos have a mating session lasting an extended period of time. Females have long gestation periods, between 8 and 22 months. Intervals between births vary between species, but the overall range is 1.3 to 4.5 years. [38] : 116–124 

They usually give birth to a single calf that is heavily reliant on females for food and protection. As they get older, the calf begins weaning while still suckling. When they reach juvenility, they are able to fend for themselves, but only to a certain extent. Females typically separate from their offspring by chasing them. Despite this, females may continue to interact with their progeny even after weaning. [38] : 133 

Lifespan and mortality

Hippopotamuses and rhinoceroses can live to be 40 years old, while elephants can live longer than 60 years. [54] Giraffes have a lifespan of around 25 years. [38] : 158 

Around 2 to 5% of adult megaherbivores die each year. Males are more likely than females to die from wounds sustained during disputes. Occasionally, in times of drought, populations may significantly reduce, with calves being the most impacted during such times. [38] : 158 

See also

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<span class="mw-page-title-main">Cenozoic</span> Third era of the Phanerozoic Eon

The Cenozoic is Earth's current geological era, representing the last 66 million years of Earth's history. It is characterized by the dominance of insects, mammals, birds and angiosperms. It is the latest of three geological eras of the Phanerozoic Eon, 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">Dinosaur</span> Archosaurian reptiles that dominated the Mesozoic Era

Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is a subject of active research. They became the dominant terrestrial vertebrates after the Triassic–Jurassic extinction event 201.3 mya and their dominance continued throughout the Jurassic and Cretaceous periods. The fossil record shows that birds are feathered dinosaurs, having evolved from earlier theropods during the Late Jurassic epoch, and are the only dinosaur lineage known to have survived the Cretaceous–Paleogene extinction event approximately 66 mya. Dinosaurs can therefore be divided into avian dinosaurs—birds—and the extinct non-avian dinosaurs, which are all dinosaurs other than birds.

<span class="mw-page-title-main">Mammoth</span> Extinct genus of mammals

A mammoth is any species of the extinct elephantid genus Mammuthus. They lived from the late Miocene epoch into the Holocene until about 4,000 years ago, with mammoth species at various times inhabiting Africa, Asia, Europe, and North America. Mammoths are distinguished from living elephants by their spirally twisted tusks and in at least some later species, the development of numerous adaptions to living in cold environments, including a thick layer of fur.

<span class="mw-page-title-main">Triassic</span> First period of the Mesozoic Era 252–201 million years ago

The Triassic is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago (Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is the first and shortest period of the Mesozoic Era and the seventh period of the Phanerozoic Eon. Both the start and end of the period are marked by major extinction events. The Triassic Period is subdivided into three epochs: Early Triassic, Middle Triassic and Late Triassic.

<span class="mw-page-title-main">Hippopotamus</span> Large semi-aquatic mammal native to sub-Saharan Africa

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

In zoology, megafauna are large animals. The precise definition of the term varies widely, though a common threshold is approximately 45 kilograms (99 lb), with other thresholds as low as 10 kilograms (22 lb) or as high as 1,000 kilograms (2,200 lb). Large body size is generally associated with other traits, such as having a slow rate of reproduction and, in large herbivores, reduced or negligible adult mortality from being killed by predators.

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<span class="mw-page-title-main">Island gigantism</span> Evolutionary phenomena leading to an increase of the size of species with insularity

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<span class="mw-page-title-main">Insular dwarfism</span> Form of phyletic dwarfism occurring on islands

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<span class="mw-page-title-main">Cypriot pygmy hippopotamus</span> Species of mammal (fossil)

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<span class="mw-page-title-main">Late Pleistocene extinctions</span> Extinctions of large mammals in the Late Pleistocene

The Late Pleistocene to the beginning of the Holocene saw the extinction of the majority of the world's megafauna, which resulted in a collapse in faunal density and diversity across the globe. The extinctions during the Late Pleistocene are differentiated from previous extinctions by its extreme size bias towards large animals, and widespread absence of ecological succession to replace these extinct megafaunal species, and the regime shift of previously established faunal relationships and habitats as a consequence. The timing and severity of the extinctions varied by region and are thought to have been driven by varying combinations of human and climatic factors. Human impact on megafauna populations is thought to have been driven by hunting ("overkill"), as well as possibly environmental alteration. The relative importance of human vs climatic factors in the extinctions has been the subject of long-running controversy.

<i>Lisowicia</i> Genus of giant dicynodont therapsid

Lisowicia is an extinct genus of giant dicynodont synapsid that lived in what is now Poland during the late Norian or earliest Rhaetian age of the Late Triassic Period, about 210–205 million years ago. Lisowicia is the largest known dicynodont, as well as the largest non-mammalian synapsid, reaching about 4.5 metres (15 ft) long, standing up to 2.6 metres (8.5 ft) tall at the hips and weighing around 5–7 metric tons, comparable in size to modern elephants. It was also one of the last dicynodonts, living shortly before their extinction at the end of the Triassic period. Fossils of a giant dicynodont were known from Poland since 2008, but Lisowicia was not named and officially described as a new species until late 2018.

<span class="mw-page-title-main">Wood-pasture hypothesis</span> Ecological theory

The wood-pasture hypothesis is a scientific hypothesis positing that open and semi-open pastures and wood-pastures formed the predominant type of landscape in post-glacial temperate Europe, rather than the common belief of primeval forests. The hypothesis proposes that such a landscape would be formed and maintained by large wild herbivores. Although others, including landscape ecologist Oliver Rackham, had previously expressed similar ideas, it was the Dutch researcher Frans Vera, who, in his 2000 book Grazing Ecology and Forest History, first developed a comprehensive framework for such ideas and formulated them into a theorem. Vera's proposals, although highly controversial, came at a time when the role grazers played in woodlands was increasingly being reconsidered, and are credited for ushering in a period of increased reassessment and interdisciplinary research in European conservation theory and practice. Although Vera largely focused his research on the European situation, his findings could also be applied to other temperate ecological regions worldwide, especially the broadleaved ones.

<span class="mw-page-title-main">Island syndrome</span> Set of phenotypical features likely to occur in geographically-isolated populations

Island syndrome describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts. These differences evolve due to the different ecological pressures affecting insular species, including a paucity of large predators and herbivores as well as a consistently mild climate.

References

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