Megaherbivores (Greek
μέγας megas "large" and Latin herbivora "herbivore"[1]) are large
herbivores that can exceed 1,000 kg (2,200 lb) in weight. They first appeared 300 million years ago in the
early Permian, in the form of
synapsids. They were then replaced by megaherbivorous dinosaurs that went extinct in the
Cretaceous-Paleogene extinction event. After this period, small mammalian species evolved into large herbivores in the
Paleogene. During the
Quaternary Extinction Event, megaherbivores disappeared on most continents on Earth. 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.
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 also include large marine herbivores.[1] They are a type of
megafauna (>45 kg), and are the largest animals on land.[3]
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 may have occurred earlier.
Flood basalts are thought to be the primary driver of the extinctions, towards the end of the
Triassic.[13][14]
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]
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]
Diprotodon was present across the entire
Australian continent by the Late Pleistocene.[23] Glyptodonts were grazing herbivores. Like many other xenarthrans, they had no incisor or canine teeth, but had cheek teeth that would have been able to grind up tough vegetation.
[24] Ground sloths were herbivores, with some being
browsers,[25] others
grazers,[26] and some intermediate between the two as mixed feeders.[27] Mammoths, like modern day elephants, have
hypsodont molars. These features also allowed mammoths to live an expansive life because of the availability of grasses and trees.[28]
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 disappearance of megaherbivores on most continents on Earth.[29] Climate change and the arrival of humans could be the causes of the extinctions.[30] 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.[31][32][33] Scientists have proposed that increasingly extreme weather—hotter summers and colder winters—referred to as "
continentality", or related changes in rainfall caused the extinctions.[34]
There are nine extant species of megaherbivores, found in Africa and Asia.[35][36] They include elephants, rhinos, hippos and giraffes.[35][37]: 1 [38] Elephants belong to the order
Proboscidea, that has been around since the late
Paleocene.[39] 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.[40][41] Giraffidae are a sister taxon to
Antilocapridae, with an estimated split of more than 20 million years ago, according to a 2019 genome study.[42] Rhinoceroses may originate from Hyrachyus, an animal whose remains dates back to the
late Eocene.[37]: 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.[43][44] 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 grow worse.[43]
Ecology of recent megaherbivory
Browsers and grazers
Living species exhibit the following adaptations: they have dietary tolerance, have a strong effect on vegetation and with the exception of calves, they face little threat from predators.[45][46][47]
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.[48] 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.[37]: 16
Due to their size, megaherbivores can defoliate the landscape. Because of this, they are considered
keystone species in their environment.[49] 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. The number of seeds that megaherbivores spread is greater than that of other frugivores.[4] 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, affecting smaller grazers in the process.[48] Their metabolic rate is lethargic, and as a result, digestion is slowed. During this prolonged digestion period, high-fiber plant matter is disintegrated.[38]
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".[50]
Interspecific interactions
Most megaherbivore species are too big and powerful for most predators to kill.[38] Calves are, however, targeted by several predator species.[37]: 158 Giraffes are susceptible to predation, and it's not rare for
lions and
spotted hyenas to hunt adult giraffes in some places. The young are especially vulnerable, with a quarter to half of giraffe calves not reaching adulthood.[51][52] In
Chobe National Park, lions have been recorded hunting young and sub-adult elephants.[53]Tigers are another known predator of young elephants.[54] Hippo calves may sometimes be prey items for lions,
spotted hyenas and
Nile crocodiles.[55]
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 react strongly to predators.[37]: 124–131
Adaptations of extant megaherbivores
Size
Elephants are the largest members, weighing between 2.5–6.0 tons. Indian rhinos, white rhinos and hippos usually weigh between 1.4–2.3 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.[37]: 14 [56]
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.[57][58] Their large size offers protection from predators, but at the same, it decreases the degree at which they reproduce due to restricted food sources.[59][60] Due to their slow population growth (elephants, for example, grow at a rate of 6–7%), populations may be drastically reduced if the population growth rate is not greater than the rate of predation.[37]: 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.[59][37]: 200 Due to the effects of human activity, population densities are currently sparse.[37]: 21
Reproduction
When females enter
estrus, males will attempt to attract and mate with them. 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.[37]: 116–124
They usually give birth to a single calf. Calves are born helpless and heavily rely 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.[37]: 133
Lifespan and mortality
Hippopotamuses and rhinoceroses can live to be 40 years old, while elephants can live longer than 60 years.[52] Giraffes have a lifespan of around 25 years.[37]: 158
Around 2 to 5% of adult megaherbivores die each year. Males are more likely than females to die from wounds sustained during disputes. Giraffes are the most preyed-upon megaherbivore species. Occasionally, in times of drought, populations may significantly reduce, with calves being the most impacted.[37]: 158
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