Permian

Dinodontosaurus skeleton

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 included dicynodonts such as Dinodontosaurus.^[7] Pareiasaurs were other large herbivores present during this time.^[8] The exact cause of the extinction remain 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 propose 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]}

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 a Paraceratheriid, 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 tonne. After this period, small mammalian species evolved into large herbivores. Herbivorous mammals had evolved to megaherbivore size 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] They 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 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]

Recent

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]

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}

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

Giraffe calves don't stay with their mothers, they sit down and hide for most of the day, and their mothers briefly visit to feed them.^[37]: 134

Black rhino calves are vulnerable to predators, and stay close to their mothers for safety for 26 to 40 months.^[37]: 136

When females enter estrus, males will attempt to attract and mate with them. Breeding opportunities may be influenced by the hierarchical system within 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