Megaherbivore
   HOME

TheInfoList



OR:

Megaherbivores (Greek μέγας megas "large" and Latin ''herbivora'' "herbivore") are large
herbivore A herbivore is an animal anatomically and physiologically evolved to feed on plants, especially upon vascular tissues such as foliage, fruits or seeds, as the main component of its diet. These more broadly also encompass animals that eat ...
s that can exceed in weight. The earliest herbivores to reach such sizes like the pareiasaurs appeared in the
Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
period. During most of the
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
, the megaherbivore niche was largely dominated by
dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
up until their extinction during the
Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event, also known as the K–T extinction, was the extinction event, mass extinction of three-quarters of the plant and animal species on Earth approximately 66 million years ago. The event cau ...
. After this period, small mammalian species evolved into large herbivores in the
Paleogene The Paleogene Period ( ; also spelled Palaeogene or Palæogene) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period Ma (million years ago) to the beginning of the Neogene Period Ma. It is the fir ...
. 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
elephant Elephants are the largest living land animals. Three living species are currently recognised: the African bush elephant ('' Loxodonta africana''), the African forest elephant (''L. cyclotis''), and the Asian elephant ('' Elephas maximus ...
s, rhinos,
hippos A hippo or hippopotamus is either of two species of large African mammal which live mainly in and near water: * Hippopotamus * Pygmy hippopotamus Hippo or Hippos may also refer to: Toponymy * The ancient city of Hippo Regius (modern Annaba, Alg ...
, and
giraffe The giraffe is a large Fauna of Africa, African even-toed ungulate, hoofed mammal belonging to the genus ''Giraffa.'' It is the Largest mammals#Even-toed Ungulates (Artiodactyla), tallest living terrestrial animal and the largest ruminant on ...
s. There are nine extant species of terrestrial megaherbivores living in
Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...
and
Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...
. The
African bush elephant The African bush elephant (''Loxodonta africana''), also known as the African savanna elephant, is a species of elephant native to sub-Saharan Africa. It is one of three extant elephant species and, along with the African forest elephant, one ...
is the largest extant species. Extant megaherbivores are
keystone species A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. The concept was introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in main ...
in their environment. They defoliate the landscape and spread a greater number of seeds than other
frugivore A frugivore ( ) is an animal that thrives mostly on raw fruits or succulent fruit-like produce of plants such as roots, shoots, nuts and seeds. Approximately 20% of mammalian herbivores eat fruit. Frugivores are highly dependent on the abundance ...
s. Extant megaherbivores, like most large mammals, are ''K-''selected species and are characterized by their large size, relative immunity to
predation Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...
, their effect on plant species, and their dietary tolerance.


Definition

Megaherbivores are large herbivores that weigh more than 1 ton when fully grown. They include both marine and terrestrial herbivores, and are classified as the largest type of terrestrial
megafauna In zoology, megafauna (from Ancient Greek, Greek μέγας ''megas'' "large" and Neo-Latin ''fauna'' "animal life") are large animals. The precise definition of the term varies widely, though a common threshold is approximately , this lower en ...
(>45 kg).


Evolution


Permian

Megaherbivores first evolved in the
early Permian 01 or 01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), ''01'' (Richard Müller album), 2001 * 01 (Urban Zakapa album), ''01'' (Urban Zakapa album), 2011 * ''01011 ...
(300 mya). The earliest megaherbivores were
synapsids Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...
; they became somewhat rare after the
Permian–Triassic extinction event The Permian–Triassic extinction event (also known as the P–T extinction event, the Late Permian extinction event, the Latest Permian extinction event, the End-Permian extinction event, and colloquially as the Great Dying,) was an extinction ...
. Taxa mainly consisted of
dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivores that typically bore a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, t ...
s, and
pareiasaur Pareiasaurs (meaning "cheek lizards") are an extinct clade of large, herbivorous parareptiles. Members of the group were armoured with osteoderms which covered large areas of the body. They first appeared in southern Pangea during the Middle Per ...
s. The exact cause of the extinction remains unknown. It is thought that the main cause of extinction was the
flood basalt A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot (geolo ...
volcanic eruptions that created the
Siberian Traps The Siberian Traps () are a large region of volcanic rock, known as a large igneous province, in Siberia, Russia. The massive eruptive event that formed the trap rock, traps is one of the largest known Volcano, volcanic events in the last years ...
, which released
sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
and
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
, resulting in
euxinia Euxinia or euxinic conditions occur when water is both anoxic and sulfidic. This means that there is no oxygen (O2) and a raised level of free hydrogen sulfide (H2S). Euxinic bodies of water are frequently strongly stratified; have an oxic, highly ...
, elevating global temperatures, and acidifying the oceans.


Triassic

'' Lisowicia'' was the last dicynodont that lived and became extinct in the Late Triassic. Some scientists have proposed that there was never a
Triassic–Jurassic extinction event The Triassic–Jurassic (Tr-J) extinction event (TJME), often called the end-Triassic extinction, marks the boundary between the Triassic and Jurassic periods, . It represents one of five major extinction events during the Phanerozoic, profoundly ...
, 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 The Triassic ( ; sometimes symbolized 🝈) 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 t ...
.


Jurassic

The taxonomic structure then switched to
sauropodomorph Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had lo ...
s. Other taxa included stegosaurs and
ankylosaur Ankylosauria is a group of herbivorous dinosaurs of the clade Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful l ...
s. 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 In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of Resource (biology), resources an ...
of extinct groups.


Cretaceous

From the
Triassic The Triassic ( ; sometimes symbolized 🝈) 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 t ...
to the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
, a diverse assemblage of megaherbivorous
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s, such as
sauropods Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their b ...
, 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.


Paleogene

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 The Yucatán Peninsula ( , ; ) is a large peninsula in southeast Mexico and adjacent portions of Belize and Guatemala. The peninsula extends towards the northeast, separating the Gulf of Mexico to the north and west of the peninsula from the C ...
. 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. The largest of these animals were ''
Paraceratheriidae Paraceratheriidae is an extinct family of long-limbed, hornless rhinocerotoids, native to Asia and Eastern Europe that originated in the Eocene epoch and lived until the end of the Oligocene. They represent some of the largest terrestrial mammals ...
'' and ''
Proboscidea Proboscidea (; , ) is a taxonomic order of afrotherian mammals containing one living family (Elephantidae) and several extinct families. First described by J. Illiger in 1811, it encompasses the elephants and their close relatives. Three l ...
''. Other taxa included '' Brontotheriidae''. The
Sirenia The Sirenia (), commonly referred to as sea cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The extant Sirenia comprise two distinct famili ...
, aquatic megaherbivores, such as ''
Dugongidae Dugongidae is a Family (biology), family in the Order (biology), order of Sirenia. The family has one surviving species, the dugong (''Dugong dugon''), one recently Extinction, extinct species, Steller's sea cow (''Hydrodamalis gigas''), and a n ...
'', '' Protosirenidae'', and ''
Prorastomidae Prorastomidae is a Family (biology), family of Extinction, extinct sirenians from Jamaica, related to the Extant taxon, extant manatees and dugong. The family includes the oldest known fossils of Sirenians, represented in two genera: *''Pezosiren ...
'' were present in the
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
. Megaherbivores inhabited every major
landmass A landmass, or land mass, is a large region or area of land that is in one piece and not noticeably broken up by oceans. The term is often used to refer to lands surrounded by an ocean or sea, such as a continent or a large island. In the fiel ...
in the
Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...
and
Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...
before the arrival of
humans Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...
.


Pleistocene

There were around 50 different species by the
Late Pleistocene The Late Pleistocene is an unofficial Age (geology), age in the international geologic timescale in chronostratigraphy, also known as the Upper Pleistocene from a Stratigraphy, stratigraphic perspective. It is intended to be the fourth division ...
: '' Diprotodon'', the largest marsupial to ever exist, was present across the entire
Australian continent The continent of Australia, sometimes known in technical contexts as Sahul (), Australia-New Guinea, Australinea, or Meganesia to distinguish it from the country of Australia, is located within the Southern and Eastern hemispheres, near t ...
by the Late Pleistocene. 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 South is one of the cardinal directions or compass points. The direction is the opposite of north and is perpendicular to both west and east. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Proto-Germanic ''*sunþa ...
and
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
. Ground sloths were herbivores, with some being
browsers Browse, browser, or browsing may refer to: Computing *Browser service, a feature of Microsoft Windows to browse shared network resources *Code browser, a program for navigating source code *File browser or file manager, a program used to manage f ...
, others grazers, and some intermediate between the two as mixed feeders. Fossilized specimens were primarily found in South and North America, with one specimen being found as far north as
Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...
. Mammoths, like modern day elephants, had
hypsodont Hypsodont is a pattern of dentition characterized by with high crowns, providing extra material for wear and tear. Some examples of animals with hypsodont dentition are cows and horses; all animals that feed on gritty, fibrous material. The oppos ...
molars. These features allowed mammoths to live an expansive life because of the availability of grasses and trees. Today, nine of the 50 species persist. The
Americas The Americas, sometimes collectively called America, are a landmass comprising the totality of North America and South America.''Webster's New World College Dictionary'', 2010 by Wiley Publishing, Inc., Cleveland, Ohio. When viewed as a sing ...
saw the worst decline in megaherbivores, with all 27 species going extinct. The
Quaternary Extinction Event The Late Pleistocene to the beginning of the Holocene saw the extinction of the majority of the world's megafauna, typically defined as animal species having body masses over , which resulted in a collapse in faunal density and diversity acro ...
is an event where many species of megafauna (particularly mammals) went extinct. This event caused the disappearances of megaherbivores on most continents on Earth. Climate change and the arrival of humans are considered likely causes of the extinctions. 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. Scientists have proposed that increasingly extreme weather—hotter summers and colder winters—referred to as " continentality", or related changes in rainfall caused the extinctions.


Recent

There are nine extant species of megaherbivores, found in Africa and Asia. They include elephants, rhinos, hippos and giraffes. Elephants belong to the order
Proboscidea Proboscidea (; , ) is a taxonomic order of afrotherian mammals containing one living family (Elephantidae) and several extinct families. First described by J. Illiger in 1811, it encompasses the elephants and their close relatives. Three l ...
; an order that has been around since the late
Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...
. Hippopotamuses are the closest living relatives to
cetaceans Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...
; soon after the common ancestor of whales and hippos diverged from even-toed ungulates, the lineages of cetaceans and hippopotamuses split apart. Giraffidae are a sister taxon to
Antilocapridae The Antilocapridae are a family of ruminant artiodactyls endemic to North America. Their closest extant relatives are the giraffids. Only one species, the pronghorn (''Antilocapra americana''), is living today; all other members of the family ...
, with an estimated split of more than 20 million years ago, according to a 2019 genome study. Rhinoceroses may originate from ''
Hyrachyus ''Hyrachyus'' (from ''Hyrax'' and "pig") is an extinct genus of perissodactyl mammal that lived in Eocene Europe, North America, and Asia. Its remains have also been found in Jamaica. It is closely related to ''Lophiodon''.Hayden, F.V''Report of ...
'', an animal whose remains date back to the
late Eocene The Priabonian is, in the International Commission on Stratigraphy, ICS's geologic timescale, the latest age (geology), age or the upper stage (stratigraphy), stage of the Eocene epoch (geology), Epoch or series (stratigraphy), Series. It spans ...
. 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 Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created ...
,
overhunting Overexploitation, also called overharvesting or ecological overshoot, refers to harvesting a renewable resource to the point of diminishing returns. Continued overexploitation can lead to the destruction of the resource, as it will be unable to ...
, and human invasion of their habitats. As a consequence of their slow reproductive rate and the preference for targeting larger species,
overexploitation Overexploitation, also called overharvesting or ecological overshoot, refers to harvesting a renewable resource to the point of diminishing returns. Continued overexploitation can lead to the destruction of the resource, as it will be unable to ...
poses the greatest threat to megaherbivores. As time progresses, it is thought that the situation will only worsen.


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. Elephants and Indian rhinoceroses exhibit both grazing and browsing feeding habits. The hippopotamus and white rhinoceros prefer
grazing In agriculture, grazing is a method of animal husbandry whereby domestic livestock are allowed outdoors to free range (roam around) and consume wild vegetations in order to feed conversion ratio, convert the otherwise indigestible (by human diges ...
herbivory, while giraffes and the three other rhinoceros species most often select
browsing Browsing is a kind of orienting strategy. It is supposed to identify something of relevance for the browsing organism. In context of humans, it is a metaphor taken from the animal kingdom. It is used, for example, about people browsing open sh ...
herbivory. Mammalian megaherbivores predominantly consume
graminoid In botany and ecology, a graminoid refers to a herbaceous plant with a grass-like morphology, i.e., elongated culms with long, blade-like leaves. They are contrasted with forbs, herbaceous plants without grass-like features. The plants most ...
s. They prefer eating the leaves and
stem Stem or STEM most commonly refers to: * Plant stem, a structural axis of a vascular plant * Stem group * Science, technology, engineering, and mathematics Stem or STEM can also refer to: Language and writing * Word stem, part of a word respon ...
of the
plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
, as well as its
fruit In botany, a fruit is the seed-bearing structure in flowering plants (angiosperms) that is formed from the ovary after flowering. Fruits are the means by which angiosperms disseminate their seeds. Edible fruits in particular have long propaga ...
s. They also exhibit both
foregut The foregut in humans is the anterior part of the alimentary canal, from the distal esophagus to the first half of the duodenum, at the entrance of the bile duct. Beyond the stomach, the foregut is attached to the abdominal walls by mesentery. ...
and
hindgut fermentation Hindgut fermentation is a digestive process seen in monogastric herbivores (animals with a simple, single-chambered stomach). Cellulose is digested with the aid of symbiotic microbes including bacteria, archaea, and eukaryotes. The microbial fe ...
, with rhinos, hippos, and elephants displaying the former and giraffes displaying the latter. Their metabolic rate is lethargic, and as a result, digestion is slowed. During this prolonged digestion period, high-fiber plant matter is disintegrated. Due to their size, megaherbivores can defoliate the landscape; because of this, they are considered
keystone species A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance. The concept was introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in main ...
in their environment. 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. In addition, megaherbivore grazers, like the
white rhino The white rhinoceros, also known as the white rhino or square-lipped rhinoceros (''Ceratotherium simum''), is the largest extant species of rhinoceros and the most Sociality, social of all rhino species, characterized by its wide mouth adapted f ...
, 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. 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".


Interspecific interactions

Most megaherbivore species are too big and powerful for most predators to kill. Calves are, however, targeted by several predator species. Giraffes are the most preyed upon megaherbivore, as it is not rare for
lion The lion (''Panthera leo'') is a large Felidae, cat of the genus ''Panthera'', native to Sub-Saharan Africa and India. It has a muscular, broad-chested body (biology), body; a short, rounded head; round ears; and a dark, hairy tuft at the ...
s to hunt adult giraffes in some places. The young are especially vulnerable, with a quarter to half of giraffe calves not reaching adulthood. In
Chobe National Park Chobe National Park is Botswana's first national park, and also the most Biodiversity, biologically diverse. Located in the north of the country, it is Botswana's third largest park, after Central Kalahari Game Reserve and Gemsbok National Park. ...
, lions have been recorded hunting young and sub-adult elephants. Tigers are another known predator of young elephants. Hippo calves may sometimes be prey items for lions,
spotted hyena The spotted hyena (''Crocuta crocuta''), also known as the laughing hyena, is a hyena species, currently classed as the sole extant member of the genus ''Crocuta'', native to sub-Saharan Africa. It is listed as being of least concern by the IUC ...
s and
Nile crocodile The Nile crocodile (''Crocodylus niloticus'') is a large crocodilian native to freshwater habitats in Africa, where it is present in 26 countries. It is widely distributed in sub-Saharan Africa, occurring mostly in the eastern, southern, and ce ...
s. 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.


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.


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. Their large size offers protection from predators, but at the same, it decreases the degree at which they reproduce due to restricted food sources. 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. 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.


Reproduction

When females enter
estrus The estrous cycle (, originally ) is a set of recurring physiological changes induced by reproductive hormones in females of mammalian subclass Theria. Estrous cycles start after sexual maturity in females and are interrupted by anestrous phas ...
, 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 Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pregn ...
periods, between 8 and 22 months. Intervals between births vary between species, but the overall range is 1.3 to 4.5 years. 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.


Lifespan and mortality

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


See also

*
Australian megafauna The term Australian megafauna refers to the megafauna in Australia (continent), Australia during the Pleistocene, Pleistocene Epoch. Most of these species became extinct during the latter half of the Pleistocene, as part of the broader global L ...
*
Deep-sea gigantism In zoology, deep-sea gigantism or abyssal gigantism is the tendency for species of deep-sea dwelling animals to be larger than their shallower-water relatives across a large taxonomic range. Proposed explanations for this type of gigantism incl ...
*
Largest and heaviest animals The largest animal currently alive is the blue whale. The maximum recorded weight was 190 tonnes (209 US tons) for a specimen measuring , whereas longer ones, up to , have been recorded but not weighed.Wood, Gerald ''The Guinness Book of Animal F ...
* List of extant megaherbivores


References

{{reflist Extinction Zoology Animal size Herbivorous mammals Ecology Megafauna