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Perissodactyla (/pəˌrɪsoʊˈdæktɪlə/, from Ancient Greekπερισσός, perissós'odd' and δάκτυλος, dáktylos'finger, toe'[3]), or odd-toed ungulates, is an order of ungulates. The order includes about 17 living species divided into three families: Equidae (horses, asses, and zebras), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs). They typically have reduced the weight-bearing toes to three or one of the five original toes, though tapirs retain four toes on their front feet. The nonweight-bearing toes are either present, absent, vestigial, or positioned posteriorly. By contrast, artiodactyls (even-toed ungulates) bear most of their weight equally on four or two (an even number) of the five toes: their third and fourth toes. Another difference between the two is that perissodactyls digest plant cellulose in their intestines, rather than in one or more stomach chambers as artiodactyls, with the exception of Suina, do.
The order was considerably more diverse in the past, with notable extinct groups including the brontotheres, palaeotheres, chalicotheres, and the paraceratheres, with the paraceratheres including the largest known land mammals to have ever existed.
Despite their very different appearances, they were recognized as related families in the 19th century by the zoologist Richard Owen, who also coined the order's name.
The largest odd-toed ungulates are rhinoceroses, and the extinct Paraceratherium, a hornless rhino from the Oligocene, is considered one of the largest land mammals of all time.[4] At the other extreme, an early member of the order, the prehistoric horse Eohippus, had a withers height of only 30 to 60 cm (12 to 24 in).[5] Apart from dwarf varieties of the domestic horse and donkey, living perissodactyls reach a body length of 180–420 cm (71–165 in) and a weight of 150 to 4,500 kg (330 to 9,920 lb).[6] While rhinos have only sparse hair and exhibit a thick epidermis, tapirs and horses have dense, short coats. Most species are grey or brown, although zebras and young tapirs are striped.
Seven figures showing the bones, blood vessels, ligaments and arteries of the hoof and pastern.
The main axes of both the front and rear feet pass through the third toe, which is always the largest. The remaining toes have been reduced in size to varying degrees. Tapirs, which are adapted to walking on soft ground, have four toes on their fore feet and three on their hind feet. Living rhinos have three toes on both the front and hind feet. Modern equines possess only a single toe; however, their feet are equipped with hooves, which almost completely cover the toe. Rhinos and tapirs, by contrast, have hooves covering only the leading edge of the toes, with the bottom being soft.
Ungulates have stances that require them to stand on the tips of their toes. Equine ungulates with only one digit or hoof have decreased mobility in their limbs, which allows for faster running speeds and agility.
Differences in limb structure and physiology between ungulates and other mammals can be seen in the shape of the humerus. For example, often shorter, thicker, bones belong to the largest and heaviest ungulates like the rhinoceros.[7]
The ulnae and fibulae are reduced in horses. A common feature that clearly distinguishes this group from other mammals is the articulation between the astragalus, the scaphoid and the cuboid, which greatly restricts the mobility of the foot.[8][9] The thigh is relatively short, and the clavicle is absent.
Tapirs are the only extant group of perissodactyls with a trunk.
Odd-toed ungulates have a long upper jaw with an extended diastema between the front and cheek teeth, giving them an elongated head. The various forms of snout between families are due to differences in the form of the premaxilla. The lacrimal bone has projecting cusps in the eye sockets and a wide contact with the nasal bone. The temporomandibular joint is high and the mandible is enlarged.
The number and form of the teeth vary according to diet. The incisors and canines can be very small or completely absent, as in the two African species of rhinoceros. In horses, usually only the males possess canines. The surface shape and height of the molars is heavily dependent on whether soft leaves or hard grass make up the main component of their diets. Three or four cheek teeth are present on each jaw half, so the dental formula of odd-toed ungulates is:
0-3 . 0-1 . 2-4 . 31-3 . 1 . 2-4 . 3 × 2 = 30-44
The guttural pouch, a small outpocketing of the auditory tube that drains the middle ear, is a characteristic feature of Perissodactyla.[10] The guttural pouch is of particular concern in equineveterinary practice, due to its frequent involvement in some serious infections. Aspergillosis (infection with Aspergillus mould) of the guttural pouch (also called guttural pouch mycosis) can cause serious damage to the tissues of the pouch, as well as surrounding structures including important cranial nerves (nerves IX-XII: glossopharyngeal, vagus, accessory and hypoglossal nerves[10]) and the internal carotid artery.[10]Strangles (Streptococcus equi equi infection) is a highly transmissible respiratory infection of horses[11][12] that can cause pus to accumulate in the guttural pouch;[13] horses with S. equi equi colonising their guttural pouch can continue to intermittently shed the bacteria for several months,[14] and should be isolated from other horses during this time to prevent transmission. Due to the intermittent nature of S. equi equi shedding, prematurely reintroducing an infected horse may risk exposing other horses to the infection, even though the shedding horse appears well and may have previously returned negative samples. The function of the guttural pouch has been difficult to determine, but it is now believed to play a role in cooling blood in the internal carotid artery before it enters the brain.[10]
All perissodactyls are hindgut fermenters. In contrast to ruminants, hindgut fermenters store digested food that has left the stomach in an enlarged cecum, where the food begins digestion by microbes, with the fermentation continuing in the large colon. No gallbladder is present. The stomach of perissodactyls is simply built, while the cecum accommodates up to 90 L (24 US gal) in horses. The small intestine is very long, reaching up to 26 m (85 ft) in horses. Extraction of nutrients from food is relatively inefficient, which probably explains why no odd-toed ungulates are small; nutritional requirements per unit of body weight are lower for large animals, as their surface-area-to-volume ratio is smaller.
Unlike artiodactyls, perissodactyls lack a carotid rete,[15] a heat exchange that reduces the dependence of the temperature of the brain on that of the body. As a result, perissodactyls have limited thermoregulatory flexibility compared to artiodactyls which has restricted them to habitats of low seasonality and rich in food and water, such as tropical forests. In contrast, artiodactyls occupy a wide range of habits ranging from the Arctic Circle to deserts and tropical savannahs.[15]
Restriction of their habitat and poaching threaten the survival of most rhino species, including the Indian rhinoceros shown here
Most extant perissodactyl species occupy a small fraction of their original range. Members of this group are now found only in Central and South America, eastern and southern Africa, and central, southern, and southeastern Asia.[16] During the peak of odd-toed ungulate existence, from the Eocene to the Oligocene, perissodactyls were distributed over much of the globe, the major exceptions being Australia and Antarctica. Horses and tapirs arrived in South America after the formation of the Isthmus of Panama around 3 million years ago in the Pliocene. Their North American counterparts died out around 10,000 years ago, leaving only Baird's tapir with a range extending to what is now southern Mexico. The tarpans were pushed to extinction in 19th century Europe. Hunting and habitat destruction have reduced the surviving perissodactyl species to fragmented populations. In contrast, domesticated horses and donkeys have gained a worldwide distribution, and feral animals of both species are now also found in regions outside their original range, such as in Australia.
Perissodactyls inhabit a number of different habitats, leading to different lifestyles. Tapirs are solitary and inhabit mainly tropical rainforests. Rhinos tend to live alone in rather dry savannas, and in Asia, wet marsh or forest areas. Horses inhabit open areas such as grasslands, steppes, or semi-deserts, and live together in groups. Odd-toed ungulates are exclusively herbivores that feed, to varying degrees, on grass, leaves, and other plant parts. A distinction is often made between primarily grass feeders (white rhinos, equines) and leaf feeders (tapirs, other rhinos).
Odd-toed ungulates are characterized by a long gestation period and a small litter size, usually delivering a single young. The gestation period is 330–500 days, being longest in rhinos. Newborn perissodactyls are precocial, meaning offspring are born already quite independent: for example,[17] young horses can begin to follow the mother after a few hours.[18] The young are nursed for a relatively long time, often into their second year, with rhinos reaching sexual maturity around eight or ten years old, but horses and tapirs maturing around two to four years old.[19][20] Perissodactyls are long-lived, with several species, such as rhinos, reaching an age of almost 50 years in captivity.[21]
Traditionally, the odd-toed ungulates were classified with other mammals such as artiodactyls, hyraxes, elephants and other "ungulates". A close family relationship with hyraxes was suspected based on similarities in the construction of the ear and the course of the carotid artery.
Molecular genetic studies, however, have shown the ungulates to be polyphyletic, meaning that in some cases the similarities are the result of convergent evolution rather than common ancestry. Elephants and hyraxes are now considered to belong to Afrotheria, so are not closely related to the perissodactyls. These in turn are in the Laurasiatheria, a superorder that had its origin in the former supercontinent Laurasia. Molecular genetic findings suggest that the cloven Artiodactyla (containing the cetaceans as a deeply nested subclade) are the sister taxon of the Perissodactyla; together, the two groups form the Euungulata.[22] More distant are the bats (Chiroptera) and Ferae (a common taxon of carnivorans, Carnivora, and pangolins, Pholidota).[23] In a discredited alternative scenario, a close relationship exists between perissodactyls, carnivorans, and bats, this assembly comprising the Pegasoferae.[24]
Internal taxonomy of the Euungulata after Welker et al. 2015[25]
According to studies published in March 2015, odd-toed ungulates are in a close family relationship with at least some of the so-called Meridiungulata, a very diverse group of mammals living from the Paleocene to the Pleistocene in South America, whose systematic unity is largely unexplained. Some of these were classified based on their paleogeographic distribution. However, a close relationship can be worked out to perissodactyls by protein sequencing and comparison with fossil collagen from remnants of phylogenetically young members of the Meridiungulata (specifically Macrauchenia from the Litopterna and Toxodon from the Notoungulata). Both kinship groups, the odd-toed ungulates and the Litopterna-Notoungulata, are now in the higher-level taxon of Panperissodactyla. This kinship group is included among the Euungulata, which also contains the even-toed ungulates (Artiodactyla). The separation of the Litopterna-Notoungulata group from the perissodactyls probably took place before the Cretaceous–Paleogene extinction event. "Condylarths" can probably be considered the starting point for the development of the two groups, as they represent a heterogeneous group of primitive ungulates that mainly inhabited the northern hemisphere in the Paleogene.[25][26]
Odd-toed ungulates comprise three living families with around 17 species—in horses, however, the exact count is still controversial. Rhinos and tapirs are more closely related to each other than to horses. According to molecular genetic analysis, the separation of horses from other perissodactyls took place in the Paleocene some 56 million years ago, while the rhinos and tapirs split off in the lower-middle Eocene, about 47 million years ago.
Internal relationships of extant Perissodactyla[27][28][29]
There are many perissodactyl fossils of multivariant form. The major lines of development include the following groups:
Scale diagram of the paraceratheriidParaceratherium, one of the largest land mammals to have ever existedBrontotherioidea were among the earliest known large mammals, consisting of the families of Brontotheriidae (synonym Titanotheriidae), the most well-known representative being Megacerops and the more basal family Lambdotheriidae. They were generally characterized in their late phase by a bony horn at the transition from the nose to the frontal bone and flat molars suitable for chewing soft plant food. The Brontotheroidea, which were almost exclusively confined to North America and Asia, died out at the beginning of the Upper Eocene.
Equoidea also developed in the Eocene. Palaeotheriidae are known mainly from Europe. In contrast, the horse family (Equidae) flourished and spread. Over time this group saw a reduction in toe number, extension of the limbs, and the progressive adjustment of the teeth for eating hard grasses.
Chalicotherioidea represented another characteristic group, consisting of the families Chalicotheriidae and Lophiodontidae. The Chalicotheriidae developed claws instead of hooves and considerable extension of the forelegs. The best-known genera include Chalicotherium and Moropus. Chalicotherioidea died out in the Pleistocene.
Rhinocerotoidea (rhino relatives) included a large variety of forms from the Eocene up to the Oligocene, including dog-size leaf feeders, semiaquatic animals, and also huge long-necked animals. Only a few had horns on the nose. The Amynodontidae were hippo-like, aquatic animals. Hyracodontidae developed long limbs and long necks that were most pronounced in the Paraceratherium (formerly known as Baluchitherium or Indricotherium), the second largest known land mammal ever to have lived (after Palaeoloxodon namadicus[30]). The rhinos (Rhinocerotidae) emerged in the Middle Eocene; five species survive to the present day.
Tapiroidea reached their greatest diversity in the Eocene, when several families lived in Eurasia and North America. They retained a primitive physique and were noted for developing a trunk. The extinct families within this group include the Helaletidae.[citation needed]
Several mammal groups traditionally classified as condylarths, long-understood to be a wastebasket taxon, such as hyopsodontids and phenacodontids, are now understood to be part of the odd-toed ungulate assemblage. Phenacodontids seem to be stem-perissodactyls, while hyopsodontids are closely related to horses and brontotheres, despite their more primitive overall appearance.
Desmostylia[31] and Anthracobunidae have traditionally been placed among the afrotheres, but they may actually represent stem-perissodactyls. They are an early lineage of mammals that took to the water, spreading across semi-aquatic to fully marine niches in the Tethys Ocean and the northern Pacific. However, later studies have shown that, while anthracobunids are definite perissodactyls, desmostylians have enough mixed characters to suggest that a position among the Afrotheria is not out of the question.[32]
Relationships within the large group of odd-toed ungulates are not fully understood. Initially, after the establishment of "Perissodactyla" by Richard Owen in 1848, the present-day representatives were considered equal in rank. In the first half of the 20th century, a more systematic differentiation of odd-toed ungulates began, based on a consideration of fossil forms, and they were placed in two major suborders: Hippomorpha and Ceratomorpha. The Hippomorpha comprises today's horses and their extinct members (Equoidea); the Ceratomorpha consist of tapirs and rhinos plus their extinct members (Tapiroidea and Rhinocerotoidea).[41] The names Hippomorpha and Ceratomorpha were introduced in 1937 by Horace Elmer Wood, in response to criticism of the name "Solidungula" that he proposed three years previously. It had been based on the grouping of horses and Tridactyla and on the rhinoceros/tapir complex.[42][43] The extinct brontotheriidae were also classified under Hippomorpha and therefore possess a close relationship to horses. Some researchers accept this assignment because of similar dental features, but there is also the view that a very basal position within the odd-toed ungulates places them rather in the group of Titanotheriomorpha.[40][44]
Originally, the Chalicotheriidae were seen as members of Hippomorpha, and presented as such in 1941. William Berryman Scott thought that, as claw-bearing perissodactyls, they belong in the new suborder Ancylopoda (where Ceratomorpha and Hippomorpha as odd-toed ungulates were combined in the group of Chelopoda).[45] The term Ancylopoda, coined by Edward Drinker Cope in 1889, had been established for chalicotheres. However, further morphological studies from the 1960s showed a middle position of Ancylopoda between Hippomorpha and Ceratomorpha. Leonard Burton Radinsky saw all three major groups of odd-toed ungulates as peers, based on the extremely long and independent phylogenetic development of the three lines.[46] In the 1980s, Jeremy J. Hooker saw a general similarity between Ancylopoda and Ceratomorpha based on dentition, especially in the earliest members, leading to the unification in 1984 of the two submissions in the interim order, Tapiromorpha. At the same time, he expanded the Ancylopoda to include the Lophiodontidae. The name "Tapiromorpha" goes back to Ernst Haeckel, who coined it in 1873, but it was long considered synonymous to Ceratomorpha because Wood had not considered it in 1937 when Ceratomorpha were named, since the term had been used quite differently in the past.[47] Also in 1984, Robert M. Schoch used the conceptually similar term Moropomorpha, which today applies synonymously to Tapiromorpha.[48] Included within the Tapiromorpha are the now extinct Isectolophidae, a sister group of the Ancylopoda-Ceratomorpha group and thus the most primitive members of this relationship complex.[44][37]
The evolutionary development of Perissodactyla is well documented in the fossil record. Numerous finds are evidence of the adaptive radiation of this group, which was once much more varied and widely dispersed. Radinskya from the late Paleocene of East Asia is often considered to be one of the oldest close relatives of the ungulates.[49] Its 8 cm skull must have belonged to a very small and primitive animal with a π-shaped crown pattern on the enamel of its rear molars similar to that of perissodactyls and their relatives, especially the rhinos.[50] Finds of Cambaytherium and Kalitherium in the Cambay shale of western India indicate an origin in Asia dating to the Lower Eocene roughly 54.5 million years ago.[51][52] Their teeth also show similarities to Radinskya as well as to the Tethytheria clade.[53][54] The saddle-shaped configuration of the navicular joints and the mesaxonic construction of the front and hind feet also indicates a close relationship to Tethytheria.[citation needed] However, this construction deviates from that of Cambaytherium, indicating that it is actually a member of a sister group. Ancestors of Perissodactyla may have arrived via an island bridge from the Afro-Arab landmass onto the Indian subcontinent as it drifted north towards Asia.[55] A study on Cambaytherium suggests an origin in India prior or near its collision with Asia.[56]
The alignment of hyopsodontids and phenacodontids to Perissodactyla in general suggests an older Laurasian origin and distribution for the clade, dispersed across the northern continents already in the early Paleocene. These forms already show a fairly well-developed molar morphology, with no intermediary forms as evidence of the course of its development.[57] The close relationship between meridiungulate mammals and perissoodactyls in particular is of interest since the latter appeared in South America soon after the K–T event, implying rapid ecological radiation and dispersal after mass extinction.[58]
Eohippus, an early relative of the horse, is one of the oldest-known perissodactyls
The Perissodactyla appeared relatively abruptly at the beginning of the Lower Paleocene about 63 million years ago, both in North America and Asia, in the form of phenacodontids and hyopsodontids. The oldest finds from an extant group originate among other sources, from Sifrhippus, an ancestor of the horses from the Willswood lineup in northwestern Wyoming.[59][60] The distant ancestors of tapirs appeared not too long after that in the Ghazij lineup in Balochistan, such as Ganderalophus, as well as Litolophus from the Chalicotheriidae line, or Eotitanops from the group of brontotheriidae.[61][62] Initially, the members of the different lineages looked quite similar, with an arched back and generally four toes on the front and three on the hind feet. Eohippus, which is considered a member of the horse family, outwardly resembled Hyrachyus, the first representative of the rhino and tapir line.[63] All were small compared to later forms and lived as fruit and foliage eaters in forests. The first of the megafauna to emerge were the brontotheres, in the Middle and Upper Eocene. Megacerops, known from North America, reached a withers height of 2.5 m (8 ft 2 in) and could have weighed just over 3 metric tons (3.3 short tons). The decline of brontotheres at the end of the Eocene is associated with competition arising from the advent of more successful herbivores.[33][64]
More successful lines of odd-toed ungulates emerged at the end of the Eocene when dense jungles gave way to steppe, such as the chalicotheriid rhinos, and their immediate relatives; their development also began with very small forms. Paraceratherium, one of the largest mammals ever to walk the earth, evolved during this era.[65][66] They weighed up to 20 metric tons (22 short tons) and lived throughout the Oligocene in Eurasia. About 20 million years ago, at the onset of the Miocene, the perissodactyls first reached Africa when it became connected to Eurasia because of the closing of the Tethys Ocean. For the same reason, however, new animals such as the mammoths also entered the ancient settlement areas of odd-toed ungulates, creating competition that led to the extinction of some of their lines. The rise of ruminants, which occupied similar ecological niches and had a much more efficient digestive system, is also associated with the decline in diversity of odd-toed ungulates. A significant cause for the decline of perissodactyls was climate change during the Miocene, leading to a cooler and drier climate accompanied by the spread of open landscapes. However, some lines flourished, such as the horses and rhinos; anatomical adaptations made it possible for them to consume tougher grass food. This led to open land forms that dominated newly created landscapes. With the emergence of the Isthmus of Panama in the Pliocene, perissodactyls and other megafauna were given access to one of their last habitable continents: South America.[67][68] However, many perissodactyls became extinct at the end of the ice ages, including American horses and the Elasmotherium. Whether over-hunting by humans (overkill hypothesis), climatic change, or a combination of both factors was responsible for the extinction of ice age mega-fauna, remains controversial.[33]
In 1758, in his seminal work Systema Naturae, Linnaeus (1707–1778) classified horses (Equus) together with hippos (Hippopotamus). At that time, this category also included the tapirs (Tapirus), more precisely the lowland or South American tapir (Tapirus terrestus), the only tapir then known in Europe. Linnaeus classified this tapir as Hippopotamus terrestris and put both genera in the group of the Belluae ("beasts"). He combined the rhinos with the Glires, a group now consisting of the lagomorphs and rodents. Mathurin Jacques Brisson (1723–1806) first separated the tapirs and hippos in 1762 with the introduction of the concept le tapir. He also separated the rhinos from the rodents, but did not combine the three families now known as the odd-toed ungulates. In the transition to the 19th century, the individual perissodactyl genera were associated with various other groups, such as the proboscidean and even-toed ungulates. In 1795, Étienne Geoffroy Saint-Hilaire (1772–1844) and Georges Cuvier (1769–1832) introduced the term "pachyderm" (Pachydermata), including in it not only the rhinos and elephants, but also the hippos, pigs, peccaries, tapirs and hyrax.[41][44][69][70] The horses were still generally regarded as a group separate from other mammals and were often classified under the name Solidungula or Solipèdes, meaning "one-hoof animal".[71][72]
In 1861, Henri Marie Ducrotay de Blainville (1777–1850) classified ungulates by the structure of their feet, differentiating those with an even number of toes from those with an odd number. He moved the horses as solidungulate over to the tapirs and rhinos as multungulate animals and referred to all of them together as onguligrades à doigts impairs, coming close to the concept of the odd-toed ungulate as a systematic unit. Richard Owen (1804–1892) quoted Blainville in his study on fossil mammals of the Isle of Wight and introduced the name Perissodactyla.[41][44]
In 1884, Othniel Charles Marsh (1831–1899) came up with the concept Mesaxonia, which he used for what are today called the odd-toed ungulates, including their extinct relatives, but explicitly excluding the hyrax. Mesaxonia is now considered a synonym of Perissodactyla, but it was sometimes also used for the true odd-toed ungulates as a subcategory (rhinos, horses, tapirs), while Perissodactyla stood for the entire order, including the hyrax. The assumption that hyraxes were Perissodactyla was held well into the 20th century.[73] Only with the advent of molecular genetic research methods had it been recognized that the hyrax was not closely related to perissodactyls but rather to elephants and manatees.[22][74]
The domestic horse and the donkey play an important role in human history, particularly as transport, work and pack animals. The domestication of both species began several millennia BCE. Due to the motorisation of agriculture and the spread of automobile traffic, such use has declined sharply in Western industrial countries; riding is usually undertaken more as a hobby or sport. In less developed regions of the world, traditional uses for these animals are, however, still widespread. To a lesser extent, horses and donkeys are also kept for their meat and their milk.
In contrast, the existence in the wild of almost all other odd-toed ungulates species has declined dramatically because of hunting and habitat destruction. The quagga is extinct and Przewalski's horse was once eradicated in the wild.
Hunting and habitat loss due to land conversion and human encroachment are the most significant threats to the three endangered species of tapir. The Malayan tapir's inland forest habitat is of particular concern, as this land is being deforested rapidly and converted into palm oil plantations. Climate change is shifting the suitable range of mountain tapirs further up the Andes Mountains, reducing their available habitat. Hunting of mountain and Baird's tapirs in Central and South America for their meat is common and is made easier by climate change, as population densities are forcibly increased. Although hunting is illegal in protected areas throughout this region, regulations are often ignored or unenforced. Conservation efforts for tapirs primarily consist of legal protections from hunting and international trade, though proposals of habitat protection and restoration at the local level are underway in all affected countries.[76]
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^Horace Elmer Wood (1934). "Revision of the Hyrachyidae". Bulletin of the American Museum of Natural History: 181–295.
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^William Berryman Scott (1941). "Part V: Perissodactyla". The Mammalian Fauna of the White River Oligocene Transactions of the American Philosophical Society. New Series. 28 (5): 747–964. doi:10.2307/1005518. JSTOR1005518.
^Leonard B. Radinsky (1964). "Paleomoropus, a new early Eocene chalicothere (Mammalia, Perissodactyla), and a revision of Eocene chalicotheres". American Museum Novitates: 1–28.
^JJ Hooker (1984). "A primitive ceratomorph (Perissodactyla, Mammalia) from the Early Tertiary of Europe". Zoological Journal of the Linnean Society. 82 (1–2): 229–244. doi:10.1111/j.1096-3642.1984.tb00545.x.
^Robert Milton Schoch (1984). "Two unusual specimens of the Yale Peabody Museum Helaletes in collections, and some comments on the ancestry of the Tapiridae (Perissodactyla, Mammalia)". Peabody Museum, Yale University: 1–20. {{cite journal}}: Cite journal requires |journal= (help)
^Malcolm C. McKenna; Chow Minna; Suyin Ting; Luo Zhexi (1989). "Radinskya yupingae, a perissodactyl-like mammal from the Late Palaeocene of China". The Evolution of Perissodactyls. Oxford University Press. pp. 24–36.
^Kenneth D. Rose (2006). The Beginning of the Age of Mammals. Johns Hopkins University Press. pp. 242–267.
^Kenneth D. Rose; Thierry Smith; Rajendra S. Rana; Ashok Sahni; H. Singh; A. Pieter Missiaen (2006). "Early Eocene (Ypresian) Continental vertebrate assemblage from India, with description of a new anthracobunid (Mammalia, tethytheria )". Journal of Vertebrate Paleontology. 26 (1): 219–225. doi:10.1671/0272-4634(2006)26[219:eeycva]2.0.co;2. S2CID86206151.
^Pieter Missiaen; Gregg F. Gunnell; Philip D. Gingerich (2011). "New brontotheriidae (Mammalia, Perissodactyla) from the Early and Middle Eocene of Pakistan with implications for mammalian paleobiogeography". Journal of Paleontology. 85 (4): 665–677. Bibcode:2011JPal...85..665M. doi:10.1666/10-087.1. S2CID129473193.
^Hlawatsch, Kerstin; Erfurt, Jörg (2007). "Zahnmorphologie und stratigraphische Verbreitung von Hyrachyus minimus (Perissodactyla, Mammalia) in den eozänen Geiseltalschichten" [Tooth morphology and stratigraphic distribution of Hyrachyus minimus (Perissodactyla, Mammalia) in the Eocene Geiseltal layers]. Hallesches Jahrbuch für Geowissenschaften. Suppl. 23: 161–173.
^Christine Janis (2008). "An evolutionary history of browsing and grazing ungulates". In Iain J. Gordon; Herbert H. T. Prins (eds.). The Ecology of Browsing and Grazing. Ecological Studies. Vol. 195. Springer. pp. 21–45. doi:10.1007/978-3-540-72422-3_2. ISBN978-3-540-72421-6.
^Benton, Michael J. (1997). Vertebrate Palaeontology. London: Chapman & Hall. p. 343. ISBN0-412-73810-4.
^Étienne Geoffroy Saint-Hilaire; Georges Cuvier (1795). "Memoire sur une nouvelle division of Mammifères, et sur les principes qui doivent servir de base dans cette sorte de travail". Magasin Encyclopédique: 164–190.
^Georges Cuvier (1817). "Le Règne Animal distribue d'après son organisation pour servir de base à l'histoire naturelle des animaux". Introduction à l'Anatomie Comparée. 1: 1–540.
^Johann Friedrich Blumenbach (1779). Handbook of Natural History. pp. 168–448.
^Donald R. Prothero; Robert M. Schoch (1989). "Classification of the Perissodactyla". The Evolution of Perissodactyls. Oxford University Press. pp. 530–537.
Martin S. Fischer: Mesaxonia (Perissodactyla) Perissodactyla. In: Wilfried Westheide, Reinhard Rieger (eds.): Systematic Zoology. Part 2: Vortex or craniotes. Spektrum Akademischer Verlag, Heidelberg and Berlin 2004, pp 646–655, ISBN3-8274-0307-3.
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Perissodactyla is an order of herbivorous ungulate mammals distinguished by their odd-toed feet, bearing either one functional toe (as in horses) or three (as in rhinoceroses and tapirs), with weight supported primarily by the central toe and hooves derived from enlarged middle claws.[1] These animals are hindgut fermenters, possessing large, grooved molars adapted for grinding tough vegetation, and they exhibit digitigrade locomotion on elongated limbs suited for speed or stability in diverse habitats ranging from open grasslands to dense forests.[1] Today, the order comprises just six genera and 17 extant species across three families—Equidae (horses, zebras, and asses), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs)—many of which are endangered due to habitat loss and poaching.[2]Perissodactyls originated in the late Paleocene or early Eocene from condylarth-like ancestors, possibly related to forms like Radinskya from Asia, and rapidly diversified during the Eocene, achieving peak generic diversity of around 33 taxa by the middle Eocene.[2] This radiation included now-extinct groups such as the massive brontotheres and the gigantic Indricotherium, the largest land mammal ever known, which roamed Eurasian woodlands and floodplains.[1] However, their abundance declined sharply starting in the late middle Eocene (around 39.9 million years ago), linked to climatic changes, including cooling following the Middle Eocene Climatic Optimum and the Eocene-Oligocene Transition, as well as increasing competition from even-toed artiodactyls.[2] By the Oligocene, perissodactyl diversity had plummeted, with further extinctions in the Miocene eliminating lineages like chalicotheres and aceratheres, leaving only the modern families.[1]Ecologically, perissodactyls play key roles as grazers and browsers, shaping vegetation structure in their native ranges across Africa, Asia, and the Americas, though their reduced numbers highlight ongoing conservation challenges.[2] Adaptations such as premolar molarization—enhancing dental efficiency for processing fibrous plants—helped surviving lineages persist through environmental shifts, underscoring the order's evolutionary resilience despite dramatic declines.[2]
Taxonomy
Outer taxonomy
Perissodactyla is an order of herbivorous mammals classified as ungulates, distinguished by their odd-toed feet bearing one or three functional digits that support the body's weight.[3] This order encompasses species adapted for terrestrial locomotion with hooves formed from enlarged toenails, and their digestive systems are specialized for processing fibrous plant material through hindgutfermentation.[4] The name Perissodactyla was introduced by the British anatomist Richard Owen in 1848 to denote these odd-toed forms, derived from the Ancient Greek words perissós (odd or uneven) and dáktylos (finger or toe), reflecting the asymmetrical digit arrangement compared to even-toed ungulates.[3][5]Within the broader mammalian phylogeny, Perissodactyla belongs to the superordinal clade Laurasiatheria, where it serves as the sister group to Cetartiodactyla, the order comprising even-toed ungulates and cetaceans.[6] This placement is supported by extensive molecular data, including analyses of nuclear and mitochondrial genes, which resolve Perissodactyla as diverging from the Cetartiodactyla lineage around 80 million years ago during the Late Cretaceous.[6] Key synapomorphies defining Perissodactyla include the mesaxonic foot structure, where the axis of symmetry runs through the enlarged central (third) digit that bears the majority of locomotor stress, and a progressive reduction in the number of toes from the ancestral five to either three (as in tapirs and rhinoceroses) or one (as in equids).[4] These adaptations enhance stability and speed on varied terrains, distinguishing perissodactyls from the paraxonic (laterally symmetric) feet of their cetartiodactyl relatives.[7]Traditional morphological classifications grouped Perissodactyla with Artiodactyla (now part of Cetartiodactyla) under a paraphyletic assemblage of Ungulata, based on shared features like hoofed feet and herbivorous diets, often tracing origins to Paleocene "condylarths."[7] However, molecular phylogenies, integrating genomic sequences from hundreds of loci, have overturned this by demonstrating that Perissodactyla forms a robust monophyletic clade, separate from other ungulate orders and more closely allied with carnivorans and bats within Laurasiatheria than previously thought.[5][8] This shift highlights how early divergences in Laurasian mammals restructured ungulate evolution, with Perissodactyla emerging as a cohesive lineage supported by both genetic and select osteological evidence.[6]
Modern members
The modern members of Perissodactyla consist of three extant families: Equidae (horses, zebras, and asses), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs), comprising a total of 17 species.[9]Equidae includes eight species, all in the genus Equus: the domestic horse (E. caballus), Przewalski's horse (E. ferus przewalskii), three species of zebra (E. quagga, E. grevyi, E. zebra), and three species of wild ass (African wild assE. africanus, onagerE. hemionus, kiangE. kiang).[10] Rhinocerotidae has five species across four genera: white rhinoceros (Ceratotherium simum), black rhinoceros (Diceros bicornis), Indian rhinoceros (Rhinoceros unicornis), Javan rhinoceros (R. sondaicus), and Sumatran rhinoceros (Dicerorhinus sumatrensis).[11] Tapiridae contains four species, all in the genus Tapirus: South American tapir (T. terrestris), mountain tapir (T. pinchaque), Baird's tapir (T. bairdii), and Malayan tapir (T. indicus).[12]These families exhibit distinct morphological features adapted to their ecological roles. Members of Equidae are characterized by a single functional toe (hoof) on each foot, enabling high-speed cursorial locomotion across open habitats.[13] Rhinocerotidae species are distinguished by prominent horns formed from keratin, varying from one to two per individual, which serve in defense and intraspecific combat, alongside massive body sizes up to 3,500 kg in the white rhinoceros.[14] Tapiridae features a short, flexible proboscis formed by an elongated snout, used for foraging in dense vegetation, with bodies marked by a distinctive black-and-white pattern in some species like the Malayan tapir for camouflage.[15]All 17 species are native to specific continents, reflecting the order's fragmented modern distribution. Equids are primarily found in Africa and Eurasia, with zebras and wild asses in sub-Saharan Africa and onagers/kiangs in central and western Asia.[16] Rhinoceroses occur in Africa (white and black species) and Asia (Indian, Javan, and Sumatran), with no overlap between continents.[17] Tapirs inhabit the Americas (the three Neotropical species in Central and South America) and Southeast Asia (Malayan tapir), representing a relict Gondwanan distribution.[18]As of 2025, conservation statuses vary markedly across families according to the IUCN Red List. In Equidae, several species are classified as Least Concern (e.g., kiang), but many face threats: Przewalski's horse is Endangered, Grevy's zebra and onager are Endangered, mountain zebra is Vulnerable, African wild ass is Critically Endangered, and plains zebra is Near Threatened, reflecting habitat loss and poaching pressures.[19] All five rhinoceros species are threatened, with black, Javan, and Sumatran rhinos Critically Endangered; Indian rhino Vulnerable; and southern white rhino Near Threatened, driven primarily by poaching for horns and habitat fragmentation.[20] Tapiridae species are all threatened: South American tapir Vulnerable, while mountain, Baird's, and Malayan tapirs are Endangered, owing to deforestation and hunting.[21] Overall, more than 80% of perissodactyl species are at risk of extinction.[22]Genetic diversity in modern perissodactyls is generally low, exacerbating vulnerability to threats. In Equidae, the domestication of horses created a severe bottleneck around 4,000–5,000 years ago, reducing effective population size and leading to decreased heterozygosity in E. caballus compared to wild relatives like Przewalski's horse.[23] Rhinoceroses exhibit inherently low genetic variation across all species, even in prehistoric populations, likely due to historical small population sizes and isolation, with modern taxa showing further declines from anthropogenic pressures.[24] Tapirs also suffer reduced diversity, particularly in fragmented wild populations and captive breeding programs, where inbreeding in species like the mountain tapir limits adaptive potential.[25]
Prehistoric members
Perissodactyla first appeared in the fossil record during the early Eocene epoch, approximately 55 million years ago, with the earliest known fossils attributed to the small, dog-sized genusHyracotherium, often referred to as the "dawn horse," which exhibited primitive odd-toed ungulate features such as three toes on the hind feet and low-crowned teeth adapted for browsing soft vegetation.[26][27][28] This genus, found in North America and Europe, represents the basal form from which the order diversified rapidly, leading to over a dozen families by the mid-Eocene, many of which were adapted to forested environments across Laurasia.[29] The order's prehistoric members spanned from the Eocene to the Holocene, showcasing remarkable morphological diversity, including browsers, grazers, and specialized forms, before declining sharply in the late Cenozoic.Among the major extinct families, the Brontotheriidae, known as brontotheres or "thunder beasts," were large, horned herbivores that dominated Eocene landscapes in North America and Asia, reaching body sizes up to 2.5 meters at the shoulder with paired nasal horns used possibly for display or combat; they became extinct by the end of the Eocene around 34 million years ago.[30][31] The Chalicotheriidae featured unique claw-like hooves on their forelimbs, suited for pulling down branches in wooded habitats, and persisted longer than many relatives, from the Eocene through the Miocene and into the early Pleistocene in Africa and Asia.[32] Other notable extinct families include the Palaeotheriidae, early European forms with tapir-like builds; the Amynodontidae, semi-aquatic hippopotamus-like rhinocerotoids; and the Hyracodontidae, small, hornless running rhinoceros relatives.[33] By the end of the Oligocene, at least 10 of the approximately 14 early families had gone extinct, reflecting a shift from over 30 genera in the Eocene to a fraction of that diversity.[34]Prominent genera among prehistoric perissodactyls highlight the order's evolutionary experimentation, such as Megacerops (formerly Titanotherium), a brontothere with a robust skull and Y-shaped cheek teeth for grinding tough plants, exemplifying the family's gigantism in late Eocene North America.[35] In the rhinocerotoid lineage, Paraceratherium (synonymous with Indricotherium), the largest known land mammal, stood over 4 meters at the shoulder and weighed up to 20 tons, browsing high foliage in OligoceneAsia (34–23 million years ago) with a long neck and trunk-like lips; its extinction underscores the vulnerability of extreme body sizes.[36][37] Equid evolution produced diverse forms like Mesohippus, a three-toed Oligocene grazer (around 40–30 million years ago) that bridged early Hyracotherium-like ancestors and later single-toed horses, with improved high-crowned teeth for abrasive grasses.[38][39]Major extinction events profoundly shaped prehistoric perissodactyl diversity, with a significant turnover at the Eocene-Oligocene boundary around 34 million years ago, known as the Grande Coupure in Europe, where cooling climates and habitat fragmentation led to the loss of many endemic lineages, including most brontotheres and early equoids.[40][26] Further declines occurred in the late Miocene and culminated in the end-Pleistocene megafaunal extinction event approximately 11,000 years ago, affecting genera like chalicotheres and woolly rhinoceroses amid global warming and increased competition from even-toed artiodactyls, which were better adapted to open grasslands through more efficient digestive systems.[41][2][42] These events reduced perissodactyls from ecological dominants to the three surviving families today.
Higher classification of perissodactyls
The higher classification of Perissodactyla divides the order into two primary extant suborders: Hippomorpha, which includes the family Equidae (horses and relatives), and Ceratomorpha, encompassing the families Tapiridae (tapirs) and Rhinocerotidae (rhinoceroses). These suborders reflect a basal split in the perissodactyl lineage, with Hippomorpha characterized by adaptations for cursorial locomotion in equids and Ceratomorpha by more generalized forms in tapirs and heavily built rhinos.At the family level, phylogenetic analyses place Equidae as the sister group to the Ceratomorpha clade, within which Tapiridae and Rhinocerotidae form sister families sharing a common ancestor in the early Eocene. This relationship is supported by both morphological and molecular data, including dental and postcranial traits that distinguish ceratomorphs from hipparions.[43]Extinct suborders include Ancylopoda, comprising claw-bearing forms such as chalicotheres (family Chalicotheriidae) and lophiodonts (family Lophiodontidae), which diverged early in perissodactyl evolution as the sister group to the crown-group suborders. Tapiroidea represents an extinct superfamily within Ceratomorpha, including early tapiromorphs like isectolophids that bridge basal forms to modern Tapiridae.[44]Cladistic analyses combining morphological and molecular datasets consistently recover Perissodactyla as monophyletic, with strong support for the Hippomorpha-Ceratomorpha dichotomy; for instance, maximum likelihood bootstrap values exceed 95% for the monophyly of Ceratomorpha in multi-gene studies. Morphological phylogenies of basal taxa further bolster this, using characters from the skull and limbs to resolve early divergences with parsimony-based bootstrap supports around 80-90% for key nodes.The placement of brontotheres (family Brontotheriidae) remains debated, with some analyses positioning them as a basal hippomorph or even outside Perissodactyla in a separate order due to their distinct horned morphology and early Eocene origins, though most recent studies retain them within the order as stem perissodactyls.[31][45]
Anatomy
Limbs
Perissodactyls are characterized by mesaxonic feet, in which the central third digit bears the majority of the body weight, providing a stable axis for locomotion along the midline of the limb.[46] This structure contrasts with the paraxonic condition in even-toed ungulates and supports efficient weight distribution in odd-toed forms. Living perissodactyls typically retain one or three functional digits: equids have a single weight-bearing digit (the third) on both fore- and hindfeet, while tapirs possess four digits on the forefeet (with the fifth reduced) and three on the hindfeet, and rhinoceroses maintain three digits on all feet.[4][47]The terminal phalanges of these digits are encased in keratinous hooves, which are enlarged, modified toenails that protect the underlying bone and facilitate unguligrade posture—walking on the tips of the toes.[4] In equids, the hoof is specialized for high-impact running, with a hard outer wall and softer internal structures like the frog for shock absorption.[47] Support for the limbs is enhanced by fusion of the fibula to the tibia in equids, creating a robust, unified structure that stabilizes the lower leg during rapid movement; this fusion is less pronounced or absent in tapirs and rhinoceroses, where the bones remain more distinct.[47][48]Limb adaptations reflect diverse locomotor demands across the order. In equids, elongated metapodial bones and a monodactyl foot enable cursorial locomotion, optimizing speed and energy efficiency through elastic recoil in tendons and ligaments.[46] Rhinoceroses, by contrast, exhibit pillar-like legs with shorter, stouter metapodials and a padded sole beneath the hooves, suited for supporting massive body weight and navigating varied terrains with stability rather than speed.[46] Tapirs show intermediate features, with broader feet aiding traversal of soft, forested ground.[47]Evolutionarily, perissodactyl limbs trace back to ancestors with five toes, as evidenced by embryonic development in modern equids, with progressive reduction to an odd number of functional digits in derived forms to enhance locomotor efficiency.[46] This trend culminated in the monodactyly of equids from earlier tridactyl ancestors.[4] An extinct variation is seen in chalicotheres, which secondarily developed large, bifid claws on their digits instead of hooves, likely for pulling down branches during browsing rather than typical unguligrade support.[49][50]
Skull and teeth
The dentition of perissodactyls is highly specialized for herbivory, featuring variations in crown height that correlate with dietary preferences among extant families. In grazing equids, the molars and premolars are hypsodont, with tall crowns that resist abrasion from abrasive grasses, whereas browsing tapirs possess brachydont teeth with low crowns suited for softer foliage.[29][16][2] In rhinocerotids, the cheek teeth are hypsodont and exhibit continuous growth to compensate for wear during processing of tough vegetation.[51][52]The typical dental formula for perissodactyls is variable but generally follows I 0–3/0–3, C 0–1/0–1, P 3–4/3–4, M 3/3 × 2 = 24–44, reflecting reductions in incisors and canines across the order, with emphasis on the postcanine dentition for mastication.[16] The cheek teeth are lophodont, characterized by transverse ridges (lophs) that enhance shearing efficiency for fibrous plant material.[16][53]Cranial morphology in perissodactyls supports these feeding adaptations, with enlarged nasal openings that accommodate expanded olfactory epithelia for detecting food sources and environmental cues.[54] Early Eocene forms, such as basal equoids, exhibited relatively reduced braincases compared to later relatives, indicative of less encephalized skulls focused on basic sensory and masticatory functions.[55][56]Evolutionarily, perissodactyl cheek teeth transitioned from bunodont cusps in Eocene ancestors, which were adapted for crushing softer fruits and leaves, to increasingly complex lophodont patterns by the Miocene, with crescentic crests (selenodont elements) in advanced equids that improved grinding of abrasive diets.[53][34] This increase in loph complexity facilitated more efficient transverse shear during occlusion.[53]In equids, jaw mechanics emphasize transverse chewing, where the mandible moves laterally during the power stroke to align lophs for optimal grinding, enabled by the mobility of the temporomandibular joint and reduced interlocking of anterior teeth.[57][58][59]
Gut
Perissodactyls exhibit a hindgut fermentation digestive system, distinguished by a simple, monogastricstomach that processes ingested forage rapidly before passing it to the small intestine for primary nutrient absorption. The hindgut, comprising the cecum and colon, serves as the main site for microbial fermentation, where symbiotic bacteria and protozoa break down recalcitrant cellulose and hemicellulose from fibrous plant material into volatile fatty acids such as acetate, propionate, and butyrate, which provide up to 70% of the animal's energy needs.[60][61] This system enables efficient utilization of low-quality, high-fiber diets typical of grazing and browsing lifestyles.[62]Anatomically, the perissodactyl gut features a relatively small stomach (9-10% of total gastrointestinal tract volume) and small intestine (about 30%), with the hindgut enlarged to accommodate fermentation volumes. In equids like horses, the cecum alone constitutes 16% of the gastrointestinal tract and holds 26-38 liters (7-10 gallons), while the large colon and related structures account for 45%, allowing substantial digesta retention for microbial action—though the entire gastrointestinal tract represents roughly 5% of body weight in mature individuals, with the hindgut comprising the majority of this mass.[60][63] Tapirs and rhinoceroses show similar proportions, with voluminous ceca and colons adapted for slower transit times to maximize fiber breakdown, contrasting with the foregut's quick passage to support continuous foraging.[60][63]Key adaptations include selective retention of particulate matter in the hindgut for prolonged fermentation and, in some species like rhinoceroses, coprophagy to recycle undigested nutrients and inoculate the gut with beneficial microbes, enhancing overall efficiency on nutrient-poor forage. Unlike artiodactyls, which employ foregut fermentation in a multi-chambered rumen for pregastric microbial digestion and superior protein recovery, perissodactyls lack a rumen and depend entirely on postgastric hindgut processes, making them better suited to abrasive, high-fiber diets but potentially less effective at extracting nitrogen from low-protein sources.[64][42] This reliance on hindgut dynamics, however, carries health risks; in equids, the convoluted colon structure promotes impactions from dehydrated or poorly fermented ingesta, leading to colic—a common and potentially fatal condition exacerbated by dietary shifts or inadequate water intake.[65][66]
Lack of carotid rete
Perissodactyls lack the carotid rete mirabile, a complex arterial network that replaces the internal carotid artery in the cerebral blood supply of most artiodactyls, and instead feature a direct pathway from the carotid arteries to the circle of Willis. This absence represents the retention of the ancestral mammalian condition, resulting in a simplified cerebral vasculature without the intricate anastomoses characteristic of the rete.[67] In contrast, even-toed ungulates (artiodactyls) evolved the rete as a derived trait, enabling countercurrent heat exchange between arterial and venous blood in the cavernous sinus.[68]The anatomical basis for this difference includes the persistence of a prominent internal carotid artery in perissodactyls, paired with enlarged jugular veins that incorporate cooled venous blood from the upper respiratory tract to facilitate overall cranial heat dissipation. This setup allows cool blood from evaporative cooling in the nasal passages to mix directly with jugular drainage, aiding in brain temperature management without a dedicated rete structure. During physical exertion, the direct arterial supply ensures that brain temperature closely tracks carotid blood temperature, with rises typically limited to about 0.2–0.5 °C above arterial levels.[69][70]The physiological advantages of lacking the carotid rete include faster thermoregulation due to the tight linkage between brain and body temperatures, which reduces fluctuations and enables rapid behavioral or physiological responses to environmental changes. Additionally, during intense activity, this configuration minimizes heat transfer specifically to the brain via countercurrent exchange, preserving overall body heat for sustained endurance rather than diverting it for selective cooling. This adaptation is particularly evident in equids, where the simplified vasculature supports prolonged locomotion in open habitats, complementing their cursorial lifestyle and hindgut fermentation-generated heat without the water-conserving but potentially limiting effects of a rete.[68] In artiodactyls, the rete's presence promotes selective brain cooling, which conserves water and energy in hot, arid conditions—especially beneficial for ruminants managing fermentation heat—but restricts temperature variability, potentially constraining activity in variable open environments.[58]
Distribution and habitat
Current geographic distribution
Perissodactyla, the odd-toed ungulates, encompass three extant families: Equidae (horses, zebras, and asses), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs). Their current geographic distribution is highly fragmented, primarily confined to Africa, Asia, and the Americas, with many species facing severe range contractions due to habitat loss from agriculture, urbanization, and poaching. Native populations are limited to specific regions, while introduced feral horse populations have established worldwide.The family Equidae has native wild populations restricted to Africa and central Asia. Zebras, comprising three species, are endemic to Africa: the plains zebra (Equus quagga) ranges across eastern and southern Africa from Sudan to South Africa; Grevy's zebra (Equus grevyi) is limited to northern Kenya and southern Ethiopia; and the mountain zebra (Equus zebra), with subspecies in southwestern Angola, Namibia, and South Africa, occupies mountainous regions in southern Africa. The African wild ass (Equus africanus) persists in small, isolated groups in the Horn of Africa, including Ethiopia, Eritrea, and possibly Sudan, with an estimated total population of around 600 individuals. The only truly wild horse, Przewalski's horse (Equus ferus przewalskii), is native to the steppes of central Asia and has been reintroduced to sites in Mongolia, China (where the population exceeds 900), Kazakhstan, and Russia, totaling approximately 2,000 wild individuals globally as of 2025.[71] Feral horse populations, descendants of domesticated horses introduced since the 16th century by European colonizers, are widespread and abundant outside native ranges, including mustangs in the western United States (estimated at approximately 73,000 as of March 2025),[72] brumbies in Australia (over 400,000, the largest feral equid population worldwide), and scattered herds in South America and New Zealand.Rhinocerotidae species are distributed across Africa and southern Asia, with two species in Africa and three in Asia. The white rhinoceros (Ceratotherium simum) and black rhinoceros (Diceros bicornis) are native to sub-Saharan Africa, spanning 13 and 12 countries respectively, from South Africa to Kenya and Namibia to Tanzania, though their ranges are highly fragmented by habitat conversion and historical poaching. The greater one-horned rhinoceros (Rhinoceros unicornis) inhabits grasslands and floodplains in northeastern India and Nepal. The Javan rhinoceros (Rhinoceros sondaicus) is critically restricted to a single population of about 50 individuals in Ujung Kulon National Park on Java, Indonesia. The Sumatran rhinoceros (Dicerorhinus sumatrensis) survives in fragmented rainforest pockets on Sumatra and Borneo, Indonesia, with 34–47 individuals remaining. The global wild rhino population stands at approximately 26,700 as of 2025, underscoring the precarious status driven by ongoing habitat degradation.Tapiridae includes four species, three native to the Americas and one to Southeast Asia. The Baird's tapir (Tapirus bairdii) ranges from southern Mexico through Central America to northwestern South America, including Colombia, with an estimated population of approximately 4,500 individuals as of 2025.[73] The mountain tapir (Tapirus pinchaque) is confined to high-altitude cloud forests in the Andes of Colombia, Ecuador, and northern Peru, with around 2,500 individuals. The lowland or Brazilian tapir (Tapirus terrestris) occupies a broad area east of the Andes in South America, from Venezuela to Argentina, though its populations are highly fragmented due to habitat loss, with no precise global estimate available (IUCN 2019). The Malayan tapir (Tapirus indicus) is found in the tropical rainforests of Myanmar, Thailand, Peninsular Malaysia, and Sumatra and Borneo in Indonesia, with fewer than 2,500 mature individuals. These distributions reflect significant contractions from historical extents, exacerbated by deforestation and human encroachment.
Habitat preferences
Perissodactyls, encompassing equids, rhinocerotids, and tapirids, exhibit diverse habitat preferences shaped by their ecological roles and physiological adaptations. Equids, including horses, zebras, and asses, predominantly inhabit open landscapes such as grasslands, savannas, and deserts, where expansive grassy areas support their grazing habits and facilitate predator evasion through speed and visibility.[74][75] These species require access to water sources and abundant forage, often selecting microhabitats with low vegetation cover to detect threats early via sight and hearing, while avoiding dense thickets that hinder flight responses.[76]Rhinocerotids show varied preferences within tropical and subtropical zones, with African species illustrating distinct niches. White rhinoceroses favor open grasslands and savannas in eastern and southern Africa, relying on grassy openings for grazing and proximity to water holes for wallowing and hydration.[77] In contrast, black rhinoceroses prefer denser bushlands, woodlands, and thickets, which provide browse vegetation and cover from predators, though they also venture into grasslands near water.[78] Asian rhinoceros species, such as the greater one-horned rhinoceros, occupy floodplain grasslands and riverine forests in subtropical regions, emphasizing the need for wetland access and tall grasses for foraging and thermoregulation. Across rhinocerotids, habitat selection prioritizes forage availability, shade, and mud wallows to mitigate heat and parasites.Tapirids are largely confined to dense, moist environments in tropical climates, underscoring their semi-aquatic lifestyles. Lowland tapirs, including the Brazilian and Baird's species, thrive in rainforests, swamps, and flooded grasslands of Central and South America, where they utilize rivers and marshes for bathing, cooling, and predator escape by submerging.[79][80] The Malayan tapir inhabits similar dense rainforests and swampy areas in Southeast Asia, favoring understory cover for concealment and fruit-rich floors for diet. The mountain tapir, adapted to higher altitudes of 2000–4000 meters in the Andes, occupies cloud forests and páramos, selecting microhabitats with epiphyte-laden vegetation for forage and rocky outcrops for protection, despite cooler subtropical conditions.[81] Overall, tapirs require consistent water access and vegetative cover to support their browsing and evasion strategies.
Behavior and ecology
Social structure and behavior
Perissodactyls exhibit diverse social structures adapted to their environments, ranging from group-living in open habitats to solitary lifestyles in dense forests. Among equids, such as horses (Equus caballus) and zebras (Equus quagga), social organization typically revolves around stable harem systems where a dominant stallion maintains a group of several mares and their offspring, fostering year-round familial bonds that enhance protection and resource access.[82] These harems often integrate into larger multilevel societies, with young stallions dispersing at 1–5 years of age to form bachelor herds—loose aggregations of unmated males that develop dominance hierarchies through ritualized displays rather than injurious fights.[82] Communication within these groups relies heavily on vocalizations; for instance, horses produce whinnies as long-distance calls to signal location, emotional state, or alarms, with acoustic variations conveying arousal levels through changing harmonics and pitch.[83] Zebras similarly use high-pitched whinnies and nickers for appeasement, affiliation, or alerting herd members to threats.[84]In contrast, rhinoceroses (Rhinocerotidae) are predominantly solitary, with adult males fiercely territorial and maintaining large, overlapping home ranges marked by dung piles and urine sprays to deter rivals.[85] Females and subadults show limited sociability, occasionally forming loose groups at water sources, but interactions are brief and agonistic, especially among males competing for mates.[78] A key behavioral trait across rhino species is mud wallowing, where individuals roll in water or mud pits to cool their thick skin, protect against parasites and sunburn, and possibly signal territory through scent deposition.[86] This thermoregulatory behavior is most frequent during hot periods and can occupy up to half the daily activity budget in some populations.[87]Tapirs (Tapiridae) also lead largely solitary lives, with adults maintaining individual home ranges that overlap minimally except during mating; however, mother-offspring pairs persist for 1–2 years, and occasional associations of two to three individuals occur at feeding sites or salt licks.[88] Males possess larger ranges that encompass multiple female territories, supporting a polygynous system with limited social bonding beyond reproduction.[88] Vocal communication plays a vital role in their elusive lifestyle, featuring a repertoire of whistles, coughs, and harmonic calls that convey individual identity, sex, and emotional state, enabling long-distance contact in dense vegetation without visual cues.[88] These calls exhibit distinct acoustic signatures, allowing recognition of familiar individuals and facilitating territorial maintenance.[88]Activity patterns among perissodactyls vary by habitat and predation pressure. Forest-dwelling species like tapirs are primarily nocturnal and crepuscular, with over 85% of activity occurring at dawn, dusk, or night to avoid diurnal predators and heat, though they may shift to more cathemeral rhythms in low-disturbance areas.[89] In contrast, open-plains equids such as plains zebras are diurnal, spending peak hours foraging and traveling, with heightened vigilance during daylight exposure.[90] Rhinoceroses display a crepuscular-diurnal pattern, most active in early morning and late afternoon for feeding and movement, while resting in shade or wallows midday to conserve energy in arid environments.[91] Some equids, notably migratory zebras, undertake seasonal long-distance treks following rainfall and vegetation pulses, covering hundreds of kilometers in predictable patterns driven by resource availability.[92]Predation defenses in perissodactyls emphasize collective and individual strategies tailored to group dynamics. In herd-forming equids like zebras, vigilance is distributed across members, with individuals scanning for threats while others graze, reducing per capita risk through the "many eyes" effect and enabling rapid group responses to alarms.[93] High sprint speeds, reaching up to 60 km/h in zebras, facilitate evasion of cursorial predators like lions, often in coordinated herd maneuvers that confuse attackers.[94] Solitary rhinos and tapirs rely more on camouflage, thick hides, and aggressive charges with horns or bulk to deter ambushes, though tapirs may vocalize to startle intruders in undergrowth.[95]
Diet and foraging
Perissodactyls are hindgut-fermenting herbivores adapted to diets high in fiber and low in protein, enabling them to process coarse, low-quality forage through microbial fermentation in the cecum and colon.[96] This digestive strategy supports consumption of fibrous plant material that ruminants might avoid, though it is less efficient at extracting protein from lush vegetation.[28]Members of the family Equidae, including horses, zebras, and asses, are primarily grazers that consume grasses as the core of their diet, often ingesting 2–3% of their body weight in dry matter daily to meet energy needs.[97] Some wild asses supplement grazing with browsing on leaves, bark, and shrubs, particularly in arid environments where grasses are scarce.[98]Rhinocerotids exhibit dietary variation by species: African species such as the white rhinoceros are primarily grazers feeding on grasses, while the black rhinoceros is a browser consuming leaves, twigs, and shoots from woody plants; Asian species like the Indian and Sumatran rhinoceroses have mixed diets.[99] The Indian rhinoceros (Rhinoceros unicornis) primarily grazes on grasses but incorporates aquatic vegetation, such as water plants and submerged herbs, as well as some terrestrial browse.[100]Tapirids, including the various tapir species, are browsers that selectively forage on fruits, leaves, stems, and aquatic plants, favoring a diverse array of understoryvegetation in tropical forests.[101] Their selective feeding targets nutrient-rich items like tender shoots and fallen fruits, often in a zig-zag pattern to sample multiple plant species within a small area.[102]Foraging techniques vary by family: equids often graze in open areas using their mobile lips to crop grasses efficiently, while rhinos and tapirs engage in solitary browsing, stripping foliage with prehensile lips or snouts.[96] Many perissodactyls exhibit seasonal diet shifts, such as equids transitioning from short grasses in wet periods to taller, more fibrous species during dry seasons to optimize nutrient intake.[103] These adaptations, supported by hindgut efficiency, allow sustained nutrition from variable forage quality without the need for constant high-protein intake.[28]
Reproduction
Mating and reproduction
Perissodactyls exhibit predominantly polygynous mating systems, where males compete intensely for access to multiple females, as seen in equids and rhinocerotids. In equids such as horses and zebras, dominant stallions form harems and defend them against rival males through aggressive displays and fights, leading to high variance in male reproductive success. Similarly, in rhinoceroses, territorial males secure mating opportunities with multiple females within their ranges, with genetic evidence confirming polygyny and elevated reproductive skew among successful sires. Male competition often involves physical confrontations, such as horn-locking in rhinos or kicking and biting in equids, which determine dominance and access to receptive females. Tapirs display more flexible systems, with males engaging in promiscuous mating but showing temporary pair bonds during courtship.Reproductive seasonality varies with habitat and latitude. In tropical environments, tapirs breed year-round without a fixed season, allowing continuous reproductive opportunities tied to resource availability. In contrast, temperate-zone equids like wild horses exhibit seasonal breeding, primarily in spring, synchronized with photoperiod changes to align foaling with abundant forage. Rhinoceroses breed opportunistically throughout the year, though peaks occur in wet seasons in some populations, reflecting adaptations to stable tropical habitats. Courtship rituals reinforce mate selection and include species-specific displays: stallions in equids prance, nuzzle, and sniff mares while occasionally rearing on hind legs to assert dominance; male rhinos perform stiff-legged approaches, ground-horn scraping, and mock charges, which females may reciprocate with bluff displays before accepting copulation. Equids uniquely feature induced ovulation, where mechanical stimulation from mating triggers the ovulatory surge, enhancing fertilization success shortly after copulation.Gestation periods are prolonged across perissodactyls, supporting the development of large, precocial young. Equids gestate for 11-12 months (approximately 330-370 days), rhinoceroses for 15-16 months (450-500 days), and tapirs for about 13 months (390-400 days). Litter sizes are invariably one, with twins being exceptionally rare—occurring in less than 1% of equid pregnancies and virtually absent in rhinos and tapirs—due to physiological constraints on multiple fetal support in these hindgut fermenters.
Development and growth
Perissodactyl offspring are precocial, born in an advanced state of development that enables rapid mobility and independence shortly after birth. In Equidae, such as horses and zebras, foals typically stand and walk within 30 to 120 minutes of birth, allowing them to follow their mothers and evade predators almost immediately.[104] Similarly, rhinoceros calves in species like the Sumatran and greater one-horned rhino stand within 20 to 195 minutes, with a median of 52 minutes, and begin nursing soon after.[105]Tapir calves also exhibit this precociality, standing and walking within 1 to 2 hours of birth.[106]A key aspect of early development in tapirs involves hiding behavior, where calves remain concealed in dense vegetation for the first few weeks to months, aided by their distinctive striped and spotted natal coat that provides camouflage against forest floors.[106] This strategy contrasts with the more mobile behavior of equid foals, which stay close to the herd, and rhinoceros calves, which bond tightly with their mothers in open habitats. Maternal protection is universal across Perissodactyla, with females providing exclusive care, including nursing and defense against threats; males play no role in rearing.[107]Weaning begins around 4-6 months but nursing often continues up to 8-24 months in wild equids and 6-12 months in wild tapirs, during which calves double their birth weight in the first 2 to 4 weeks, but extends to 2 to 3 years in rhinoceroses to support slower initial growth in larger-bodied species.[108][109][106][110][111]Growth is rapid in the first year, particularly in equids, where foals reach approximately 50% of adult body weight and 80 to 90% of adult height by 12 months, driven by high-energy milk and early foraging.[112]Rhinoceros and tapir growth is comparatively steadier, with calves achieving 30 to 40% of adult size in the first year before accelerating toward maturity. Sexual maturity is attained between 2 and 6 years across the order, with females as early as 2-3 years and males 3-6 years in equids, 2-4 years in tapirs, and 4-6 years in rhinoceroses, aligning with body size and environmental demands.[108][109][106][113]In the wild, perissodactyls have lifespans of 20 to 40 years, varying by family—20 to 25 years for equids, 25 to 30 years for tapirs, and 30 to 40 years for rhinoceroses—with juveniles facing the highest mortality from predation and environmental stressors during their vulnerable early stages.[114]Senescence in older individuals is often marked by excessive tooth wear from lifelong abrasive foraging on grasses and browse, which erodes hypsodont molars and limits grazing efficiency, contributing to weight loss and increased susceptibility to starvation.[115][116]
Evolutionary history
Origins
Perissodactyla originated around 55 million years ago in the early Eocene epoch, evolving from condylarth ancestors within the family Phenacodontidae during the late Paleocene to early Eocene transition in North America and Asia.[26] These archaic ungulates, such as Phenacodus, exhibited primitive dental and locomotor features that perissodactyls adapted for herbivory and terrestrial locomotion.[26] Recent fossil evidence from early Eocene deposits in western India, including the cambaytheriid genus Cambaytherium, suggests that the order may have arisen on the drifting Indian Plate, potentially representing a sister group to crown perissodactyls with transitional traits between condylarths and later odd-toed ungulates.[117]The earliest definitive perissodactyl fossils belong to Hyracotherium, a small, dog-sized browser approximately 0.4–0.6 meters in length, characterized by four toes on its front feet and three on its hind feet, adapted for navigating forested undergrowth.[26] These remains, dating to about 55 million years ago, were first discovered in the Eocene formations of Wyoming, such as the Clarks Fork Basin, where they indicate an animal that foraged on soft vegetation in subtropical woodlands.[118]Hyracotherium represents the basal equoid lineage and exemplifies the order's initial morphology, with low-crowned teeth suited for browsing rather than grazing.[119]In the wake of the Cretaceous-Paleogene mass extinction, which eliminated non-avian dinosaurs and opened ecological niches, perissodactyls rapidly radiated during the Eocene, adapting to warm, humid forest habitats across Laurasia and filling roles as medium-sized herbivores where large competitors were scarce.[26] This adaptive shift involved enhancements in limb cursoriality and dental lophodonty for processing foliage, enabling diversification into browsing niches unoccupied by other ungulate groups. The lack of early competitors facilitated an explosive Eocene boom, with perissodactyls achieving peak diversity of approximately 13 families by the middle of the epoch, dominating herbivorous mammal assemblages in North America, Europe, and Asia.[16][26]Early perissodactyls dispersed within Laurasia during the Eocene, with subsequent migrations southward to Africa in the early Miocene (around 20 million years ago) and to South America in the Pliocene (around 3 million years ago) via the formation of the Isthmus of Panama.[26][120] This expansion allowed colonization of diverse paleoenvironments, setting the stage for further phylogenetic branching while maintaining a primarily Holarctic center of abundance during the Paleogene.[121]
Phylogeny
Perissodactyla comprises three extant families—Equidae, Tapiridae, and Rhinocerotidae—organized into two primary suborders: Hippomorpha, which includes equids and the extinct palaeotheres, and Ceratomorpha, encompassing tapirs, rhinoceroses, and the extinct indricotheres.[5] These clades reflect a deep evolutionary bifurcation, with molecular and morphological data consistently supporting their monophyly. Within Ceratomorpha, Tapiridae and Rhinocerotidae form sister groups, while Equidae stands alone in Hippomorpha.[122]Molecular clock analyses, calibrated against the fossil record, estimate the divergence between Hippomorpha (Equidae and palaeotheres) and Ceratomorpha (Tapiroidea) at approximately 50–56 million years ago (Mya) during the early Eocene.[123] Within Ceratomorpha, the split between Rhinocerotidae and Tapiridae occurred around 40–44 Mya in the middle Eocene to early Oligocene, marking a key radiation event.[124]Fossil morphology, including dental and postcranial traits, alongside DNA sequences such as mitochondrial DNA (mtDNA), provide robust evidence for these relationships; for instance, mtDNA analyses strongly support Ceratomorpha monophyly with high bootstrap values.[125]A pivotal divergence in equid evolution was influenced by the Eocene-Oligocene cooling event around 34 Mya, which promoted the expansion of grasslands and selected for hypsodont teeth and high-crowned molars adapted to abrasive foraging.[126] This environmental shift drove diversification within Equidae, contrasting with the more conservative adaptations in tapirs and rhinos.[58]However, they highlight novel insights into centromere evolution in equids, revealing rapid chromosomal rearrangements and satellite-free neocentromeres linked to speciation events.[127][128]
History of research
Early studies
Early studies of perissodactyls were pioneered by Georges Cuvier in the early 19th century through his examinations of fossil remains from European and Siberian deposits. In his foundational 1812 publication Recherches sur les ossemens fossiles des quadrupèdes, Cuvier systematically described extinct forms such as the woolly rhinoceros (Coelodonta antiquitatis, first described by Johann Friedrich Blumenbach in 1799 based on a skull found in 1769 near the Lena River in Siberia) and clarified distinctions among large mammal fossils, including resolving early confusions between mammoth and rhinoceros remains unearthed from permafrost layers where they often occurred together. These efforts established perissodactyl fossils as evidence of extinction, shifting focus from mythical interpretations to scientific anatomy.Building on Cuvier's comparative approach, Richard Owen formalized the classification of odd-toed ungulates in 1848 by coining the order Perissodactyla, derived from Greek roots emphasizing their characteristic odd number of functional toes (one or three) on hind feet, distinguishing them from even-toed artiodactyls. This naming occurred in Owen's detailed analysis of Eocene dental fossils from the Isle of Wight, where he grouped extant and fossil forms like horses (Equidae), rhinoceroses (Rhinocerotidae), and tapirs (Tapiridae) based on shared ungual and skeletal features, providing a foundational taxonomic framework for the order. Owen's work integrated living and fossil evidence, highlighting perissodactyls' evolutionary continuity despite their diversity.The mid-to-late 19th century saw intensified fossil exploration in North America, epitomized by the "Bone Wars" rivalry between paleontologists Othniel Charles Marsh and Edward Drinker Cope from the 1870s to the 1890s. Their competitive expeditions across the western United States unearthed vast quantities of perissodactyl remains, particularly from Eocene and Oligocene formations, yielding key specimens that outlined the evolutionary sequence of horses from small, multi-toed Eohippus (now Hyracotherium) to larger, single-toed forms like Miohippus and Merychippus. Marsh's descriptions, such as his 1874 report on new equine mammals, documented over 20 intermediate species, establishing a linear progression that became a cornerstone of paleontological evidence for gradual change.Early classifications harbored significant misconceptions, such as treating equids as a distinct order separate from rhinoceroses and tapirs due to their specialized cursorial adaptations, a view prevalent before Owen's synthesis and persisting in some regional taxonomies. Brontotheres (extinct perissodactyl relatives like Menodus and Megacerops), initially described in the 1850s by Joseph Leidy and later expanded by Marsh, were recognized as basal perissodactyls based on their odd-toed limb morphology and dental features, though limited comparative material initially emphasized superficial resemblances in size and horn structures.A pivotal advancement came with Henry Fairfield Osborn's 1910 publication The Age of Mammals in Europe, Asia and North America, which presented the first detailed phylogeny of titanotheres (brontotheres), reconstructing their diversification from small Eocene ancestors to late Oligocene giants across four major lineages based on cranial and dental variations. Osborn's analysis, drawing on American Museum collections, firmly placed titanotheres within Perissodactyla as a basal group, illustrating adaptive radiation in horn development and body size without reliance on orthogenetic trends. This work synthesized 19th-century discoveries into a coherent framework, influencing subsequent perissodactyl studies.
Modern developments
In the 20th century, George Gaylord Simpson's seminal 1945 classification of mammals provided a foundational framework for understanding ungulate orders, including Perissodactyla as a distinct group characterized by odd-toed ungulates with mesaxonic feet.[129] This work emphasized morphological traits and fossil evidence to delineate Perissodactyla from even-toed artiodactyls, influencing subsequent taxonomic studies. Simultaneously, museum exhibits popularized the evolutionary narrative of horses (Equidae), presenting a linear progression from small, multi-toed ancestors like Eohippus to modern single-toed forms, which engaged public interest in perissodactyl adaptation despite later revisions revealing more complex branching patterns.[130]The advent of molecular phylogenetics in the 1990s revolutionized perissodactyl systematics by confirming Perissodactyla's placement within the Laurasiatheria superorder through analyses of mitochondrial genomes. Waddell et al. (1999) proposed this clade by integrating complete mitochondrial DNA sequences from multiple mammals, demonstrating shared ancestry among perissodactyls, carnivores, and cetartiodactyls, thus resolving long-standing debates on placental mammal interordinal relationships. This molecular evidence complemented fossil records, highlighting rapid diversification post-Cretaceous-Paleogene boundary.Recent advances from 2023 to 2025 have leveraged environmental DNA (eDNA) surveys to map tapir distributions in biodiverse regions like the Colombian Amazon, where metabarcoding of water samples detected lowland tapirs (Tapirus terrestris) across fragmented habitats, aiding non-invasive monitoring of elusive species.[131] In equids, centromere studies have uncovered rapid evolutionary dynamics, with uncoupling of CENP-A and CENP-B proteins facilitating chromosomal rearrangements and contributing to karyotype instability observed in horse lineages.[132] Field research employing GPS tracking has illuminated rhino movement ecology; for instance, collar data from greater one-horned rhinos (Rhinoceros unicornis) revealed home range sizes averaging 20-30 km² in floodplain grasslands, informing habitat management.[133] Conservation genetics efforts address low-diversity populations, such as northern white rhinos showing elevated inbreeding coefficients (F > 0.1) and genomic erosion, prompting strategies like assisted reproduction to mitigate extinction risks.[134]Updated phylogenies integrating fossil-calibrated molecular data have addressed historical gaps, with genomic analyses of ancient and modern rhinoceros samples refining divergence estimates for Ceratotherium at around 5-7 million years ago and underscoring low heterozygosity as a persistent trait across Perissodactyla.00891-6) These integrative approaches, combining Bayesian tip-dating with whole-genome sequences, yield robust timelines for perissodactyl radiation, estimating the crown group origin at approximately 55-60 million years ago and highlighting Eocene diversification events.[135]
Interactions with humans
Domestication and use
Among the perissodactyls, domestication has primarily succeeded with equids, particularly horses and donkeys, transforming human societies through their utility in labor and mobility. Horses (Equus caballus) were first domesticated approximately 4,200 years ago in the Pontic-Caspian steppe, where genetic evidence indicates origins from pastoralist groups managing them for riding and herding.[136] From these origins, domestic horses spread rapidly across Eurasia and beyond via trade routes and military campaigns, enabling expanded mobility and cultural exchange by the early Bronze Age.[137]Historically, domesticated horses revolutionized transportation by pulling wagons and chariots, facilitated agriculture through plowing and hauling, and dominated warfare as cavalry mounts, with chariot use emerging prominently in Bronze Age societies around 2000 BCE.[138] In modern contexts, horses continue to serve in sports such as racing and equestrian events, as well as recreational riding and limited draft work in some regions.[139]Donkeys (Equus asinus) were domesticated earlier, around 7,000 years ago in East Africa from wild ass populations, primarily for their endurance as pack animals in arid environments and trade caravans.[140] Efforts to domesticate zebras (genus Equus, subgenus Hippotigris) have met with limited success despite historical attempts, including 19th-century initiatives by European colonizers in Africa to train them as draft animals, owing to their aggressive temperament and social structure less amenable to human control.[141] Hybrids like mules, offspring of horses and donkeys, have proven valuable for their strength and hardiness in agriculture and transport, though sterility limits further breeding.[142]In contrast, rhinoceroses (family Rhinocerotidae) and tapirs (family Tapiridae) have never been domesticated due to their solitary habits, size, and ecological needs, but both faced extensive historical hunting by humans. Rhinoceros populations were targeted for their horns, valued in traditional Asian medicine and as status symbols since at least 2600 BCE in China, leading to widespread exploitation across Africa and Asia.[143] Tapirs were hunted primarily for meat and hides in indigenous cultures of Central and South America, with occasional use of their teeth in artifacts, though not true ivory trade.[144]Perissodactyls have held profound cultural symbolism in human societies, often embodying power, purity, or mystery in art and mythology. Horses frequently represent nobility, speed, and conquest in Eurasian folklore and heraldry, while the unicorn—a mythical creature likely inspired by rhinoceros horn trade in medieval Europe—symbolizes chastity and grace in Western traditions.[145]
Conservation
Perissodactyl species face significant anthropogenic threats, including habitat loss due to deforestation and agricultural expansion, poaching driven by demand for rhino horns and other body parts, human-wildlife conflicts such as crop raiding by tapirs and equids, and climate change impacts that degrade grassland habitats essential for grazing.[146][147][148] These pressures have led to population declines across the order, with emerging risks like infectious diseases in tapirs, including screwworm infestations and theileriosis, exacerbating vulnerability in fragmented habitats.[149][150]According to the IUCN Red List 2025 updates, three rhino species (black, Sumatran, and Javan) are classified as Critically Endangered, the greater one-horned rhinoceros is Vulnerable, and the white rhinoceros is Near Threatened (though its northern subspecies is functionally extinct).[146] The northern white rhino (Ceratotherium simum cottoni) is considered functionally extinct, as only two females remain in captivity with no viable wild population. Black rhinos (Diceros bicornis) number approximately 6,788 individuals and remain Critically Endangered, while Javan rhinos (Rhinoceros sondaicus) persist at around 50 individuals, Critically Endangered following a recent decline due to poaching. The Sumatran rhino (Dicerorhinus sumatrensis) is Critically Endangered with fewer than 50 individuals remaining, and the greater one-horned rhinoceros (Rhinoceros unicornis) is Vulnerable with an estimated 4,075 individuals.[146][151][11] Among tapirs, three of the four species are Endangered—Malayan (Tapirus indicus) with fewer than 2,500 mature individuals, mountain (Tapirus pinchaque), and Baird's (Tapirus bairdii)—while the lowland tapir (Tapirus terrestris) is Vulnerable.[152] Equids are generally less threatened, with most classified as Least Concern or Near Threatened, though Grevy's zebra (Equus grevyi) is Endangered with about 2,500 individuals remaining.[153]Conservation efforts for perissodactyls emphasize protected areas, such as Kruger National Park in South Africa, which safeguards significant rhino populations through intensive management despite ongoing poaching challenges.[154] Anti-poaching initiatives have incorporated advanced technologies like AI-powered drones and thermal cameras to monitor and deter incursions, achieving notable success in reserves such as Sabi Sand, where poaching incidents have dropped dramatically.[155][156] Breeding and rewilding programs are also critical, including reintroductions of Przewalski's horses (Equus ferus przewalskii) into steppe habitats in Kazakhstan and Spain to restore wild populations and genetic diversity.[157][158]Successes include the recovery of black rhinos, which increased by 5.2% since 2023 to over 6,700 individuals, rebounding from a low of fewer than 2,500 in the 1980s through translocation, habitat protection, and reduced poaching.[146][159] However, failures persist, as exemplified by the Javan rhino's stagnant population below 75 and vulnerability to localized threats, highlighting the need for enhanced international cooperation and addressing gaps in disease surveillance for tapirs.[151][149]
