| Odd-toed ungulates|
Temporal range: 56–0 Ma ?Late Paleocene - Recent
|Hoof of a horse|
An odd-toed ungulate is a mammal with hooves that feature an odd number of toes. Odd-toed ungulates comprise the order Perissodactyla (Greek: περισσός, perissós, "uneven", and δάκτυλος, dáktylos, "finger/toe"). The middle toe on each hoof is usually larger than its neighbours. Odd-toed ungulates are relatively large grazers and, unlike the ruminant even-toed ungulates (artiodactyls), they have relatively simple stomachs because they are hindgut fermenters, digesting plant cellulose in their intestines rather than in one or more stomachs. Odd-toed ungulates include the horse, tapirs, and rhinoceroses.
By the start of the Eocene 55 million years ago (Mya), they had diversified and spread out to occupy several continents. Horses and tapirs both evolved in North America; rhinoceroses appear to have developed in Asia from tapir-like animals and then recolonised the Americas during the middle Eocene (about 45 Mya). Of the approximately 15 families, only three survive (McKenna and Bell, 1997; Hooker, 2005). These families were very diverse in form and size; they included the enormous brontotheres and the bizarre chalicotheres. The largest perissodactyl, an Asian rhinoceros called Paraceratherium, reached 12 short tons (11 t), more than twice the weight of an elephant.
Perissodactyls were the dominant group of large terrestrial browsers right through the Oligocene. However, the rise of grasses in the Miocene (about 20 Mya) saw a major change: the even-toed ungulates with their more complex stomachs were better able to adapt to a coarse, low-nutrition diet, and soon rose to prominence. Nevertheless, many odd-toed species survived and prospered until the late Pleistocene (about 10,000 years ago) when they faced the pressure of human hunting and habitat change.
The members of the order fall into two suborders:
- Hippomorpha are odd-toed ungulates that are, today, fast runners with long legs and have only one toe. The only extant family of this suborder is Equidae (whose sole surviving genus is Equus), comprising the horse, zebra, donkey, onager, and allied species. The extinct, rhinoceros-like brontotheres are also included in this suborder. Both families probably descended from palaeotheres.
- Ceratomorpha have several functional toes; they are heavier than and move more slowly than the Hippomorpha. This suborder has two extant families: Tapiridae (tapirs) and Rhinocerotidae (rhinoceroses). The extinct chalicotheres may belong to this suborder as well.
The three surviving families of odd-toed ungulate are classified as follows.
- ORDER PERISSODACTYLA
- Suborder Hippomorpha
- Family Equidae: horses and allies, seven species in one genus
- Wild horse, Equus ferus
- African Wild Ass Equus africanus
- Domesticated Ass (Donkey) Equus africanus asinus
- Onager or Asiatic Ass, Equus hemionus
- Kiang or Tibetan Wild Ass, Equus kiang
- Plains Zebra, Equus quagga
- Mountain Zebra, Equus zebra
- Grevy's Zebra, Equus grevyi
- Family Equidae: horses and allies, seven species in one genus
- Suborder Ceratomorpha
- Family Tapiridae: tapirs, four species in one genus
- Family Rhinocerotidae: rhinoceroses, five species in four genera
- Suborder Hippomorpha
It was long thought, based on morphology, that odd-toed ungulates form a clade with even-toed ungulates. Recent phylogenetic studies have lacked full confidence in this conclusion however; some studies link Perissodactyla with Ferae into a proposed clade Zooamata while the Pegasoferae proposal goes further, suggesting that Chiroptera (bats) are more closely related to odd-toed ungulates than even-toed ones. The most recent study, by Zhou et al. (2011), finds better (but not full) support for the traditional view, uniting Perissodactyla with Cetartiodactyla into a clade of "true ungulates," Euungulata.
The living perissodactyls are a diverse group. At one extreme are the lithe and graceful horses; on another, the huge, tank-like rhinoceroses; and in the middle, the vaguely pig-like tapirs. All extant perissodactyls are large, from the 180-kg Mountain Tapir to the 2,300-kg White Rhinoceros.
Extinct perissodactyls possessed a far more diverse range of forms, too, including the tiny, vaguely tapir-like paleotheres, the monstrous brontotheres, the knuckle-walking chalicotheres, and the gigantic rhinoceros Indricotherium, which dwarfed even elephants.
However, all perissodactyls, extinct and extant, have a mesaxonic foot structure. In other words, the symmetry of the foot passes through the third digit. This means that this digit holds the animal's weight. In equines, the mesaxonic foot has been modified so that the non-weight bearing digits have atrophied away, while the third toe has enlarged, so that modern equines have only one toe. Also, all perissodactyls are hindgut fermenters. Hindgut fermenters, in contrast to the ruminants, store digested food which has left the stomach in a pouch-like extension of the large intestine called the cecum (literally "cave"), where the food is digested by bacteria. No gallbladder is present.
Today, the equines are the only social perissodactyls still extant. Horses organize themselves into small bands with a dominant mare at the top of the pecking order, as well as a resident stallion. Several bands will share a common territory, with some members of one band joining another band, every so often. These bands, in turn, form a herd.
Huge fossil beds made of the bones of hundreds or thousands of individuals suggest that many of the larger brontothere species were social animals at least some of the time. Some prehistoric rhinoceroses, such as Diceratherium, were also social animals which organized themselves into herds. However, modern-day rhinoceroses are solitary animals which maintain territories, often attacking members of their own species when their space has been invaded. Tapirs, too, are solitary animals, though they are shy, retiring creatures which do not defend or maintain territories.
Mating and reproduction
As with the males of many other animal groups, male perissodactyls often spar with each other for the privilege to mate with receptive females. A male which has found a female will attempt to taste her urine to see if she is in estrus. The female may also signal that she is in estrus, such as the whistling of cow Indian rhinoceroses and tapirs. Perissodactyls tend to have one foal or calf at a time. Very rarely, the female may have twins. Gestation is very long, from about 11 months in horses to 16 months for rhinoceroses. The calf or foal is capable of standing within moments of birth, but is very dependent on its mother. The young stays with its mother even after weaned, usually until it is chased off by the mother upon the birth of a new foal or calf. At this time, in horses, the foal will enter into the herd proper, later, young stallions are often chased off and join bachelor herds. With rhinos and tapirs, the newly weaned calf wanders away to search for new feeding grounds.
Humans and conservation
Humans have a historically long interaction with perissodactyls. The wild ass was the first equid to be domesticated, around 5000 BC in Egypt. Horses were domesticated 1000 years later. The zebroid, that is, a zebra hybrid, began appearing in zoos and menageries during the 19th century. During the 16th century, the Spaniards brought horses with them, and inadvertently reintroduced horses back into North America. While no rhinoceros has been domesticated, they have been captured for zoos and menageries since ancient times.
The odd-toed ungulates have been among the most important herbivorous mammals, at times they have been the dominant herbivores in many ecosystems. However, over the course of millions of years, many species went extinct due to climatic change, newer, coarser-leaved plants, predators, disease, and competition from other herbivores, particularly the artiodactyls. The Chalicotheriidae was the most recent family of perissodactyl to become entirely extinct. The perissodactyls' decline continues even today. Most species are listed as threatened species, and although no species are confirmed to be extinct, some subspecies have gone extinct. The quagga was hunted for its meat, the tarpan were hunted for sport, and a subspecies of Black Rhinoceros was hunted for its horn (as with all other African rhinoceros species).
Perissodactyls tend to do well in captivity, and there are many breeding programs in place to help replenish wild populations. The Przewalski's horse has been recently released back to the wild. Some of the captive breeding programs for some equids are unusual, in that breeders have been carefully selecting specimens to recreate various recently extinct equids, such as the Tarpan and Quagga. Most wild rhinoceroses are monitored, and some have their horns trimmed off to discourage horn-poachers. Even so, if conservations do not improve, it may very well be that the only living perissodactyls left will be the domesticated horse and donkey.
Two recently extinct equids
- Quagga photo.jpg
The quagga became extinct in 1883
The Tarpan went extinct, due to sport hunting.
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