Perissodactyls were the dominant group of large terrestrial browsers right through the Oligocene. However, the rise of grasses in the Miocene (about 20 million years ago) 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 three surviving families of odd-toed ungulate are classified as follows.
It was thought that odd-toed ungulates are closely related to even-toed ungulates. But recent molecular comparisons show that even-toed and odd-toed ungulates may not form a clade. Instead, perissodactyls may be more closely related to carnivorans, bats and pangolins (and, by default, the Creodonts and Cimolestids). So, some biologists group the orders Perissodactyla, Carnivora, Chiroptera, Pholidota, Creodonta, and Cimolesta as the clade Pegasoferae (Nishihara et al., 2006; see Matthee et al., 2007 and Springer et al., 2007 for alternate views).
The living perissodactyls are a diverse group, with no generalized appearance. 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 2,273-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 the digits hold 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 caecum (literally "cave"), where the food is digested by bacteria.
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 in order 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 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 in order to recreate various recently extinct equids, such as the Tarpan and Quagga. Most wild rhinoceroses are monitored, and some have their horns trimmed off in order 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