Odd-toed hoofed mammals (Perissodactyla) are a group of mammals that are largely defined by their feet. Members of this group-horses, rhinoceroses, and tapirs-bear the bulk of their weight on their middle (third) toe. This distinguishes them from the even-toed hoofed mammals, whose weight is carried by their third and fourth toes together. There are about 19 species of odd-toed hoofed mammals alive today.
The details of foot anatomy vary between the three groups of odd-toed hoofed mammals. Horses have lost all but a single toe, the bones of which have adapted to form a sturdy base on which to stand. Tapirs have four toes on their front feet and only three toes on their hind feet. Rhinoceroses have three hoofed toes on both their front and back feet.
The three groups of living odd-toed hoofed mammals are varied in their body structure. Horses are long-legged, graceful animals, tapirs are smaller and rather pig-like in body structure and rhinoceroses are very large and bulky in build.
Like the even-toed hoofed mammals, odd-toed hoofed mammals are herbivores but the two groups differ significantly with respect to stomach structure. Whereas most even-toed hoofed mammals (with the exception of pigs and peccaries) have a multi-chambered stomach, odd-toed hoofed mammals have a pouch that extends from the large intestine (called the caecum) where their food is broken down by bacteria. Many even-toed hoofed mammals regurgitate their food and re-chew it to aid in digestion. But odd-toed hoofed mammals do not regurgitate their food, it instead is broken down slowly in their digestive tract.
Odd-toed hoofed mammals inhabit Africa, Asia, North America and South America. Rhinoceroses are native to Africa and southern Asia. Tapirs live in the forests of South America, Central America, and Southeast Asia. Horses are native to North America, Europe, Africa and Asia and are now essentially worldwide in their distribution, due to domestication.
Some odd-toed hoofed mammals, such as rhinoceroses, have horns. Their horns form from an outgrowth of skin and consist of compressed keratin, a fibrous protein that is also found in hair, nails, and feathers.
Odd-toed hoofed mammals are classified within the following taxonomic hierarchy:
Animals > Chordates > Vertebrates > Tetrapods > Amniotes > Mammals > Odd-Toed Hoofed Mammals
Odd-toed hoofed mammals are divided into the following taxonomic groups:
- Horses and relatives (Equidae) - There are 10 species of horses alive today.
- Rhinoceroses (Rhinocerotidae) - There are 5 species of rhinoceroses alive today.
- Tapirs (Tapiridae) - There are 4 species of tapirs alive today.
It was previously thought that odd-toed hoofed mammals were closely related to even-toed hoofed mammals. But recent genetic studies have revealed that the odd-toed hoofed mammals may, in fact, be more closely related to carnivores, pangolins, and bats than to the even-toed hoofed mammals.
Odd-toed hoofed mammals were far more diverse in the past than they are today. During the Eocene the were they dominant land herbivores, vastly outnumbering the even-toed hoofed mammals. But ever since the Oligocene, odd-toed hoofed mammals have been in decline. Today, all odd-toed hoofed mammals except domestic horses and donkeys are sparse in number. Many species are endangered and at risk of extinction. Odd-toed hoofed mammals of the past included some of the largest land mammals ever to have walked the Earth. Indricotherium, an herbivore that inhabited the forests of central Asia between 34 and 23 million years ago, was three or four times the weight of modern-day African savannah elephants. The most primitive of the odd-toed hoofed mammals are believed to be the brontotheres. Early brontotheres were about the size of modern-day tapirs, but the group later produced species that resembled rhinos.