Odd-Toed Ungulates: Perissodactyla

Ungulates (UNG-gyuh-luhts) are hoofed mammals. What makes perissodactyls (puh-RIH-suh-dack-tuhlz) different from artiodactyls (ar-tee-oh-DACK-tuhlz), is the number of toes. The presence of a single toe links the horse family (including horses, zebras, and asses), tapir, and rhinoceros together. This single toe is actually a combination of three toes that bear the weight together, with the middle toe being the largest of the three. Tapirs have four toes on the front feet and three on the back, while rhinoceroses (frequently called "rhinos") have three on all feet, and horses have just one.

The smallest perissodactyl is the mountain tapir, which weighs up to 485 pounds (220 kilograms). The white rhinoceros is the largest and can weigh more than 7,700 pounds (3,500 kilograms). Male rhinos and horses are bigger than females, but the opposite is true for tapirs.

Horses are medium sized with long heads and the ears stand up. The long neck is covered by a short-haired mane except in the domestic horse, whose mane falls to one side. All horses have long tails, and the ass and zebra have short hair at the tip.

The large, heavy body of the rhino sits on top of short, thick legs. The eyes are small and located on each side of the head. Though their vision is not well developed, their hearing is excellent and their erect ears are rather big. Some rhinos' skin is all but naked, while other rhinos are covered with fine hair. The horns of the white rhino can grow to reach 70 inches (175 centimeters). Rhinos' horns continue to grow throughout their lifetimes, and if lost, will grow back.

Tapirs are heavy with short, fat limbs, a short tail, and medium-sized ears that grow out and up. Their eyes are small. The hind legs of the tapir are about 4 inches (10 centimeters) higher than the front legs. Due to this difference, most of the weight is supported by the longer hind legs. Tapir skin is tough and sparingly covered with hair except for the mountain tapir, whose hair is thick to protect against the cold.

Because perissodactyls eat large quantities of hard-to-chew food, their lower jaw is deep and the mouth muscles are large. The lips are thick and flexible. The stomach is simple and food passes through the digestive system quickly. This makes digestion less efficient than in other animals with more than one stomach, such as the cow. In fact, a horse digests food only 70 percent as efficiently as a cow does.