Mammal Classification
Ungulates
Hooves are modified toenails, and the mammals that walk on them are called ungulates. Ungulates are typically herbivorous and are adapted for running. They are classified into two orders: Perissodactyla and Artiodactyla. Although the perissodactyls were originally — during the early Tertiary period — the more abundant of the two orders, they later declined for unknown reasons, while the artiodactyls flourished and diversified. Today there are 17 perissodactyl species and more than 220 species of artiodactyls.
Perissodactyls have either one weight-bearing toe, such as the horse, or three weight-bearing toes, such as the rhinoceros and the tapir, which also has a fourth toe and vestiges of a fifth toe on the front foot.
The tapir's largest relative is the 2.3-ton white rhinoceros (Ceratotherium simum), which, unlike the aggressive black rhinoceros, is mild and inoffensive. It has even bigger ancestors, including the largest known land mammal, Indricotherium, a brute of a herbivore that may have weighed 20 tons. Although this ancient giant lacked horns, today's rhinos have either single or double horns, both of which lack a bony core and are made up of countless hollow filaments extending from the skin.
Horns and antlers are perhaps the most visually outstanding feature of many artiodactyls. Giraffes, pronghorns and all wild bovids are among those that have bony horns sheathed in keratin or covered in furred skin. The horns themselves are never shed, although pronghorns shed their sheaths annually.
Antlers, however, are shed annually in the winter. Mostly males grow antlers, and their presence is found in all but two species in the family Cervidae, a group that includes deer, wapiti and caribou. Secretions from the pituitary gland initiate the growth of the antlers in the spring, and they continue to grow until the rut in the fall, when the males use them in clashes with each other during competition for females.
Artiodactyls are defined by the structure of their feet. Unlike perissodactyls, they bear their weight primarily on toes three and four; the big toe is always missing. Some species, including pigs, peccaries and hippopotamuses, possess two additional complete digits on each foot. On others, namely bovids, deer, giraffes and camels, these lateral toes are either absent or incomplete. The artiodactyls' articulation of bones and ligaments allows them to flex and extend the foot and digits — perfect for springing away from predators. However, not all artiodactyls get around in the same way: Possibly as an adaptation for locomotion on shifting sands, the camel has developed a digitigrade posture, meaning it moves on the "balls" of its feet like cats and dogs.
Many artiodactyls belong to the suborder Ruminantia, and they have a complex digestive system, characterized by the regurgitation and rechewing of food in a three- or four-chambered stomach, which harbors microorganisms for breaking down cellulose (see Eating).
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