Members of the deer family live in a wide variety of places, including forests, swamps, deserts, and tundra. They feed exclusively on such plant materials as grass, young shoots, twigs, and bark. Some deer travel in herds and go on seasonal migrations. Deer are extremely cautious animals with keen senses of smell and hearing. Most deer reach maturity in one to three years, and the female gives birth to one or two young or, occasionally, to triplets. The offspring nurse for several months.

Deer are mammals belonging to the family Cervidae in the order Artiodactyla (the even-toed, hoofed mammals). The order Artiodactyla also includes pigs, cattle, goats, sheep, camels, antelope, giraffes, and hippopotamuses.

White-tailed deer are larger in the northern part of their range where bucks, or males, often weigh more than 470 pounds (213 kilograms). As in most deer species, the does, or females, are somewhat smaller. Both sexes are reddish brown in summer and gray in winter. When they run, they lift their tails straight up like white flags. The young, called fawns, have reddish coats with white spots.

A buck develops a pair of spiked antlers by the fall of its second year. Between January and April, after mating, the buck sheds his antlers and grows a larger set. The autumn mating season transforms timid bucks into fierce fighters, though, in contrast to many European and Asian species, the North American deer do not utter sounds. The winner in a contest of clashing antlers inherits mating privileges with the does in the vicinity.

A closely related species, the mule deer (O. hemionus) lives in western North America from the southern Yukon to northern Mexico. It has large ears, and the tail is tipped with black. Both white-tailed and mule deer can run as fast as 40 miles (64 kilometers) per hour and are good swimmers.

Roe deer (Capreolus capreolus) are small and reddish with no visible tail. They are found from the British Isles to China. Fallow deer (Dama dama) are a woodland species now mostly in Europe that are more restricted in their range than red or roe deer.

The bull moose has a large head with a broad muzzle that curves downward. Beneath his neck hangs a hairy fold of skin called a bell. The antlers of a moose are flattened and may spread 6.6 feet (2 meters) across. The antlers are shed in winter, sprout again in spring, and reach full size by early summer. In late spring the cow gives birth to one or two calves that stay with her until the following spring.

Moose occur in forested areas, particularly near water. They feed on willow tips, saplings, and bark. They wade along the shores and thrust their heads under water for mouthfuls of tender plants that grow on the bottom. Moose swim very well.

Wapiti roam in herds, moving from the mountains to the valleys in winter. During the mating season in the fall, fights between males are common. They challenge one another with a loud bellow. Two animals face each other from a distance of about 20 feet (6 meters), paw the ground, and then charge and crash their antlers together. Bellowing as they fight, they continue the struggle until one goes down. The loser usually survives but leaves the area. Occasionally the antlers of the fighters lock together so that neither can eat, and both animals die from starvation. In March the bulls lose their antlers, but they can still ward off predators with their sharp front hooves. In early summer each cow gives birth to one or two white-spotted calves.

Caribou move continually, migrating in herds that travel south for winter and north in summer. In the warmer months they can be found in the farthest northern reaches of the Arctic tundra.

Caribou have thicker bodies and shorter legs than do most deer. Their hooves are broad. Their coats are brown. They have white tails, necks, and sides. The colors vary with the seasons, as do those of many other deer, becoming lighter in color during winter. Those in the northernmost parts of their range may be almost white.

Both sexes of caribou have large, irregularly branching antlers, though those of the female are somewhat smaller and more slender. The males also reach larger sizes, with some reaching weights of more than 700 pounds (318 kilograms) and shoulder heights of up to 4.6 feet (1.4 meters).

Caribou eat grasses and browse on low-lying vegetation. They are noted for consuming large quantities of a lichen called reindeer moss that grows in the tundra regions. When alarmed, caribou break into a clumsy gallop, changing to a steady trot that carries them across the tundra. Their large, spreading hooves, with sharp cup-shaped edges, give them a firm footing on the soft, mucky surface of their summer homes and on winter ice and snow.