The largest populations are usually in mixed wooded areas and open land, although deer also live in swamps, on mountains, and on northern tundras. Deer species range in size from the European elk, or moose which may reach a shoulder height of 7.7 ft, to the South American pudu, which can be as little as 10 in high at the shoulder. The red deer, which are gregarious, associate in small to large herds, each led by a mature female. Caribou migrate between the forest and the treeless tundra in herds of a thousand or more. Deer forage on twigs, leaves, bark, herbs, and buds of bushes and saplings and on grasses and other plants, feeding most actively at twilight.
Females give birth once a year, usually to one or two fawns.
The gestation period lasts from 160 days in musk deer to ten months in roe deer. Fawns are kept hidden in thickets, camouflaged by their usually dappled markings. In the United States deer now have few natural predators, they often over browse their territory and die of starvation, especially harsh winters. Hunting seasons are adjusted to allow for harsh conditions. They are also excellent swimmers.