The reindeer (Rangifer tarandus), also known as the caribou when wild in North America, is an Arctic and Subarctic-dwelling deer, widespread and numerous across the northern Holarctic.

Distribution and habitat

Around 4,000 reindeer have been introduced into the French subantarctic archipelago of Kerguelen Islands.

Biology and behavior

Anatomy

The weight of a female varies between . In some subspecies of reindeer, the male is slightly larger; in others, the male can weigh up to . Both sexes grow antlers, which (in the Scandinavian variety) for old males fall off in December, for young males in the early spring, and for females, summer. The antlers typically have two separate groups of points (see image), a lower and upper. Domesticated reindeer are shorter-legged and heavier than their wild counterparts.

Reindeer have specialized noses featuring nasal turbinate bones that dramatically increase the surface area within the nostrils. Incoming cold air is warmed by the animal's body heat before entering the lungs, and water is condensed from the expired air and captured before the deer's breath is exhaled, used to moisten dry incoming air and possibly absorbed into the blood through the mucous membranes.

Reindeer hooves adapt to the season: in the summer, when the tundra is soft and wet, the footpads become spongy and provide extra traction. In the winter, the pads shrink and tighten, exposing the rim of the hoof which cuts into the ice and crusted snow to keep it from slipping. This also enables them to dig down (an activity known as "cratering")"In the winter, the fleshy pads on these toes grow longer and form a tough, hornlike rim. Caribou use these large, sharp-edged hooves to dig through the snow and uncover the lichens that sustain them in winter months. Biologists call this activity "cratering" because of the crater-like cavity the caribou's hooves leave in the snow." All About Caribou. - Project CaribouImage of reindeer cratering in snow. through the snow to their favorite food, a lichen known as reindeer moss. The knees of many species of reindeer are adapted to produce a clicking sound as they walk.

The reindeer coat has two layers of fur, a dense woolly undercoat and longer-haired overcoat consisting of hollow, air-filled hairs.