The Caribou

Description: The Porcupine caribou are barren ground caribou, Rangifer tarandus-granti and are well adapted to their northern environment. They have short, stalky bodies to conserve heat, long legs for movement through the snow, and a thick winter coat for insulation.

Their unique hooves are large, broad and concave with sharp edges to support them and provide traction in the snow and ice. The hooves also function as scoops for digging craters in the snow to uncover lichens and other plants. Caribou are also excellent swimmers due to their hooves functioning as paddles.

Caribou are the only ungulate where both males and females grow antlers. This is probably so that females can defend their winter-feeding craters.

Diet: During the winter, caribou primarily feed on lichens which is a unique characteristic among large mammals that enables them to survive in the harsh northern climate. The caribou have an excellent sense of smell to locate these lichens underneath the snow. These lichens provide a good source of energy for the caribou but they are not a good source of protein. Therefore, in the spring and summer caribou feed on other plants and mushrooms.

The Herd

The Porcupine Caribou herd is a large population of approximately 123,000 caribou that have been named after the major river that flows through its range. The range of the herd covers about 290,000 sq. km and the survival of the herd depends on annual migrations between their winter range and calving grounds (approximately 900km per year). During the year, the caribou go through seven stages in their life cycle, some of which include long migrations.

Spring Migration: The spring migration begins in early March as the caribou make their way to the northern limits of their wintering grounds to their spring calving grounds. The pregnant cows lead the migration, followed by juveniles, bulls, and non-pregnant cows a few weeks later. This is the worst time for the caribou as lichens are scarce above treeline and new plant growth has not yet begun.

Calving and Summer: Calving takes place during the end of May and the beginning of June. Calving usually occurs within the 10-02 area of the Arctic National Wildlife Refuge on Alaska's North Slope. The synchronization of calving (80% born within a week of June 2nd) is probably an adaptation to reduce predation - there are more calves than the predators can kill. This is a critical period for calf growth and survival. Half of calf deaths occur during the month of June.

Post Calving Aggregation: In late June and early July, mosquitoes emerge to harass the herd. This harassment disrupts the herd by reducing their feeding time and increasing their time spent running and walking. As a result, the caribou gather into enormous polt-calving aggregations that number in the tens of thousands to reduce the impact of the mosquitoes on individual caribou.

Dispersal: The caribou disperse once the mosquitoes have declined in numbers. However, they are then plagued by warble and nose-bot flies. These cannot be avoided by moving or by the wind. The caribou must hold their heads down low, violently shake their head, stamp their feet, and race across the terrain.

Fall Migration: The fall migration occurs in late August to mid October as caribou move southwards into Alaska and the Yukon. This is a time when caribou put on a lot of fat reserves and males become increasingly aggressive. Fall is also the time when most of the harvesting of the herd takes place, which amounts to about 2-3% of the herd.

Rutting: This is a two week period in the middle of October when bulls fast and breed. This occurs during the migration to the wintering grounds.

Winter: The herd tends to spend their winters in northeastern Alaska and in the Peel and Porcupine River watershed of the Yukon. Caribou tend to lose weight during this period as they must sniff and dig out their food from under the snow. Unlike other times of the year, the caribou do not move around much during the winter.