Named for the Yukon River tributary that drains most of their range, the Porcupine Caribou Herd inhabits northeastern Alaska and the northern Yukon Territory of Canada. According to the last official count, in the summer of 2010, the herd is now composed of about 169,000 individuals, making this one of the most spectacular concentrations of migrant mammals on our planet.
After wintering in mountainous regions south and east of the Brooks Range, the Porcupine Caribou begin their spring migration in March, led by cows and yearlings (bulls and juveniles follow a few weeks later). Using three primary routes across the Brooks Range of Alaska and British Mountains of the northern Yukon, pregnant cows reach the North Slope by late May and calving begins, synchronized to reduce the impact of predation by wolves, grizzlies and golden eagles; the formation of nursery groups and the rapid development of calves (able to run within 24 hours) also serve to diminish loss of the newborns. As the bulls and juveniles arrive, the caribou assemble in huge herds, feeding on the nutritious tundra and moving about to escape hordes of mosquitoes.
By late July, the Porcupine herd begins to leave the vast tundra plain and heads to the northern foothills of the Brooks Range where they now endure biting flies (warble and nose-bot flies) that deposit larvae in their hide and nostrils, respectively. The fall migration ensues by late August and the 2-week rut occurs in October, as frenzied bulls forego eating to gather and impregnate their harem. While some of the herd remains within the Brooks and British Mountains, most head for the Richardson and Ogilvie Mountains of the Yukon Terrority, in the upper reaches of the Porcupine River. Their neighbors, the Central Arctic Caribou herd, numbering about 20,000 individuals, have a much shorter migration, wintering in the Brooks Range and summering on the Coastal Plain. Both herds are threatened by global warming and by human development across their territories, especially the ongoing political pressure to drill on the Arctic NWR.
