Within a range almost twice the size of New England, the Porcupine Caribou circulate in an annual rhythm set by the ice ages. Preferring the tundra, the caribou stay north of the treeline until fall snowstorms force them southward to winter ranges in eastern Alaska and the Yukon. However, as soon as possible they begin to drift northward again – the cows in particular – and by May the spring migration towards the calving grounds is in full swing.
Spring migration and calving
Because weather has such a strong influence on caribou migration, each year's pattern of movements is unique, with one exception – the calving grounds. No matter where caribou have spent the rest of the year, they unfailingly head for the same calving area each spring unless snow conditions are so bad they must calve elsewhere. Even in that case, they will still complete their migration to the calving and post-calving areas in the '1002' section of the Arctic National Wildlife Refuge on Alaska's North Slope. There, the food is best for nursing cows, predators like wolves and grizzlies are relatively scarce, and there is some sanctuary from the biting flies which descend on them in July.
Faithfulness to this calving area is what defines the Porcupine Caribou as a herd and has been the cornerstone of its survival for thousands of years. Eviction from the calving grounds for whatever reason would throw the herd into chaos, from which it may never recover. Research indicates that in years when weather conditions prevent the herd from calving in the 1002 area, fewer calves tend to survive.
Fall migration and the rut
In the fall, the herd moves from the summer ranges in the northern Yukon and Alaska in a southerly direction to the winter ranges. In late September and October, the bulls begin to undergo physical changes. Their necks begin to swell, they almost completely stop eating. By the peak of the rut in late October or early November, they begin to stink, and hunters avoid killing them because their meat is lean, stringy, and inedible. If hunters must harvest during this period, the PCMB recommends they select young bulls that are not rutting instead of taking cow caribou during this time.
Rutting behaviour begins in September. Bulls become restless and aggressive. They fight with other bulls in an attempt to prove their dominance. As the bulls approach the rutting season, the fighting becomes increasingly violent, and sometimes bulls can be severely wounded or even killed. There is a high mortality rate for bulls because after the exertion of the rut, the weaker ones are not physically prepared to meet the harsh winter and many die.
The winter is a critical time period for caribou as they have to deal with low temperatures and continuous snow cover.
"Whatever happens in the winter affects the calf survival, affects the pregnancy the next year and virtually every aspect of productivity from age of first reproduction to calf survival."
Quote from Don Russell, Canadian Wildlife Service, from 2000 presentation: Porcupine Caribou Habitat and Oil and Gas Development in the North Yukon.
Caribou feed on lichens under the snow and tend to occupy areas with favourable long-term snow conditions. In the Yukon, the winter range is situated in the snow shadow region of the Ogilvie and Hart River basins. Pacific storms from the south and southwest deposit most of the moisture on the south slopes of the Ogilvie and Wernecke Mountains. Storms from the Bering Sea do not affect these headwater basins. The Richardson Mountains winter region has less snowfall, as it is affected by high winds that redistribute the snow.
Although caribou use different parts of the wintering range each year, the portion of the wintering range that the caribou do use is of vital importance for the caribou for that year.