The calves' precociousness is an obvious advantage where cover is sparse and predators common. Many cows with calves assemble to form small nursery bands, and these groups move slowly through the calving grounds, where the cows graze, favoring new-growth cottongrass shoots. As the mosquitoes emerge in late June and early July, the caribou gather into enormous post-calving aggregations, sometimes numbering in the tens of thousands. For example, in 1987, over 93,000 caribou assembled in one group south of Camden Bay. The caribou seek areas where breezes and cooler temperatures reduce the harassment by mosquitoes, and when there is no wind, the caribou move continually.
Cold winds offer relief from the mosquitoes and permit the caribou to rest and feed freely.
Although some of the Porcupine Caribou occasionally remain on the North Slope for the winter, they usually travel south and east to Canada. When they do stay on the North Slope, the Porcupine Caribou usually move westward from the 1002 area and mingle with caribou from the Central Arctic Herd. The warble fly, which looks like a small yellow and black bumblebee, lays its eggs in the fur of the legs or abdomen of the caribou. The larvae soon hatch, burrow under the skin, and travel to the back. Here they encapsulate and cut a breathing hole in the skin.