Caribou, or reindeer, migrate north every summer to feed, on paths used for generations. Even summer temperatures are freezing where the caribou makes its home, but it is well prepared.
Its large hooves serve as brilliant snow-shoes, as paddles for crossing rivers and as snowploughs to scoop away snow when foraging. A foot joint clicks when it walks, keeping herds together when visibility is poor, and its coat has a warm underlayer and an outer, insulating one. To cope with the dark winter months, the backs of their eyeballs change from gold in the summer to blue in the winter, reflecting less light away from the eye, and the network of blood vessels in its velvety nose warms the air before it reaches its lungs.
A migrating herd may number thousands, a huge mass moving across the land. With global warming, longer summers mean more grass; the herd packs on the pounds and the females carry more than one calf. But then warmer winters bring rain rather than snow, which freezes and forms a barrier to the juicy grasses and plants below. The females cannot eat enough and many babies do not survive; those that do are much smaller, and would find it hard to pull Father Christmas’s sleigh.