Have you ever wondered how animals survive the bitter harshness and perpetual darkness of an Arctic winter?
I remember someone once asking me how a caribou can find its food in the dark. It turns out that they have eyes like cats that glow in the dark. A layer of cells at the back of the eye behind the retina, known as the tapetum lucidum, reflects light back through the retina and helps animals see in the dark.
However, even more interesting is the fact that caribou (reindeer) can change the tapetum lucidum to reflect different wavelengths of light going back through the retina. During summer the tapetum lucidum is golden, so the eyes appear golden. After the sun sets for winter, it changes to deep blue as less light is reflected out of the eye possibly due to increased pressure as the pupil dilates to let in as much light as possible.
It is thought that the increased pressure might compress the tapetum lucidum and reduce the space between the cells, thus causing shorter wavelengths of light to be reflected back through the retina.
Not only is this useful for the caribou to find food, but it also might provide an advantage in avoiding predators in the dark.
Caribou also have a strong sense of smell and can locate food, for example, ground lichens, beneath snow. Musk oxen do the same. Polar bears and wolves also use smell to locate food and use light from the stars and moon to hunt.
There are many other adaptations that animals use to survive the long cold dark of the Arctic. The options are to leave the area (migrate), wear a thick coat and stick it out, or live underneath the snow and remain active or hibernate.