Two dissected reindeer eyes, showing the tapetum lucidum. The left one comes form an animal killed in winter; the right one, in summer. The bit that actually changes colour is the tapetum lucidum or “cat’s eye”—a mirrored layer that sits behind the retina. It helps animals to see in dim conditions by reflecting any light that passes through the retina back onto it. In dark conditions, muscles in your irises contract to dilate your pupils and allow more light into your eyes. When it’s bright again, the irises widen and the pupils shrink. The same thing happens in reindeer, but the interminable Arctic winter forces their pupils dilate for months rather than hours.