All animals, including humans, can adapt their eyes to the changing level of light. In dark conditions, muscles in the irises contract to dilate the pupils and allow more light into the eyes. When it’s bright again, the irises widen and the pupils shrink. The same thing happens to reindeers. But when the arctic winter brings perpetual darkness for months on end, something more happens to reindeer’s eyes causing them to change color.

The arctic reindeer is the only mammal whose eyes shine a different color depending on the season. In summer, when the sun is bright, the eyes are dazzling golden, but as the sun prepares to set for the winter, the eyes become less reflective until it turns dark blue in the dark winter months.

Two dissected reindeer eyes. The left one comes from an animal killed in winter. The right one was killed in summer. Photo credit: Glen Jeffery