The Arctic winter doesn't simply get dark. For weeks in the far north, the Sun can remain below the horizon, leaving animals to navigate a world dominated by prolonged twilight and dramatically reduced light. Reindeer ( Rangifer tarandus ) have an extraordinary way of dealing with this seasonal transformation: part of their eyes changes color. During summer, a reflective layer behind the retina appears mostly golden. In winter, it becomes deep blue. Researchers have traced this transformation to microscopic changes in collagen inside the tapetum lucidum, a structure that acts rather like a biological reflector. Far from being cosmetic, the seasonal switch appears to alter how light behaves inside the eye and may help reindeer see during the Arctic's darkest months.
