Reindeer may have a unique way of coping with the perpetual darkness of Arctic winters: During that season, their eyes become far more sensitive to light. Like many vertebrates and most mammals, especially those that are nocturnal, reindeer (Rangifer tarandus, main image) have a light-reflecting layer of collagen-containing tissue behind the retinas of their eyes. This structure, called the tapetum lucidum (Latin for “bright tapestry”), gives the eye’s light-sensitive neurons a second chance to detect scarce photons in low-light conditions. (The layer also produces the “eyeshine” that can make animal eyes appear to glow in the dark.) During sunny months, reindeer have yellow eyeshine (upper right). But in the wintertime, light reflected from the tapetum lucidum takes on a decidedly bluish sheen (lower right)—a seasonal shift that hasn’t been noted in other mammals, the researchers say. To study this unusual color change, the researchers brought some disembodied reindeer eyeballs into the lab and placed small weights on them. When under pressure, the eyeballs changed the color of eyeshine almost immediately. That fits with what happens in the wild over the course of seasons, the researchers say. In winter, reindeer pupils are constantly dilated, which increases fluid pressure. That, in turn, decreases the spacing of collagen fibers in the tapetum lucidum, further increasing the scattering of light within the eye and shifting the reflected light toward the lower wavelengths of light which are predominant at dusk. These changes make the reindeer’s eyes between 100 and 1000 times more light-sensitive, the researchers report today in the Proceedings of the Royal Society B. Although this decreases the creature’s sharpness of vision, it’s a tradeoff that, on the whole, probably boosts reindeer survival by helping them better detect predators in the dark, the researchers contend.