An Arctic reindeer has three eyelids on each eye.
Like other reindeer, it has two ordinary eyelids plus a third, translucent membrane called the nictitating membrane. This extra eyelid can sweep across the eye, helping protect it from snow, wind, dust, and intense glare from bright winter landscapes. It also helps keep the eye moist.
Reindeer are adapted to dramatic seasonal changes in Arctic light. In summer, their eyes are adapted for bright conditions; in winter, a reflective layer behind the retina changes colour and improves sensitivity in dim light. This unusual seasonal change helps reindeer see during both the bright polar summer and the dark winter.
Reindeer are called caribou in North America, although the names can refer to different populations or usage traditions. They are the only deer species in which both males and females naturally grow antlers, a fact separate from their specialised eye structure.