Why Do Reindeer Eyes Change Color? Gold to Blue in Winter

What actually changes color?

The part that changes is not the iris, the colored ring you see from the front, but the tapetum lucidum, Latin for “bright carpet.” It is a mirror-like layer behind the retina found in many mammals, including cats, dogs, horses and deer. Light that passes through the retina without being absorbed is reflected back through it, giving the light-sensitive cells a second chance to catch it. This is why many animals’ eyes glow in car headlights or camera flashes. People do not have one.

In most animals the tapetum keeps the same color all year. In Arctic reindeer, known as caribou in North America, it is different:

The 2013 “shifting mirrors” study

The color change was described in a 2013 paper in the Proceedings of the Royal Society B by researchers from the University of Tromsø in Norway and University College London, including Karl-Arne Stokkan and Glen Jeffery. They examined the eyes of reindeer from northern Norway, where the sun does not rise for weeks in midwinter and does not set for weeks in midsummer.

They found that the tapetum was golden in summer animals and deep blue in winter animals. Measurements showed that the retinas of winter reindeer were significantly more sensitive to light than those of summer animals. The researchers described it as the first known example of a mammal’s retina changing its structure with the seasons.

How the color change works

The tapetum of reindeer is made of fine collagen fibers arranged in regular layers. The color it reflects depends on how closely these fibers are packed, in the same way that the spacing of tiny structures gives butterfly wings or soap bubbles their colors. Wider spacing reflects longer wavelengths such as gold; tighter spacing reflects shorter wavelengths such as blue.

The study found that in winter the fibers are packed more tightly. The likely cause is pressure. In the dark winter months, reindeer pupils stay widely dilated almost all the time to let in as much light as possible. This appears to block the normal drainage of fluid from the eye, which raises the pressure inside the eyeball and squeezes the collagen layers closer together. In spring, as days lengthen and the pupils contract again, the pressure falls and the tapetum gradually returns to gold. The change happens slowly over weeks to months rather than overnight.

Why blue helps in the dark

A golden tapetum acts like a flat mirror, bouncing much of the light straight back out of the eye. A blue tapetum scatters more light sideways within the retina, so light passes through more photoreceptor cells before it escapes. More light is captured, which makes the eye more sensitive in dim conditions.

The cost is sharpness. Scattered light blurs the image a little, so winter vision is less detailed. For a reindeer, the trade is worth it. Spotting the movement of a gray wolf or other predator in the blue twilight of an Arctic winter matters more than seeing fine detail. The gray wolf is the most important natural predator of adult reindeer.

Reindeer can also see ultraviolet light

A 2011 study by many of the same researchers found another winter advantage. Unlike most mammals, reindeer have a cornea and lens that let ultraviolet (UV) light through, and their retina responds to wavelengths down to about 320 nanometers. Humans see only from about 400 nanometers upward.

The Arctic is rich in UV light, especially when snow reflects it. Snow reflects UV strongly, while many objects absorb it and stand out as dark shapes. The researchers suggested that this may help reindeer pick out:

• Lichens, their main winter food, which absorb UV and appear dark against the snow. A 2023 study of reindeer in Scotland’s Cairngorms supported this idea.
• Urine, which may signal the presence of other animals.
• Predators, such as wolves, whose fur may contrast with UV-bright snow.

It has also been proposed that reindeer avoid some power lines because the lines give off faint flashes of UV light that reindeer can see, though this idea is still being studied.

Do only Arctic reindeer do this?

The color change has been documented in reindeer living far north, where the seasonal change in light is most extreme. How strongly it occurs in reindeer and caribou farther south, where winter days are longer, is less well studied. No other mammal is currently known to change the color of its tapetum with the seasons.

The eyes are one of many cold-weather adaptations in this species, alongside hollow-haired fur, heat-saving noses and hooves that change with the seasons. To learn more about how reindeer survive winter, see what do reindeer eat?