The Reason Why Reindeer Eyes Change Color

It may sound unbelievable, but reindeer eyes change from a blazing gold in the summer to a deep, dark blue in the winter. However, this isn’t the kind of color change you might expect, as it usually can’t be seen in most photographs. So, what exactly is happening? Read on to discover why reindeer eyes change color and how this adaptation benefits them in the wild.

Turning Their Brown Eyes Blue

When we hear that reindeer eyes change color, most of us think of the iris—the colored part of the eye. While reindeer typically have reddish-brown to brown irises, this part of the eye does not change color. The part that changes color is called the tapetum lucidum. The tapetum lucidum is a reflective membrane located behind the retina. It reflects visible light that has already passed through the retina, sending it back through the retina a second time. This gives the light-sensitive cells in the retina, called photoreceptors, another chance to detect the light.

Humans do not have a tapetum lucidum, but it is a common adaptation in dogs, cats, raccoons, and other nocturnal animals. It helps animals see better in the dark and in low-light conditions. When light from the flash of a phone camera, flashlight, or other light source passes through the eyes of an animal with a tapetum lucidum, it makes their eyes appear to glow. This is because the light is being reflected back at us. While the eyes of other animals may glow gold like reindeer eyes in the summer, scientists have not yet found any other animal whose tapetum lucidum changes color. So why does the tapetum lucidum in reindeer turn blue?

Changing Colors with the Changing Seasons

Not all reindeer eyes change color. To date, the phenomenon has only been observed in Arctic reindeer. This is due to the extreme environmental changes they experience at such high latitudes. Despite the stories, reindeer don’t actually live at the North Pole. However, Arctic reindeer do live in the forests and tundra surrounding it. The closer you get to the poles, the longer the periods of continuous daylight and darkness become. So, reindeer eyes change color to adapt to the amount of light they receive in the summer versus the winter.

In the summer, reindeer are exposed to extended sunlight. So, the gold tapetum lucidum reflects most of the light back out of their eyes. However, in the winter, when the sun remains below the horizon for months on end, the blue tapetum lucidum reflects less light out of their eyes. Researchers theorize that the blue reflection may scatter more light through the photoreceptors, instead of directly reflecting it. This allows the retinas to capture more light during the winter, as well as during the extended twilight of spring and fall.

How This Adaptation Benefits Reindeer in the Wild

Researchers believe the reindeer’s corneas and lenses do not block all ultraviolet light, extending their visual range into the ultraviolet. In the Arctic, the shorter wavelengths of UV light reflect off the snow and ice. This extended vision allows reindeer to make the most of the limited, but UV-rich, light available in dark and near-dark conditions.

Reindeer achieve this by altering the wavelengths of light reflected by the tapetum lucidum. The reindeer’s pupils remain dilated for extended periods, causing swelling in the eyes and increasing pressure on the tapetum lucidum. In reindeer, the tapetum lucidum layer is primarily made of collagen fibers. As pressure builds and fluid between the collagen fibers is squeezed out, the fibers become more tightly packed. Normal spacing between the fibers reflects the golden yellow wavelengths, while tighter spacing reflects blue.

Although the increase in sensitivity to UV light is offset by reduced visual acuity, researchers believe this tradeoff is important. Lichens, a major source of food for Arctic reindeer, absorb UV light, making them appear dark against the snow, which reflects UV light. The fur of predators such as wolves and lynxes also absorbs UV light, creating a contrast with the snow that helps reindeer more quickly recognize and respond to threats.