Wild · IN60SECONDS

Why do reindeer’s eyes change color in winter?

So they can see in the Arctic’s dim, blue winter light. A reflective layer behind a reindeer’s retina shifts from gold in summer to deep blue in winter, which researchers think helps it catch more light.

Quick Answer

Reindeer eyes change color with the seasons because a reflective layer at the back of the eye, the tapetum lucidum, changes in winter. In summer it reflects gold to turquoise light, and in the dark of winter it reflects deep blue. A 2013 study in Proceedings of the Royal Society B by Karl-Arne Stokkan and colleagues found that the winter blue sends less light back out of the eye and is thought to scatter it so more reaches the retina’s light-sensing cells, which raises sensitivity at the cost of sharpness, possibly a good trade in the dim Arctic winter. A 2022 study by Robert Fosbury and Glen Jeffery of University College London reported that reindeer are the only mammal known to adapt their eyes seasonally to the blue color of the long Arctic twilight, and suggested that changes in pressure inside the eye may shift the spacing of tiny collagen fibers in the tapetum. Scientists are still working out the details.

Key Facts

What changes: The reflective layer behind the retina, called the tapetum lucidum
Summer color: Gold to turquoise
Winter color: Deep blue
Leading explanation: It helps reindeer see in the Arctic’s dim, blue winter light

A reindeer’s eye in summer and winter

	Summer	Winter
Light conditions	Near-constant daylight	Long twilight and darkness
Color the tapetum reflects	Gold to turquoise	Deep blue
What it does with light	Reflects most light straight back through the retina	Reflects less light out, and scatters it so more reaches the retina
The trade-off	A sharper image	More sensitivity, less sharpness

Sources: Stokkan et al. (2013) and Fosbury and Jeffery (2022), both in Proceedings of the Royal Society B. Researchers are still working out the details.

A mirror behind the retina

Many animals have a tapetum lucidum, a reflective layer behind the retina that bounces light back through the light-sensing cells for a second chance to be detected. It’s what makes some animals’ eyes shine in a flashlight beam. In reindeer, which live through months of continuous daylight in summer and months of darkness in winter, this layer changes color with the seasons.

From gold to blue

The 2013 study found that the reindeer’s tapetum reflects gold in summer and deep blue in winter. In winter, less light is reflected back out of the eye and more is scattered inside it, which should raise the retina’s sensitivity at some cost to the sharpness of the image. The researchers proposed that this is an advantage in the dim winter light.

How it might work

The 2022 study by Robert Fosbury and Glen Jeffery proposed that reindeer are the only mammal known to adapt their eyes this way to the blue color of the Arctic twilight. The researchers suggest that pressure inside the eye, raised by long periods with the pupil wide open in the dark, may change the spacing of the collagen fibers that form the reflector. It’s a proposal, and researchers continue to study it.

Myth vs. Fact

MythThe color change is just a trick of the light.

FactIt’s a physical change in the tapetum lucidum, the reflective layer behind the retina, documented in studies of reindeer eyes in summer and winter.

MythReindeer eyes look the same all year.

FactStudies found the tapetum reflects gold to turquoise light in summer and deep blue light in winter.

The Bottom Line

Reindeer eyes change from gold in summer to deep blue in winter because the reflective layer behind the retina shifts. Researchers think it helps them see in the dim, blue light of the Arctic winter.