Reindeer (caribou) living above the 70th parallel adapt to prolonged Arctic winters by seasonally changing their tapetum lucidum, a reflective layer behind the retina. A 2022 study compared eyes from two winter-killed and two summer-killed reindeer and found structural differences consistent with fluid movement and denser collagen packing in winter. This change increases reflection of blue twilight, giving photons another chance to reach the retina and improving low-light vision. The adaptation appears unique among mammals but may exist in other polar species.
How Reindeer Turn Their Eyes Into 'Night Vision' During Arctic Winters
The Weird Thing That Happens to Reindeer Eyes During Winter
The reindeer, also called the caribou (Rangifer tarandus), is a real Arctic mammal found across northern North America, Northern Europe and Siberia. Although folklore often places them at the North Pole, their true marvel is biological: during long polar winters reindeer effectively switch their eyes into a low-light mode that improves vision in near-dark conditions.
Why This Matters
Reindeer inhabit circumpolar regions above roughly the 70th parallel. Because of the Earth’s axial tilt, those latitudes experience prolonged periods of deep blue twilight — roughly eight to 11 hours a day from September through April — and in some places the Sun remains below the horizon for months. Surviving in that dim environment requires excellent low-light vision for finding food and avoiding predators.
How Their Eyes Change
The effect depends on a reflective layer behind the retina called the tapetum lucidum. This structure, familiar to anyone who’s seen a cat’s eyes shine in flashlight beam, acts as a retroreflector: it bounces incoming light back through the retina to increase photon capture. Uniquely among mammals, reindeer can alter the optical properties of their tapetum lucidum seasonally.
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The 2022 Study
In 2022 scientists tested a hypothesis that seasonal reshaping of the tapetum is driven by movement of interstitial fluid inside the eye. Researchers collected eyes from recently deceased reindeer at a slaughterhouse — two killed in winter and two in summer — and fixed the samples in 4% paraformaldehyde in phosphate buffer before dissecting them to examine tapetum structure.
Proposed Mechanism
While the exact mechanism is not yet proven, one plausible explanation starts with sustained pupil dilation in low light. Prolonged dilation could raise intraocular pressure and displace fluid out of the tapetum lucidum. As fluid leaves, collagen fibers in the layer pack more tightly, altering the reflector’s geometry and shifting its spectral sensitivity so it reflects more blue light — the dominant wavelength of Arctic twilight. That gives photons a second chance to be absorbed by photoreceptors and produces a brighter, clearer image in dim conditions.
Broader Implications
To date, reindeer are the only known mammals shown to make this seasonal optical change, but researchers suggest that other high-latitude species exposed to extended twilight or polar night could have evolved similar strategies. The finding also underscores how specialized sensory adaptations can be in extreme environments.
Whether or not any reindeer are flying, their seasonal eye transformation is an impressive example of nature’s ingenuity for seeing in the dark.
