Reindeer eyes change color in winter — from gold to blue

In summer, reindeer eyes have a golden-orange sheen. In winter, under conditions of near-constant polar darkness, that same sheen turns deep blue.

It comes down to a layer called the tapetum lucidum — a structure behind the retina that reflects light back through the photoreceptors, giving the animal better night vision (it's also what makes a cat's eyes "glow" in flashlight beams).

In winter, in response to months of darkness, this layer physically compresses at the microscopic level, which changes the wavelength of light it reflects — hence the shift from orange to blue.

The mechanism: the blue variant reflects less light but scatters it more broadly, trading sharpness for sensitivity — a useful trade-off when you need to spot a predator in near-zero light.