Why reindeer eyes in winter reflect deep blue

In summer, the reflective layer inside a reindeer eye has a gold-turquoise appearance. But reindeer eyes in winter reflect a striking deep blue. It is not the iris that changes colour. The transformation takes place in a biological mirror behind the retina.

This structure is the tapetum lucidum. Many animals that function in dim light have one, which is why a cat’s or dog’s eyes can appear to glow at night. Light that passes through the retina is reflected back through it, giving photoreceptors another opportunity to capture photons.

Reindeer take this familiar low-light adaptation one step further. Their tapetum changes its optical properties with the seasons.

Gold in summer, blue in winter

The Arctic light cycle is unlike the ordinary rhythm of day and night at lower latitudes. Summer can bring weeks of almost continuous daylight. Winter brings polar night, when much of the available illumination comes from prolonged blue twilight.

The spectrum of that twilight is unusual as well. When the Sun lies below the horizon, sunlight travels through a long path in the atmosphere. Ozone removes much of the yellow-orange and red part of the spectrum, while atmospheric scattering further favours shorter wavelengths. The result can be hours of intensely blue-violet ambient light.

The reindeer’s winter reflector is remarkably well matched to this environment.

Earlier measurements showed that the summer tapetum has a reflectance maximum around 541 nanometres, in the green-yellow region of the spectrum. In winter, the peak moves toward approximately 444 nanometres — deep into the blue.

The winter tapetum reflects less light directly back out of the eye, but more light appears to scatter within the retina. Previous physiological measurements found substantially greater retinal responses in winter animals. The likely benefit is increased sensitivity in darkness, although it comes with some loss of visual acuity.

The deeper question was how a biological mirror could physically switch colour.

A photonic material inside the eye

In a 2022 study, Robert Fosbury and Glen Jeffery investigated the physics behind the seasonal transformation. The reindeer tapetum is built from highly organized collagen fibres surrounded by fluid.

At this scale, colour does not have to come from a pigment. It can emerge from the physical arrangement of microscopic structures. This is known as structural colour: particular spacings between components selectively reinforce some wavelengths of reflected light while weakening others.

The collagen fibres in the reindeer tapetum behave as a two-dimensional photonic crystal. Change the distances and order among the fibres, and the wavelengths reflected by the structure change as well.

During summer, the fibres are more widely spaced and less tightly ordered. The reflector therefore covers a broader range of wavelengths and appears gold or turquoise.

In winter, the fibres move closer together and become more regularly packed. That changes the optical properties of the structure and shifts its reflection toward shorter wavelengths, producing the deep blue appearance.

Fosbury and Jeffery tested this model by slowly drying samples of summer and winter tapetum while continuously measuring their reflectance. As fluid was lost and the collagen fibres moved closer together, the reflected spectrum shifted toward shorter wavelengths. The experiment supported the idea that seasonal changes in interstitial fluid and fibre packing can tune the colour of the reflector.

Exactly what drives the seasonal transformation inside a living reindeer is not yet fully settled. One proposed mechanism involves changes in pressure within the eye. During prolonged winter darkness, the pupil remains widely dilated for long periods, which may alter fluid movement and help compress the tapetal structure.

An eye that retunes its own mirror

The remarkable part is not simply that the tissue changes colour.

The winter tapetum is spectrally well matched to the blue light that dominates Arctic twilight. The authors propose that this tuning helps reindeer use scarce winter photons more effectively and may improve visual contrast when light levels are extremely low.

That adaptive interpretation is plausible, but the ecological advantage cannot be measured directly from an experiment on isolated eye tissue. Exactly how much the change improves the ability to find food or detect predators in the wild remains harder to establish.

Reindeer are currently the only mammals known to show such a pronounced seasonal restructuring of this reflective part of the eye.

Their visual system therefore does something unusually physical. In bright summer conditions, one optical configuration is useful. As polar night arrives, the same eye reorganizes the nanoscale structure of its internal mirror.

For a reindeer, winter does not only change the landscape. It changes the optics inside the eye.