This is how Santa’s reindeer find all those houses! Better put up your lights ✨
Reindeer’s eyes are golden in the summer, but turn a deep blue in the winter to capture more light – making them the only mammal that does this.
Scientists investigated a reflective layer behind the retina of the reindeer’s eyes. The retina, located on the back of the eyeball, contains the eye’s light-sensitive cells.
The color of the light reflected by reindeer eyes is related to the spacing of collagen fibers in the reflective layer, technically known as the tapetum lucidum. Reindeer apparently increase pressure inside the eyeball during the winter to compress these fibers together, and reducing the spacing between these fibers makes the eyes reflect bluer light.
The reflective layer bounces light back through the eye. In the summer, it turns golden in many reindeer, reflecting most light through the retina, helping the reindeer deal with the nearly continuous Arctic summer daylight. However, in the winter, it turns deep blue, reflecting far less light out the eye, helping the reindeer cope with the nearly continuous Arctic winter darkness.
Moreover, the shift to blue may also scatter light through larger numbers of light-sensitive eye cells rather than directly reflecting it out from the eye through fewer numbers of these cells. As such, their eyes end up capturing more light in the winter when they need it most, with a trade-off of less visual sharpness.
These findings are the first known instance of mammal retinas altering their structure due to seasonal changes in environmental light.