Reindeer eyes change from gold in summer to blue in winter

Behind the retina, many mammals have a reflective layer called the tapetum lucidum. It's the reason animal eyes glow in your headlights.

In Arctic reindeer, that layer changes with the seasons. A 2013 study led by Karl-Arne Stokkan and Glen Jeffery found it is golden in summer and turns deep blue during the dark Arctic winter.

The blue version scatters light across more of the retina, which makes the eye far more sensitive in dim light, at the cost of some sharpness. The researchers think pressure inside the eye squeezes collagen fibers closer together and shifts the color toward blue, though that part is still being studied.

Bonus fact: reindeer are the only deer species in which females regularly grow antlers too.

Wear the Reindeer Christmas

A shaggy reindeer hauls a little red sled piled with a fresh-cut fir and wrapped gifts through a snowy birch forest at a rosy-gold sunset.

Get the shirt on EtsySee the design