Reindeer eyes turn from gold to deep blue for the Arctic winter
Shine a torch at a cat at night and its eyes blaze back, thanks to a mirror behind the retina called the tapetum lucidum. Reindeer take the trick further: each winter theirs shifts colour from gold-turquoise to deep blue, tuning itself to the long blue twilight of the Arctic.
The tapetum lucidum, Latin for bright tapestry, sits just behind the retina and bounces light back through it, giving the light-sensing cells a second chance to catch each photon at the cost of slightly blurring the image. Because it returns light along the same path it arrived, the reflection lines up with the original picture and preserves contrast. In cats it lowers the dimmest light the animal can detect without really altering which colours it sees. Humans, like other daytime haplorhine primates, lack one; our red-eye in flash photos is a weak reflection from the choroid instead.
Evolution has built these mirrors from surprisingly varied materials. Crocodiles use guanine, marsupials lipid spheres, dogs and ferrets zinc, cats riboflavin plus zinc, and lemurs riboflavin alone, while cows, sheep and horses use arrays of collagen fibres. A cat's tapetum has 15 to 20 cell layers and reflects nearly 130 times as much as a human eye, a brilliance that led ancient Egyptians to think cats' eyes held the sun at night. In dogs, some drugs strip zinc from the tapetal cells and weaken night vision.
The reindeer mechanism was worked out with reflectance spectroscopy. Their tapetum is a crystal-like lattice of parallel collagen fibrils, and seasonal changes in the fluid between those fibrils alter their spacing and orderliness. That shifts the reflected colour toward the blue that dominates polar twilight, helping reindeer spot predators in winter gloom, though at the price of blurrier sight. They are the only mammal known to adapt their eyes this way.
The idea turns up far beyond mammals. Owls, kiwi, the kakapo and nightjars have one; most spiders carry reflective layers in their smaller side eyes; and transparent mantis shrimp larvae use eyeshine as camouflage against bright water. The biologist Nathan H. Lents has proposed that vertebrates evolved the mirror to offset their backward-facing photoreceptors, a flaw the otherwise similar squid eye avoids.
Source: Tapetum lucidum
