September 7 – Light Of My Life

Today’s factismal: Many experts can identify animals by the color of their “eyeshine”, the light reflected by their eyes.

Shine a light outside at night and you are almost certain to see something startling: two glowing dots staring back at you. Even when the light is too faint to reveal the animal’s shape, you can still see those eerie glowing dots. But why should an animal’s eyes shine in the dark? And why can some people tell which animal it is just from the color of the reflection?

Eyeshine from a zebra (Image courtesy Instant Wild)

Eyeshine from a zebra
(Image courtesy Instant Wild)

That reflected light is known as eyeshine, and not all animals have it; for example, humans don’t have eyeshine, nor do squirrels. In general, most diurnal animals don’t have eyeshine and most nocturnal animals do (though exceptions abound on both sides). And that is the reason for the eyeshine: it is caused by a reflecting layer known as the tapetum lucidum (“bright tapestry” in geek-latin). That layer bounces light that passes through the retina back for another pass, and increases the eye’s sensitivity in low light situations (i.e., at night). But, evolution being what it is, the tapetum lucidum isn’t the same in all nocturnal animals. In some animals, it is made up of a layer of guanine crystals. In others, it is parallel fibers. And in yet others, it is a mish-mash of different structures. And that’s why different species have different colors of eyeshine; the tapetum lucidum reflects light back, but the color of the light that gets reflected back depends on the type of the tapetum, just as the color reflected off of a wall depends on the color of the wall. As a result, owls and other birds have red eyeshine, as do foxes and rabbits. But cats and frogs have a greenish eyeshine, and raccoons have yellow eyeshine.

Eyeshine from coyotes (Image courtesy Instant Wild)

Eyeshine from coyotes
(Image courtesy Instant Wild)

Now this is more than just a fun way to identify animals at night; it is also a way to identify animals in night-time cameras, like those at Instant Wild. They’ve got a collection of photos taken by camera traps across the globe, and they need your help in identifying the animals. To pitch in (and see some great examples of eyeshine), set your browser to:

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