Mirror, mirror, on the wall…

Has this happened to you? In the darkness, on a lonely country road, your headlights play across a pair of bright green, almost iridescent, spots of light; they seem to float eerily across the road without support. These furtive spots seem to move and blink, clearly alive. Eyeshine. It is all you see of the creature, as the rest of the body seems to disappear into the darkness that surrounds it. But, what exactly is eyeshine, and why did it evolve?

The tapetum lucidum (Latin, carpet shining) is a reflective structure found in the eyes of many diverse creatures and represents convergent ocular evolution solely for maximising photon capture. Surprisingly, the techniques for the production of these reflective mechanisms are variable, and much like the crystalline lens, seem to be drawn from whatever materials the evolutionary process found at hand.

Guanine crystals, for example, provide biological reflection and support camouflage by making a fish glisten and gleam, and hence appear invisible or at least present confusing reflections to a predator. Although we can never know for sure, this coating probably appeared very …