Picture ancestral humans huddled around a fire, peering beyond the edge of the firelight into a nighttime world dotted with the golden eyes of other animals. Welcome to the gleaming world of eyeshine, where wildlife, or perhaps your dog caught in the beam of a flashlight on the last walk before bed, stares at you with glowing eyes.
A buck showing eyeshine in a meadow at night
A second chance for sight
Foxes romp in the snow
In a diurnal animal’s eye, like yours and mine, light enters the pupil and focuses on the light-sensitive retina at the back of the eye. Some light is detected, and some passes through the retina. In our diurnal eyes, this pass-through light goes to waste, while in a deer’s or dog’s eye, this light is reflected back through the retina by a mirror-like layer at the back of the eye called the tapetum lucidum,4 Latin for “shining tapestry.” Essentially, eyeshine gives the retina a second chance to detect light, which may be crucial for nocturnal and crepuscular animals in dim light. Every little boost can make the difference in being able to catch prey if you are a predator or avoid being eaten if you are not. There must be some light present, or there will be no reflected light to help. In other words, animals with tapeta lucida didn’t evolve the superpower of vision in total darkness.5
“Come into my parlor,” said the spider to the fly6
There's eyeshine, and then there's eyeshine
To a casual observer, the eyes of animals can look very different (think of a dragonfly versus a cat). If we unweave the shining tapestry to look closer, we see that the underlying “fabric” is very diverse.
The tapeta lucida of whales, horses, and deer have sheets of reflecting, parallel fibers. Carnivores like cats have arrays of reflecting vitamin B crystals. Sharks and spiders use crystals of the chemical guanine. Whatever the mechanism, the result is the same – a reflecting structure that enhances visual sensitivity.
What about Uncle Fred's red-eye?
All that glitters is not eyeshine
Strictly speaking, eyeshine can only be said to arise from a tapetum lucidum if there is a tapetum in the eye. That may seem obvious; however, some animals exhibit eyeshine for whom the jury is still out on the actual mechanism involved. Various sources state that amphibians do or do not have tapeta, yet it is clear that many have eyeshine. You can demonstrate this for yourself by going to a pond inhabited by frogs in the late spring or summer when they are active and shining a flashlight along the shore. You will be rewarded with the eyeshine from frogs like wood frogs and bullfrogs.
Eyeshine from a bullfrog and a wood frog
Breaking the nocturnal rule
Some primarily diurnal animals like crab spiders dip their toes into nighttime predation when nocturnal prey – for example, moths in late summer – become particularly abundant. Not surprisingly, crab spiders also have tapeta lucida, presumably to increase the odds of their hunting success. Nevertheless, there is a trade-off between increased sensitivity in dim light and decreased sharp vision in daylight that the tapetum would cause. There’s no free lunch in nature!
Eyeshine is like a box of crayons
In the dark, animal eyes have specific wavelengths (colors) they are most sensitive to, often blue-green. One might expect that the color of the eyeshine would match the color the eye is most sensitive to; after all, the point of the eyeshine is to maximize dim-light sensitivity. However, eyeshine colors in different species can range from blue to red, and it isn’t likely that these colors match the eyes’ dim-light sensitivities in all cases. The explanation for this apparent discrepancy awaits further study. Suggestions are welcome in the comment section below!
Even within a single species, eyeshine color can vary. The tapetal reflections in dogs range from blue-green to green to yellow-green to yellow and orange, with nearly 80% recorded as yellow-green or orange.10 There is some correlation between a dog’s coat and eyeshine color. For example, a dog with a red coat will likely have orange eyeshine while a dog with a white coat will probably have yellow-green eyeshine.
Herschel, Poppy, and Pippin
The next time you are out at night with your flashlight and see the reflecting eyes of a deer, fox, moth, or spider, realize that you are seeing the glowing products of a remarkable convergence that has wended its way to the present for millions of years.
1 The red-eye seen in a flash photograph of Uncle Fred is a different phenomenon and will be discussed below.
2 Crepuscular animals are primarily active at dawn or dusk.
3 The eyes don’t really “glow.” They reflect light from a light source like a car’s headlights or a flashlight.
4 Pronounced “tuh-PEE-tum LOO-sid-um.”
5 For those animals living in total darkness, producing light can be their key to success. You can read my blog on Bioluminescence for more on this at https://forestbarkdollweil.com/bioluminescence/.
6 Thanks to the poet Mary Howitt.
7 Think of Thorin Oakenshield’s description of the Arkenstone gem in J.R.R. Tolkien’s The Hobbit: “It shone like silver in the firelight.”
8 A few birds have them, but most birds do not.
9 If you have dogs in your household, you’ve probably noticed that they can see things in the dark much better than you can, partly because they have tapeta lucida.
10 I have been able to distinguish between our dogs at night based on the color of their eyeshine. Even the same species may have color variation.