When an animal's eyes shine in the dark, it's because of a structure inside the eye that helps them see better in dim conditions. Learn what that structure is and how it works.
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● Ollivier, F. J., Samuelson, D. A., Brooks, D. E., Lewis, P. A., Kallberg, M. E., & Komáromy, A. M. (2004). REVIEW Comparative morphology of the tapetum lucidum (among selected species). Veterinary Ophthalmology, 7(1), 11. doi:10.1111/j.1463-5224.2004.00318.x
There’s something really spooky about the way some animal’s eyes shine in the dark. My grandmother… she called it The Shining! But most people refer to it as Tapetum Lucidum, a reflective surface behind an animal’s retinas. It’s a common feature in vertebrates that do stuff in low-light environments. It enhances their visual sensitivity so they can see better in near darkness.
When light enters an eye, lots of it hits photoreceptors that then transmit visual information to the brain. But some photons slip past the receptors, so the tapetum lucidum acts like a mirror, bouncing the light back for a second chance at being caught.
The tapetum lucidum is usually located behind the retina and is made up of mirrored crystals that reflect light. Lots of different animals have these, so there’s a broad diversity of how they’re structured. Depending on the animal, there may be several layers of both high- and low-refractive index materials in the tapetum. These layers are compatible with different wavelengths of light. And depending on the structure of these layers and the particular light wavelengths in play, they’ll either enhance or interfere with night vision.*
And the more layers there are, the more effective it is at reflection. A tapetum with five layers would only reflect up to 75% of the light that hits it. Between 10 and 20 layers means that an eye could approach a reflection rate of 100%. Keep in mind, however, that blood vessels, cells and imperfections in the tapetum layers can all interfere with the reflectance.
But what about all the animal variations out there? Well, among mammals there are more variations within the carnivore family, probably because most of these animals are active at night. Herbivores are more diurnal, meaning they sleep at night and party in the day. As such, they have less evolved tapetum than their nocturnal, blood-thirsty cousins.
There are three basic tapetal morphotypes in vertebrates, classified based on where they’re located and the composition of their reflective layers. First there are animals without tapeta lucida. This includes humans, other primates, squirrels, pigs and birds. In fact, birds are the only large group of animals where the tapetum is consistently absent. If you see any reflection in these animals’ eyes – like when you have redeye in a photo – that’s light reflecting off blood vessels at the back of the eye.
The second type are animals with tapetal material located within the retina itself, making it closer to their eye’s photoreceptor cells. This is most common in fish and some reptiles and only occurs rarely in mammals like fruit bats and opossum.
The last type are animals where the tapetal material is inside the choroid, vascular layer of the eye that’s over the retina. Lemurs have this, as do cows, sheep, horses and the aforementioned harmless kittens. You know what else has eyes like this? Sharks! And the tapetum lucidum in a shark makes it so they see 10 times better than humans can in dim light.
Here’s another difference: Not all tapetum lucidum shine with the same color. Whether they’re yellow, green, blue or whatever, the hue comes from different substances within the layers, like riboflavin or zinc in the tapetum or varying pigments in the retina. And these colors aren’t necessarily the same within each species or even each breed. For instance, age can change the color – so two Chihuahuas’ eyes might shine differently.
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