The science behind eyeshine

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Here in the northern hemisphere, the autumn days are getting shorter and shorter. It’s getting darker earlier in the day, and our eyes have to adjust to dim conditions.

Without the help of fire or electric lights, we humans are pretty bad at night vision. Unlike many other animals, our eyes lack a specialized reflective surface that aids sight at night and in low light environments (caves, under water, etc.). This surface, called a tapetum lucidum, located behind the retina, acts as a mirror to reflect light photons.

Humans lack the tapetum lucidum located in between the retina and choroid in the eyes of many nocturnal animals (Source).

Light enters the eye and hits photo receptors in the retina. Some light, however, will miss the photo receptors and pass past the retina. The tapetum lucidum reflects that light and gives it a second chance to hit the photo receptors and illuminate the scene.

Some of this light is reflected back out of the eye, which is why some animals’ eyes appear to glow in nighttime trail camera photos.  All types of camera flash, even the low-glow infrared flash of the Snapshot Wisconsin cameras, can reflect off the tapetum lucidum and cause an animal’s eyes to light up. (This is not the same as the red-eye effect seen in photos of human eyes which is caused by light reflecting off the blood vessel-rich choroid behind the retina.)

There is variation in mineral content and structure of the tapetum lucidum, which causes eyeshine in different species – and even different breeds of dog – to look different. Eyeshine may appear white, blue, green, yellow, pink or red.  It’s too bad nighttime trail cam photos are in black and white and we can’t see these color differences!

Animals having a tapetum lucidum (not extensive):

• deer
• carnivores: canids and felids
• grazing animals: sheep, goats, cattle, horses
• ferrets
• lemurs
• marsupials
• fruit bats
• ray-finned fishes and cartilaginous fishes including sharks
• owls and a few other nocturnal birds
• crocodilians including alligators (bright red eyeshine – spooky!)

Animals lacking a tapetum lucidum (not extensive):

• higher apes including humans
• kangaroos
• pigs
• squirrels*

*I’ve read that squirrels don’t have a well developed tapetum,  but flying squirrel eyes certainly glow in our nighttime trail cam photos. Anyone who can shed some light on this mystery, please leave a note in the comments!

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Sources:

• What causes the red eye effect? Yale Scientific Magazine.
• Candid Creatures: How Camera Traps Reveal the Mysteries of Nature. Johns Hopkins University Press.
• Why do animals’ eyes glow in the dark? NPR All Things Considered.
• Comparative morphology of the tapetum lucidum (among selected species). Veterinary Ophthalmology 7(1):11-22.
• Crystals of riboflavin making up the tapetum lucidum in the eye of a lemur. Letters to Nature.
• Ocular comparative anatomy of the family Rodentia. Veterinary Ophthalmology.