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Q -- I have noticed that my cat's eyes glow at night when he stares at my headlights but that human eyes do not. I suspect that this glow is due to cats being nocturnal and needing better night vision while hunting.

A. -- You are absolutely correct in regard to the glow from your cat's eyes begin due to his nocturnal nature. In fact, the feline eye has three adaptations that improve its nocturnal vision.

Here is a quick review of anatomy to explain how the eye works. Light first passes through the cornea (the clear part of the eye where people place their contacts), then through the pupil, is refracted by the lens, and absorbed by special cells called rods and cones that make up the retina. The resulting signal is sent to the brain via the optic nerve.

As noted, cats are nocturnal and have adaptations that help them function in low-light conditions. The glow you noted is caused by the tapetum lucidum, which is a reflective surface lying just behind the retina and is found in many nocturnal animals. As light reaches the retina, energy is absorbed by the rods and cones. Light energy not absorbed on the first pass is reflected by the tapetum lucidum back to the rods and cones again, thus doubling the retina's exposure to the light and resulting in that eerie glow.

The second nocturnal adaptation of the feline eye is an increase in the number of rods in the retina. Rods are cells that are stimulated in low-light conditions while cones allow an individual to see color in daytime conditions. The feline eye has 25 rods for every cone whereas the human eye has five rods for every cone.

The third ocular adaptation is an increase in relative cornea size. Increasing their size and the curvature of the lens allows more light to reach the retina and creates a more intense signal in the low-light condition.

All these adaptations aid nocturnal animals in their night vision, which is crucial to their ability to survive.

-- Timm Otterson, DVM
Prepared as a public service by the Niagara Frontier Veterinary Society. Send questions to Pets, P.O. Box 403, East Aurora, N.Y. 14052-0403. Sorry, personal replies cannot be provided.

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