Why do animals' eyes glow at night,
while human eyes don't?


Human eyes do reflect light, as all flash photographers know ... they just don't do it very well. Our eyes look red in some photographs. But it's not a reflector. It's the camera flash bouncing off the red blood vessels and red tissue in the back of our eye. The distinctive "eyeshine" given off by wolves and raccoons and crocodiles, among many other species, comes from the "tapetum lucidum", a mirror-like layer of cells in or behind the retina. This structure is found mostly in nocturnal animals, for whom it serves as a kind of light amplifier. The retina captures some of the light that enters the eye, but some passes through. The tapetum lucidum bounces it back at the retina, giving the animal a second chance to "see" it. It's this ricocheting light that gives off the vaguely eerie glow we've come to associate with scary tales ... all those sinister pairs of eyes staring out from gloomy thickets.

How about human ability to see at night? Cats are much better adapted for performance under low light than we are. They have larger corneas and pupils than us to collect more light in dim conditions. They also have the tapetum which reflects light back out of the eye. This way, their retinae get two chances to capture each photon of light. A cat's tapetum reflect 130 times more light than the human eye. This is why cats' eyes glow in the dark in photographs.

In low light levels the cats pupil must be able to open as wide as possible, but also be able to contract to very small size to protect the sensitive retina in bright sunlight. In human eyes, this size variation of the pupil is controlled by a circular ciliary muscle, but this limits the amount of size variation. In cats however, the same process is controlled by two, shutter-like ciliary muscles, which gives the cat it’s characteristic slit-like pupil in bright light conditions. All cats pupils are therefore elliptical, however some, notable the ‘Big Cats’, appear more circular when dilated.

Glowing Cat's Eyes

The size of the cat's eye is relatively larger than those found in human, this enables a larger pupil and therefore more light to enter the eye. Generally, the lens is more curved enabling sharper focusing even at the edges of the lens. The size of the anterior chamber and the curvature of the cornea is also greater, which helps more light to be refracted onto the light-sensitive retina.

Another feature, which enabled the amount of light hitting the retina to be increased, is the tapetum lucidum. This is positioned at the back of the eye, behind the retina and acts like a mirror, reflecting light back onto the light sensor cells in the retina. This gives the cats eyes the characteristic night-time glow when they are caught in a beam of light.

There are two distinct types of light receptor cell on the retina – Cones, which are sensitive to high levels of light, used in colour vision and Rods, sensitive in low light conditions. In cats, there is a greater concentration of Rods, aiding their night-time vision. As in humans, there is a greater concentration of receptor cells at the centre of the eye, leading to the optic nerve. In cats, however, these a concentrated along a broader, horizontal band. This gives the cat far more sensitivity to movement along the horizontal axis and they are therefore more able to detect prey movement along the ground at greater distances.

You may say that humans are not night vision animals, but we must have some of the best DAY time vision. Well, its hard to compete with the eagle. Eagle eyes are approximately the same size and weight as human eyes, even though a full grown adult bald eagle weighs no more than 14 pounds. But an eagle eye has a much different shape. The back of their eye is flatter and larger than the back of our eye, giving an eagle a much larger image than we can see. The back layer of the human eye has a special area on the retina called the fovea where there is an enormous concentration of vision cells. Our human fovea has 200,000 cones/mm which is an incredible number. But the eagle easily outdoes us. The fovea of the eagle has about a million cones per millimeter. This allows an eagle to see a mouse a mile away. That is far beyond our capacity.

~From: OcularOlympics.com

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