Forest Preserve District of Cook County, Illinois
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Eyes of Animals
Nature Bulletin No. 403-A   January 23, 1971
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

The eyes of an animal tell the story of the creature's life: its sources of food, its habits, its fears, and the history of its kind. Stop and think about that. Our modern civilization has come about because man has a complex brain that reasons; hands with the all-important thumbs; and eyes which, because of their position and construction, can be instantly focused on any object whether near or far -- "binocular vision" -- and which see all the colors of a rainbow.

Only the higher apes have such eyes. The lower apes and monkeys have eyes similarly placed, which enable them to examine things and swing from tree to tree, but they, like most mammals, see colors as shades of gray. A bull does not see red. Birds and lizards apparently see colors about the same as we do. Color vision is also found in turtles and the higher fishes -- such as black bass and bluegills -- but the fish see colors as we do when wearing amber sunglasses. Not much is known about insects except that ants, honeybees and butterflies see ultraviolet light, which is invisible to us, but are color-blind to red: it's dark gray or black to them.

The predatory mammals -- flesh-eating hunters such as the cats the dogs, wolves and foxes; the bears; and the weasel family -- have binocular vision: eyes in the front of their heads, with powerful eye muscles that enable them to focus very rapidly and to contract or dilate the pupils according to whether the light is bright or dim. An animal that is preyed upon by many enemies has its eyes out on the sides of its head: each eye with its own field of vision. For example, a cottontail rabbit can see what is above, behind, on either side, and in front of it except what is right before its nose. He probably does not see the clover leaf he eats. Species which are hunted by other animals but which are predators themselves at times, commonly have eyes that are a compromise. A possum's eyes are located at an angle of about 30 degrees with the axis of the body.

In most mammals, the pupil of the eye is round but in many of the flesh- eaters such as the domestic cat, it becomes a vertical slit in bright sunlight and widens to cover most of the pupil in darkness. That is why they see so well at night. In the eyes of the kangaroo and some hoofed animals, notably the goat, the pupil is a horizontal slit. The eyes of many animals shine at night because of a peculiar mirror, on the retina at the back of the eyeball, which reflects light; so that a cat's eyes have a green glare and those of alligators and crocodiles shine red. Such eyeshine is found in many other kinds of animals including various mammals, sharks, sturgeons, and some moths, but not in birds nor amphibians: and in all colors except violet and white.

Swift animals, like the antelope, usually have large eyes. Nocturnal animals like the deer mouse and flying squirrel, have also developed very large eyes which enable them to see better and evade their many enemies. Others, which live mostly in tunnels, like the meadow mice, shrews and moles, have tiny weak eyes. Some, like the earthworm, are blind. Land snails have eyes on tentacles with which they can see around a corner, but leeches merely have "eye spots " or none at all. Crawfish and crabs have eyes on short stalks. Spiders have from 2 to 8 eyes. Many insects have huge compound eyes -- the dragonfly has from 20 to 25 thousand tiny lenses in a honeycomb pattern -- and some, like the grasshopper, also have three little "simple" eyes for close vision. Birds have the keenest eyes and those of the hawks, eagles and vultures can see small objects at incredible distances.

By their eyes ye shall know them.

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