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

****:EYES OF ANIMALS

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.