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
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.
their eyes ye shall know them.
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Update: June 2012