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American Bats
Nature Bulletin No. 686-A   September 23, 1978
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

A bat is one of the strangest of all animals. Like the opossum, it is a survivor from prehistoric times and strikingly similar to ancestors found as fossils in rocks formed at least 50 million years ago. During the ages a bewildering variety of bats developed, and now there are nearly 2000 species, but they all have certain unique characteristics.

Bats are the only mammals that have wings and can fly. Others, such as the flying squirrel, can glide through the air for short distances but bats have remarkable powers of sustained flight. Further, they have specialized ears and other equipment that enable them to navigate, in total darkness, far more accurately than man's aircraft and seacraft dependent upon electronic devices such as radar and sonar.

A bat's wing consists of a thin membrane which joins the long forearm, the greatly elongated fingers, and the hind leg, to the side of the body. The thumb ends in a hooked claw that is very useful. The elbow and the knee bend only backward. On many species, extending from each hind leg, there is also a triangular membrane attached to the tail.

Bats are not blind but their eyesight is poor and it is the unique sound equipment that enables them to hunt for food at twilight or at night when flying insects are most plentiful. They emit ultrasonic beeps and evaluate the echoes detected by their marvelously adapted ears. Apparently a bat recognizes an echo that bounces back from an insect, changes direction and captures the prey -- all within a split second.

In a laboratory, when deprived of hearing in only one ear, Little Brown Bats avoid large obstacles but are unable to catch small insects. When deafened in both ears they blunder about with no sense of direction. With normal hearing but both eyes taped shut, they can catch insects and fly through a maze of dangling threads without touching one of them.

Bats are distributed over most of the earth but most kinds inhabit the tropical or subtropical regions. However, 12 species occur in Illinois and one of them, the Silver-haired Eat, ranges from ocean to ocean and from Mexico to the northern limit of trees in Canada.

There are two main groups. One comprises the large fruit-eating bats, such as the flying fox which may be 12 inches long and have a 5-foot wingspread, found only in tropical regions of the South Pacific. Most bats are in the other group and the great majority are insectivorous. However, in the American tropics, there are some kinds which also eat fruit; some that prey on rodents, frogs and smaller bats; a few that catch little fish; and the highly specialized vampire bats that live on blood they secure from other animals.

The bats in the United States, all insect eaters, are of two general types. In one they tend to be solitary; to roost in trees and to migrate southward in autumn. In the other they tend to live in colonies; to be non-migratory; to hibernate in caves or abandoned mines during winter; and to roost in buildings, caves, or hollow trees the rest of the year. Carlsbad Caverns in New Mexico are inhabited by hundreds of thousands of bats.

Those in Illinois mate during autumn but the young are not born until late spring or early summer. A female usually produces only one but in certain species she may have two or more. Suckled by the mother and carried with her until too heavy, they grow rapidly and some kinds are able to fly when only three weeks old.

Scientists in search of better electronic devices hope to discover how a bat distinguishes between echoes from an insect and those from an obstacle.

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