Bats Have Built-in Radar

Bats “see” with their ears. Echoes of sounds inaudible to man enable the flying mammals to find their way through the almost absolute darkness of deep cavern or jungle. These creatures might be considered inventors of the Navy’s sonar device by which underwater obstacles are located by echoes—or even, in a sense, of radar.

Almost entirely creatures of night and late twilight, bats have small and poorly developed eyes. When one is on the wing it emits an almost constant succession of inaudible “squeaks” at a sound frequency of between 25,000 and 70,000 vibrations a second. The human hearing range reaches only to 30,000. Each squeak, according to measurements by Dr. Donald R. Griffin of Cornell University, lasts about two-hundredths of a second. In ordinary flight over open country it is repeated about ten times a second. By means of the echoes it apparently is possible to detect and avoid any obstacle, even one as small as a strand of silk thread strung across the path, within a distance of ten or twelve feet.

The bat does not hear its own squeaks. Each time one is uttered an ear muscle contracts automatically, thus momentarily shutting off the sound itself so that only the echo can be heard. It is possible that each animal has its individual sound pattern and is guided only by its own echoes. Otherwise, it would seem, there would be complete confusion from the echoes of several hundred bats moving in a flock.

Largest of the bats are northern India’s flying foxes. The body is shaped almost precisely like that of a small fox and is covered with fine, dark-brown hair. The wing spread is about three feet. These flying foxes move in flocks of thousands. They are exclusively fruit eaters and forest dwellers. They are the only bats eaten by man. Their flesh is said to resemble chicken.

Insect-eating bats are prisoners of the air. Once on the wing they must remain in flight all night until they return to the dark caves where they sleep all day, suspended head downwards. Flying from dusk to dawn requires an enormous amount of energy for which a lot of food is required. One of these animals probably must eat about a third of its own weight in insects each night. Thus it is a good friend of the farmer and one of the potent factors in keeping the balance of nature.

If a bat lit on the ground or on any solid object it would be very difficult, perhaps impossible, to get it on the wing again. This is accomplished only by falling from its sleeping place.

The hibernation of temperate zone bats appears very close to complete lifelessness and is probably the most deathlike sleep experienced by any mammal. Animals close to a cave entrance have been found completely coated with ice, as moisture has congealed on the fur. Yet when they wake in the spring they appear none the worse for the experience.