National Geographic : 1961 Apr
LINCOLN LABORATORY,M.I.T. Hungry bat broadcasts signals that bounce back from target TINY LARYNX emits up to 200 beeps a second as the brown bat homes in on an insect. Big ears detect the returning echoes, and a brain weighing a few hundredths of an ounce computes the data and controls the hunter's speed and direction. Like an FM radio station, the bat modu lates the frequency of his signals. In each cruising beep, he starts broadcasting at 100,000 cycles per second but quickly slides down to 40,000. Near the target, he starts at 30,000 and slides to 20,000, approximately the shrillest pitch heard by humans. Individ ual variations in sliding frequencies may ex plain how he recognizes his own echo in a caveful of bats. He judges direction by comparing the echo at one ear with that at the other. Brown bats 578 deprived of hearing in one ear can avoid large obstacles, but they are unable to catch small insects. Range to a target or barrier is judged by a bat as it is by a radar set, by sensing the time delay between the outgoing pulse and the echo. For a target six inches from the bat's mouth, the time delay for the round trip is about a thousandth of a second. Though undetected by human ears, a bat's beeps measured a few inches from his mouth register as much sound intensity as the roar of a four-jet airliner a mile away. He can hear an echo from a target as tiny as a mos quito, recognize it in a split second, and swoop to the attack - a feat of nature that man, with all his electronic skill, must still hold in awe.