Science: Sound & Survival

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Far beyond the limit of human hearing, the high-pitched hunting cry of the bat makes the dark dangerous for night-flying insects. Bat chirps bounce off their tiny bodies like sonar pulses, giving their position away to the swooping enemy. Yet despite the bat's delicate detection equipment, many an insect escapes—and scientists have long wondered why. In the current issue of the American Scientist, Biologists Kenneth Roeder and Asher E. Treat explain how they pried into the defensive secrets of the noctuid moth, an insect that has demonstrated singular evasive skill.

Warning Wiggles. Working with infinite care, Roeder and Treat took a live moth, attached delicate wires to the nerves leading out of one of its ears, and connected the little insect to an amplifier and an oscillograph. Then they turned on an electronic generator that gave out brief bursts of ultrasonic sound—a reasonable imitation of a prowling bat. Even where the man-made beeps were too weak to be detected by man-made microphones, the moth's ear responded with electrical signals. When the imitation bat sounded louder, as if it were closing in, the moth's ear responded more strongly, covering the face of the oscillograph with trains of wiggly warning lines.

Having analyzed the moth's sonar-detecting apparatus in the laboratory, Roeder and Treat tried it out in the field against real bats. They set up their apparatus on a Massachusetts hillside, and at nightfall their wired moth began to detect the ultrasonic cries of bats. From the traces on their oscillograph, the biologists could tell whether an invisible bat was approaching or flying away. Later, when Roeder and Treat turned on a powerful floodlight, they could watch the bats diving on their prey and hear, through the captive moth's ear, the bats' searching sonar beeps and their final triumphant buzz. Sometimes they saw free-flying moths take evasive action, but the motions of both hunter and hunted were too fast to follow with the naked human eye.

Dives & Loops. To learn more about the moth's methods of escape, the two scientists set up a floodlight and trained a camera on its beam. When an insect flew across the floodlit area, the operators opened the camera's shutter and turned on their electronic beeper to simulate a cruising bat. "Many insects." say Roeder and Treat, "showed no change in flight pattern when they encountered the sound. In others, the changes in flight path were dramatic in their abruptness and bewildering in their variety. One of the commonest reactions was a sharp power dive into the grass. Almost as frequently the dive was prefaced or combined with a series of tight turns, climbs and loops" (see cut).

In order to measure the value of such tactics, Roeder and Treat studied 402 floodlit meetings between moths and bats. They noted which moths made evasive maneuvers and which were caught. "The selective advantage of evasive action," they concluded dryly, "was 40%, meaning that for every 100 reacting moths that survived, there were only 60 surviving non-reactors. This figure is very high when compared with similar estimates of survival value for other biological characteristics. It seems more than adequate to account for the evolution of the moth's ear through natural selection."