Big Brown Bat (Eptesicus fuscus)

Early oscilloscope images of the sounds produced by bats, from "Listening in the Dark", by Donald Griffin, Cornell University Press, 1958

Little Brown Bat (Myotis lucifugus)

In 1938, a student called Donald Griffin, who worked with bats, wanted to solve Spallanzani's Bat Problem for good. He heard about a pioneering professor called J.W. Pierce, who was investigating, for the first time, sounds of frequencies beyond the human range of hearing - ultrasounds.

Pierce had built a "sonic detector" to detect ultrasound, perhaps the only one in the world at that time. It consisted of a specialised microphone and an electrical circuit that converted ultrasound into sound that humans can hear. He used it to listen to the ultrasound of insects.

What would happen if Griffin and Pierce used the sonic detector on bats? They found that bats in fact emit powerful pulses of ultrasound continuously, especially while in flight. Later, Griffin measured the sounds with an oscilloscope (right).