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)

Meanwhile, the dutch zoologist Dijkgraaf was coming to the same conclusions as Griffin. He worked in war time (1939-1945), and the usual channels of scientific communication were closed. The world got to hear about his work in 1946.

He noticed that bats emit a very faint ticking sound when in flight, like the ticking of a watch. He called this the ticklaut. The bats made more ticklauts when they neared obstacles, or performed difficult manouevres. The ticklaut is actually what we can hear of the bat's ultrasonic sonar sounds: because the ultrasonic sonar sounds are produced so quickly, a tiny piece of energy leaks down into the audible range.

It seems amazing that nobody noticed these ticklauts before. They should have provided a clue to earlier biologists that the bats were making sounds, possibly outside the usual hearing range of humans. Why couldn't Spallanzani hear them, for instance? Perhaps because he was so old. When we get older, we lose our high-frequency hearing. The ticklaut, although in the usual audible range, is still very high in frequency.