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)

Were these the sounds bats used for orientation in darkness? Griffin needed to make sure by holding the mouths of bats shut with tape. The bats were useless at orientation, and bumped into obstacles all the time!

So to avoid obstacles in darkness, bats need (1) their hearing, and (2) the ability to emit these pulses of ultrasound. This seems like enough evidence that bats use biosonar, doesn't it? A bat must emit these sounds, and then wait for the echoes to return, which would give the bat information on the distances to obstacles.

But this wasn't enough evidence for Griffin, or the rest of the scientific community. Griffin and others spent the next 20 years gathering evidence of the sonar abilities of bats. Griffin wrote about it all, for a non-specialist audience, in his book "Listening in the Dark" (Cornell University Press, 1958).