Prey evasiveness and masking noise jointly promote the ultrahigh call rate in echolocating bats

Among mammals, only bats and toothed whales are known to produce > 160 calls per second, which occurs shortly before these echolocators capture their prey. This high call rate phase, dubbed terminal buzz, is powered by rare superfast muscles. The enigmatic terminal buzz of bats is hypothesized to have evolved in response to the challenges of capturing flying insects. However, it remains unclear why some bats produce a full terminal buzz sequence when capturing stationary, inescapable prey, such as insect larvae suspended in the air. Here, we recorded bats performing foraging and orientation tasks in a laboratory setting, both with and without acoustic masking noise.

We discovered that masking noise selectively enhanced the call rate of Pratt's roundleaf bats during foraging, but not during landing tasks. We found that the prevalence and elaboration of terminal buzz in these bats were determined by prey evasiveness, with the most frequent and elaborated terminal buzz occurring for bats capturing flying insects. Unlike other reported aerial hawking bats, the Pratt's roundleaf bats did not exhibit a terminal buzz when capturing stationary, inescapable insect larvae. Surprisingly, the absence of terminal buzz can be instantly restored by exposing bats to masking noise. In contrast, masking noise had only a minimal effect on the buzz-like calls produced during a landing task.

We posit that prey evasiveness and masking noise, two primary challenges faced by both echolocating bats and toothed whales, jointly drove the evolution of the ultrahigh call rate of terminal buzz.