Just as conversationalists at a loud cocktail party may raise their voices to be heard over the din, so do echolocating bats, according to a new study released Monday by the Proceedings of the National Academy of Sciences.
The effect is known as the Lombard effect, and it’s found in a wide range of chattering and chirping critters, from zebra finches to marmosets to humans. Often the effect registers not as turning up the volume (an increase in the sound wave’s amplitude), but in raising the pitch of one’s voice (a rise in frequency).
For this study, researchers from UCLA and Northeast Normal University in Jilin tested the reactions of three horseshoe bats collected from China. Bats make excellent test subjects: The nocturnal mammals have a fine-tuned sense of hearing and use echolocation to navigate in the dark.
Bats can hear (and use) a wide range of frequencies, stretching 100 Hz, but they have a particular resting frequency that they use when perched. The researchers introduced back noise above, below and right on top of that resting frequency, and recorded more than 83,000 echolocation calls to see how the bats reacted.
In almost all cases, the bats raised their frequency (the pitch) of their calls so they could pick it out against the noise. The only time they increased the amplitude of their calls (as in, called more loudly) was when the noise lay right over the bats’ dominant resting frequency. If they couldn’t work around the background noise effectively, only then would they try to talk over it, cocktail-party style.
The study provides some insight into how their brains must be wired, too: The animals didn’t need to wait for their voices to come back to shift their pitch or amplitude -- they did so as soon as they heard the background noise. There must be a direct link between their brains’ auditory systems (which pick up the noise) and their vocal-motor systems (which adjust their calls accordingly).
The findings may also have implications for animals living near populated urban areas, where white noise can often affect the calls -- and thus the sex lives -- of nearby bird populations.