Bats Can Make Calls More Intense Than Rock Concerts

Brian Handwerk
for National Geographic News
April 30, 2008
Bats may seem silent to human ears, but their calls can reach sound pressures greater than those emitted at rock concerts, a new study says.

The mammals use their natural sonar, called echolocation, to find prey by putting out high-pitched sounds and listening for the noise reflections.

The intensity of such sounds, which are beyond the range of human hearing, were extensively studied for the first time as part of the new research.

At close range—some four inches (ten centimeters) from the bat's mouth—bat cries exceeded 140 decibels (dB), which is a measurement of sound pressure.

That's a higher level than has ever been observed from any airborne animal.

A loud rock concert, by comparison, measures 115 to 120 dB—just under the threshold of pain for humans.

(Related: "Vampire Bats Hunt by Sound of Victims' Breath, Study Says" [June 19, 2006].)

Changing Tunes

Annemarie Surlykke of the University of Southern Denmark and Elisabeth Kalko from the University of Ulm, Germany, used microphones to track 11 species of insect-eating bats in tropical Panama.

(See photos of Panama's bats in National Geographic Magazine.)

The researchers modified a method for capturing whale songs to accurately measure the bats' vocal output and find out exactly where the animals were flying at any given moment.

"Of course you can't see very well at night, and they fly very fast and change directions all the time," Surlykke said.

Although the scientists studied only insect-eating bats with comparable feeding habits, they found major differences in the species' voices.

Different species called in different frequencies, and some were much louder than others.

(Listen to calls of Panama's bats.)

"We found these extreme differences in emitted intensity, up to 140 dB for the loudest down to 120 dB for the most silent of the bats in our study," she said.

A 20 dB difference equals a tenfold increase in sound pressure, according to Surlykke.

Yet louder and quieter bats all seemed to detect their insect prey at about the same distance—typically some seven to ten feet (two to three meters) away.

"That surprised us because the differences in immediate intensities are quite big," Surlykke said.

Results of the study were published this week in the journal PLoS ONE.

Loud, But Not Far

Some bats apparently turn up the volume in order to compensate for their high-frequency calls, which don't carry far and weaken quickly.

"Ultimately there are limits to what the mammalian ear can detect," said Cynthia Moss, principal investigator at the University of Maryland's Auditory Neuroethology Laboratory.

"Bats that produce high-frequency sounds can't push their hearing to detect sounds that are really weak in the environment," said Moss, who was not involved in the new research.

"They push the output of the vocalization to end up hearing echoes at about the same level," she said.

Study co-author Surlykke noted that the ability is likely a critical survival adaptation.

"They may have to adjust the volume to get a reasonable detection distance to operate," she said.

"If the [distance] was too short they wouldn't be able to react if they did detect prey."

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