If you think rock concerts are loud, you should hear the bats. An article published in the open-access journal PLoS ONE finds that bats emit sounds that are above the human threshold of pain.

Researcher Annemarie Surlykke (Institute of Biology, SDU, Denmark) and her colleague Elisabeth Kalko (University of Ulm, Germany) investigated the patterns and behavior of echolocation in 11 species of insect-eating tropical bats from Panamá. The bats use a sonar system to capture insect prey at night, and the researchers reconstructed these flight paths using arrays of microphones and photographic methods. These models were used to estimate the intensity of sound that was emitted during the flights.

Surlykke and Kalko found that bats emit a sound that is stronger than any other animal in air. The exceptionally loud sounds exceed 140 decibels (dB) SPL (Sound Pressure Level measured at 10 cm from the bat's mouth). This is louder than the 115 - 120 dB that is emitted at a loud rock concert and the 120 dB human threshold of pain.

Humans, however, cannot hear the echolocation call since the bats emit sounds at ultrasonic frequencies. They do this in order to find the location of small insects that would be missed using lower frequencies. These high frequencies, though, do not travel as far in air as low frequencies because of air's ability to attenuate the sound. The researchers used estimates of the detection range for typical insect prey to conclude that extremely intense, ultrasonic sounds are necessary in order to counteract this attenuation.

Another interesting finding was that bats that emit the highest frequencies were also the ones emitting the highest intensities. This first comparative field study of bat echolocation sounds found a wide variation of signal intensities and frequencies that converged on similar detection ranges.

"Overall, our study underlines the importance of intensity measures in the field as source level plays a crucial and so far largely underestimated role in bat echolocation. If we want to further understand which ecological and evolutionary factors shape echolocation signal design, an even larger variety of call parameters need to be considered, including sound duration and pulse interval, which may create call-echo overlap or other masking effects," conclude the authors.

Echolocating Bats Cry Out Loud to Detect Their Prey
Surlykke A, Kalko EKV
PLoS ONE (2008). 3(4): e2036.
doi:10.1371/journal.pone.0002036
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PLoS ONE is the first journal of primary research from all areas of science to employ both pre- and post-publication peer review to maximize the impact of every report it publishes. PLoS ONE is published by the Public Library of Science (PLoS), the Open-access publisher whose goal is to make the world's scientific and medical literature a public resource.

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: Peter M Crosta