Bat that roared

Bats using sound to find their way in the dark boom louder than home fire alarms and rock concerts, according to new measurements.

Fortunately all that noise stays at frequencies too high for human hearing, or bats would drive people batty.

Measurements of sounds from 11 species of tropical bats revealed that all animals emitted extremely loud sounds, reports Annemarie Surlykke of University of Southern Denmark in Odense. She and Elisabeth K.V. Kalko of the University of Ulm in Germany recorded and analyzed the yells bats emitted while hunting outdoors at the Smithsonian Tropical Research Institute’s field station on BarroColoradoIsland in Panama.

Species from four bat families made sounds that, at a distance of 10 centimeters, ranged between 122 and 134 decibels (measured on a scale that sets the threshold of human hearing at 0 decibels), the researchers say in the April PLoS One. Two species flying over open water made the loudest sounds yet recorded for any bat, averaging around 137 decibels and even hitting 140.

Home fire alarms reach about 108 decibels, says bat behavior specialist Brock Fenton of the University of Western Ontario in London, Ontario. Previous methods had suggested bat echolocation sounds would be loud, around 110 decibels, but this field study has topped even that number. “That’s what’s really amazing,” he says.

The intensity of bat sounds has been challenging to measure because animals in the lab don’t sound off as the way they do in natural situations. Surlykke and Kalko worked with free-ranging bats that were dipping and diving to hunt insects. These night hunters find insects by echolocation, belting out high-pitched sounds and listening for the echoes bouncing off obstacles and edibles.

Bats may be loud because they have to be. Their high frequency sounds fade quickly with distance. The researchers calculate that the two loudest species, which echolocate at the highest frequencies, also have the shortest detection ranges, a mere nine meters for a large moth.

The researchers set up an array of microphones and recorded the sounds bats made while swooping by. By measuring the difference in time of sound arrival at the different microphones, the researchers worked out where each bat was when it roared. Knowing the location let the researchers account for the way sound fades with distance, and report the intensity at 10 centimeters from each bat.