Using microphone arrays and photographic methods to reconstruct flight paths of bats in the field when they find and capture prey in air using their sonar system, Annemarie Surlykke from the Institute of Biology, SDU, Denmark, and her colleague, Elisabeth Kalko, from the University of Ulm, estimated the emitted sound intensity and found that bats emit exceptionally loud sounds exceeding 140 dB SPL (at 10 cm from the bat's mouth), which is the highest level reported so far for any animal in air.

For comparison, the level at a loud rock concert is 115-120 dB and for humans, the threshold of pain is around 120 dB.(1)



Measured call intensity and source level as bats approach the array. Two individual approach flights of N. leporinus (upper panel) and N. albiventris (lower panel). The flight paths (left panels) with arrows indicating the flight directions are shown as seen from above as the bats approached the three microphones (red circles on the x-axis). For N. leporinus, blue circles show positions based on photos. Source levels were estimated for the search calls marked by red in the flight path. The last calls in this recording were approach/terminal calls of a pursuit for which source level was not estimated. For N. albiventris, all calls were search calls and source levels were estimated for the whole sequence. The right panels show the recorded sound level and estimated source level (SL) as a function of distance between bat and microphone. At long distances the source level is constant, while at short distances the bat reduces the source level, such that the recorded level is constant as the bat gets closer.

Bats emit their echolocation calls at ultrasonic frequencies, i.e. above the human hearing range. This is necessary to get echoes from small insects, but the draw-back of high frequencies is that they do not carry far in air as they are attenuated faster than low frequencies. By estimating detection range for typical insect prey, Surlykke and Kalko conclude that these extreme intensities are essential for the bats as they serve to counteract attenuation.

Their examination of the echolocation behavior in 11 species of insect-eating tropical bats from Panamá is the first comparative field study of bat echolocation sounds focusing on intensity and the results revealed, very interestingly, that although signal intensities (and frequencies) of bats vary widely, they appear to converge on similar detection ranges, because the bats emitting the highest frequencies were also the bats emitting the highest intensities.

Thus, the study illustrates the value of an interdisciplinary approach combining bat biology, ecology, behavioral biology and acoustics.

(1) Who concert reference may not be relevant to anyone under the age of 40. Trust us, they were louder than a 747 and Pete Townshend definitely has tinnitus.

Citation: Surlykke A, Kalko EKV (2008) Echolocating Bats Cry Out Loud to Detect Their Prey. PLoS ONE 3(4): e2036. doi:10.1371/journal.pone.0002036