Most echolocating bats exhibit a strong correlation between body size and the frequency of maximum energy in their echolocation calls (peak frequency), with smaller species using signals of higher frequency than larger ones1,2. Size–signal allometry or acoustic detection constraints imposed on wavelength by preferred prey size have been used to explain this relationship1,3. Here we propose the hypothesis that smaller bats emit higher frequencies to achieve directional sonar beams, and that variable beam width is critical for bats. Shorter wavelengths relative to the size of the emitter translate into more directional sound beams4. Therefore, bats that emit their calls through their mouths should show a relationship between mouth size and wavelength, driving smaller bats to signals of higher frequency. We found that in a flight room mimicking a closed habitat, six aerial hawking vespertilionid species (ranging in size from 4 to 21 g, ref. 5) produced sonar beams of extraordinarily similar shape and volume. Each species had a directivity index of 11 ± 1 dB (a half-amplitude angle of approximately 37°) and an on-axis sound level of 108 ± 4 dB sound pressure level referenced to 20 μPa root mean square at 10 cm. Thus all bats adapted their calls to achieve similar acoustic fields of view. We propose that the necessity for high directionality has been a key constraint on the evolution of echolocation, which explains the relationship between bat size and echolocation call frequency. Our results suggest that echolocation is a dynamic system that allows different species, regardless of their body size, to converge on optimal fields of view in response to habitat and task.
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We thank H. Baagøe and B. Fenton for access to the bat collections at the Natural History Museum of Denmark and the Royal Ontario Museum, respectively, and R. Fisher and J. Hallam for providing gape heights of bats in flight. S. Brinkløv, C. Elemans, B. Falk, B. Fenton, J. Galef, P. Madsen, C. Moss, H.-U. Schnitzler and M. Wahlberg provided detailed comments that improved the manuscript. This study was funded by the Danish Council for Natural Sciences (FNU), Carlsberg, Oticon and the European Commission via the Seventh Framework Programme project ChiRoPing, Information Society Technologies Contract 215370. Animal capture and experimentation was approved by Skov- og Naturstyrelsen (Denmark).
The authors declare no competing financial interests.
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Jakobsen, L., Ratcliffe, J. & Surlykke, A. Convergent acoustic field of view in echolocating bats. Nature 493, 93–96 (2013). https://doi.org/10.1038/nature11664
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