Does echolocation wavelength restrict bats’ choice of prey?

Abstract

The size range of insects encountered by bats that use narrowband echolocation signals (FM-CF or quasi-CF) extends to sizes much smaller than the wavelengths used by these bat species. It was hypothesized that the expected Rayleigh scattering sets a lower limit on the size of prey which can be detected. To test the Rayleigh scattering theory empirically, insects were ensonified with pure-tone pulses ranging from 20 to 85 kHz and their target strengths were measured. The target strength of the smaller insects was frequency-dependent within the frequency range used by aerial-hawking bats. The target strength diminished sharply as the wing length to wavelength ratio decreased below unity. A model was developed to predict maximum detection distances, which lead to an estimate of the optimum frequency to be used for detecting each insect size. In a dietary analysis, it was found that the bat species using the shortest wavelengths took the smallest insects. The minimum prey size detectable by each bat species was predicted. The longest wavelength bats (Vespertilio murinus, Nyctalus noctula) took smaller prey than were estimated. The diets of the shorter wavelength bats agreed with predictions of the model. [Work supported by BBSRC.]