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.]

Implications of sensory ecology for species coexistence: biased perception links predator diversity to prey size distribution

“…Our fifth prediction is that bat species are passively specialized on prey size by sensory bias (but see Houston et al 2003). Such a specialization could represent an important selective pressure on insect body size evolution given the abundance and diversity of bats in many ecosystems and the huge amounts of prey that they consume (Kalka et al 2008).…”

“…These metric prey sizes can be interpreted as body or wing size. Based on empirical measurements of insect echo reflectance (Kober and Schnitzler 1990;Waters et al 1995;Siemers et al 2001;Houston et al 2003), we assumed a TS of -70 dB for our smallest prey size and a TS of -30 dB for the largest. We estimated TS for all intermediate prey sizes by a logarithmic interpolation:…”

“…We combined this knowledge to estimate prey perceptibility for a hypothetic community of bats that echolocate using different call frequencies. Several previous studies have estimated echolocation range and prey detectability as a function of call frequency for bats (e.g., Barclay and Brigham 1991;Waters et al 1995;Holderied and von Helversen 2003;Houston et al 2003;Jung et al 2007). We extended these in two ways: first, we aimed at realistically modeling the abiotic environmental conditions, i.e., by integrating the complex dynamical influences of atmospheric conditions and their interaction with call frequency on sound propagation and attenuation.…”

“…Third, physical knowledge of sound propagation and echo generation (Mohl 1988;Crocker 1998;Stilz 2004) as well as an extensive body of literature on call production and echo perception in bats is available. The latter include quantifications of source levels (call amplitudes) in the field (Holderied and von Helversen 2003;Surlykke and Kalko 2008) and measurements of reflection properties of natural prey objects (Kober and Schnitzler 1990;Waters et al 1995;Houston et al 2003). We combined this knowledge to estimate prey perceptibility for a hypothetic community of bats that echolocate using different call frequencies.…”

Seasonal Change in the Diet Composition of the Asian Parti-Coloured BatVespertilio sinensis

“…Moreover, some insects may not be "available" to bats. For example, detectable insect size is restricted by echolocation call structure ( Jones and Rydell 2003;Houston et al 2004), and some insect taxon have evolved ears that enable them to detect bat echolocation calls and avoid capture (Corcoran et al 2009). In the present study, we compared diet composition with relative insect abundance on the basis of assumption that our insect data reflected the whole flying insect community structure (not availability) around the roost.…”