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, 3 (4), e2036

Echolocating Bats Cry Out Loud to Detect Their Prey

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Echolocating Bats Cry Out Loud to Detect Their Prey

Annemarie Surlykke et al. PLoS One.

Abstract

Echolocating bats have successfully exploited a broad range of habitats and prey. Much research has demonstrated how time-frequency structure of echolocation calls of different species is adapted to acoustic constraints of habitats and foraging behaviors. However, the intensity of bat calls has been largely neglected although intensity is a key factor determining echolocation range and interactions with other bats and prey. Differences in detection range, in turn, are thought to constitute a mechanism promoting resource partitioning among bats, which might be particularly important for the species-rich bat assemblages in the tropics. Here we present data on emitted intensities for 11 species from 5 families of insectivorous bats from Panamá hunting in open or background cluttered space or over water. We recorded all bats in their natural habitat in the field using a multi-microphone array coupled with photographic methods to assess the bats' position in space to estimate emitted call intensities. All species emitted intense search signals. Output intensity was reduced when closing in on background by 4-7 dB per halving of distance. Source levels of open space and edge space foragers (Emballonuridae, Mormoopidae, Molossidae, and Vespertilionidae) ranged between 122-134 dB SPL. The two Noctilionidae species hunting over water emitted the loudest signals recorded so far for any bat with average source levels of ca. 137 dB SPL and maximum levels above 140 dB SPL. In spite of this ten-fold variation in emitted intensity, estimates indicated, surprisingly, that detection distances for prey varied far less; bats emitting the highest intensities also emitted the highest frequencies, which are severely attenuated in air. Thus, our results suggest that bats within a local assemblage compensate for frequency dependent attenuation by adjusting the emitted intensity to achieve comparable detection distances for prey across species. We conclude that for bats with similar hunting habits, prey detection range represents a unifying constraint on the emitted intensity largely independent of call shape, body size, and close phylogenetic relationships.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Echolocation calls.
Time-signals and spectrograms of search phase echolocation calls of the 11 bat species studied. Dashed vertical lines separate species. The 20 ms time scale applies to both time signals and spectrograms, but pulse intervals between calls are collapsed. Calls from species emitting more than one call type were consecutive calls from one recording. Molossus molossus (M. m.) is an open air forager. Cormura brevirostris (C. b.), Centronycteris centralis (C. c.), Saccopteryx bilineata (S. b.), S. leptura (S. l.), Pteronotus gymnonotus (P. g.), Lasiurus ega (L. e.), Myotis albescens (M. a.), and M. nigricans (M. n.) are edge-space foragers. Noctilio leporinus (N. l.) and N. albiventris (N. a.) are trawling bats.
Figure 2
Figure 2. Echolocation call source levels.
Scatter plots of estimated source levels (SL), i.e. emitted intensity in dB SPL 10 cm from the bat's mouth, as a function of distance between the bat and the array. Logarithmic trend lines (R2 values annotated) are shown for source level values as a function of distance at short distances, i.e. up to 5 m. Trend lines and R2 are only shown for those bat species, where correlations were statistically significant (P<0.001, t-test, [41]). The figure includes the eight species that were recorded over several nights.
Figure 3
Figure 3. 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.
Figure 4
Figure 4. Removing interference from water reflections in echolocation calls.
N. leporinus approached the array as shown by the multi-flash photos and reconstructed flight paths (seen from above) in the upper panel. The arrow indicates the flight direction towards the array with the three microphones, blue, red, and green circle, on the x-axis. The flight path based on photo-reconstructions (red) fits closely with the path based on sound (blue curve). The middle microphone was in the acoustic axis when the analyzed signal (indicated by a big red filled circle) was emitted. The color coded time signals illustrate how different simultaneous recordings of the same signal may be on the three microphones in the array. The color of time signals and spectra indicates the recording microphone shown by colored circles at 0, 1 and 2 m on the x-axis; blue: left microphone, red: middle microphone, green: right microphone. Spectra of recorded N. leporinus signals with notches from interference from water reflections are shown in the middle panel. The lower panel shows the effect of mathematically removing the reflections to leave only the smooth spectra of the emitted calls.
Figure 5
Figure 5. Methods for recording and photographing bats.
Sounds were recorded with a linear microphone array with three microphones 1 m apart. The time-of-arrival-difference (TOAD) of sound on the microphones was determined by cross correlation between the three recordings using one (green part of the signal on M, middle microphone) as the model. TOADs were used to reconstruct flight paths. Some flight paths were also reconstructed from stereo photographs with a custom-made multiflash unit and two analogue 35-mm cameras (CL, CR). Flight paths from photographic reconstruction corroborated the positions based on array recordings. In the example shown here, the bat approached the left microphone before turning to the right (the bat's left) in front of the array. The echolocation call shown was emitted from a position 10 cm to the left of the left microphone, 1 m away from the array (x,y = −0.1, 1 m). Accordingly, the time-of-arrival differences show that the signal arrived first at the left microphone.

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