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The acoustic advantage of hunting at low heights above water: behavioural experiments on the European ‘trawling’ bats Myotis capaccinii, M. dasycneme and M. daubentonii

Björn M. Siemers^*, Peter Stilz and Hans-Ulrich Schnitzler

Department of Animal Physiology, Zoological Institute, University of Tübingen, Auf der Morgenstelle 28, 72076 Tübingen, Germany

View larger version (22K): [in a new window]   Fig. 1. Mealworms were presented consecutively on a linoleum screen and on a clutter screen with an area of 1.2 mx1.4 m. During experimental sessions, the two Myotis capaccinii (A, cap1; B, cap2), the two M. daubentonii (C, dau1; D, dau2) and the three M. dasycneme (E, dasy1; F, dasy2; G, dasy3) frequently passed over both the linoleum and the clutter screen without capturing prey (filled columns). All seven bats frequently performed capture attempts (grey columns) on mealworms on the linoleum screen. They rarely or never aimed capture attempts at a mealworm on the clutter screen. All seven bats aimed more capture attempts at mealworms on the linoleum screen than at mealworms on the clutter screen (P-values from Pearson 2-test and sample size are given on the graphs; d.f.=1 in each case).

View larger version (38K): [in a new window]   Fig. 2. In low searching flight over the experimental area, all bats emitted short, downward-modulated echolocation signals. Representative calls for Myotis capaccinii (A), M. daubentonii (B) and M. dasycneme (C) are presented as sonagrams with the averaged power spectrum on the right and the oscillogram below. The artificial probe signal that we used for ensonification (D, measured 1 m from the speaker) was designed to be similar to the signals used by the bats.

View larger version (50K): [in a new window]   Fig. 3. Six different experimental settings were ensonified with a probe signal, and the echoes were recorded. Representative recordings are plotted as oscillogram (above) and spectrogram (below) representations. In all recordings, the outgoing signal and its second harmonic can be seen as recorded by the microphone positioned 5.6 cm laterally to the speaker. These are distorted because of the frequency-specific directionality and lateral position of the speaker and microphone. A good recording of the outgoing signal is shown in Fig. 2D. We ensonified the nylon threads in the air in the absence of a mealworm (A) and in the presence of a mealworm (B), of the linoleum screen in the absence of a mealworm (C) and in the presence of a mealworm (D) and of the clutter screen in the absence of a mealworm (E) and in the presence of a mealworm (F). In all the recordings depicted here, the mealworm was positioned at approximately 90° to the impinging sound. In air and on the clutter screen, the mealworm reflected back a good copy of the ensonification signal (B,D), whereas the echo from the clutter screen did not differ when a mealworm was absent (E) or present (F). Threads in air did not reflect any conspicuous echo (A), and the recording was identical to a recording of the noise floor of our apparatus. The level of clutter echoes reflected off the linoleum screen (C) was low in comparison with that reflected off the clutter screen (E). The latter yielded a smeared echo that extended over a wide time period. The linoleum screen reflected a strong ‘ground echo’ only from immediately below the ‘artificial bat’.

View larger version (18K): [in a new window]   Fig. 4. Cross-correlation of the artificial sweep with the echoes (the right-hand column shows the time window of expected correspondence on an enlarged scale). (A) The echo of the nylon threads in air yielded no conspicuous correspondence with the sweep. (B) The echo of a mealworm suspended in air had a distinct correspondence peak with the sweep at the expected time. (C) The echo of the empty linoleum screen yielded a conspicuous correspondence with the sweep only from immediately below the ‘artificial bat’. No distinct peak was present in the time window where the mealworm echo would be expected. (D) The echo of the mealworm on the linoleum screen had a distinct correspondence peak with the sweep at the expected time. (E) The cross-correlation of the artificial sweep with the echo of the empty clutter screen was rippled because of the many overlapping echoes. (F) The cross-correlation with the echo of the mealworm on the clutter screen showed no obvious match.

View larger version (35K): [in a new window]   Fig. 5. Mean sound pressure levels of the echoes (significance levels are given in Table 2). (A) In the air, the sound pressure level (SPL) in the 2 ms time window of the expected echo (enclosed by vertical dotted lines) was significantly higher in the presence of a mealworm (red) than in its absence (black). (B) On the linoleum screen, the SPL in the same 2 ms time window was significantly higher in the presence (red) than in the absence (black) of the mealworm. (C) On the clutter screen, the SPL in the presence of the mealworm (red) did not differ significantly from that in the absence of the mealworm (black). (D) A, B and C (with mealworm) plotted on one graph for comparison. The SPL reflected by the mealworm on the linoleum screen (red) is higher than that reflected by the mealworm in air (grey). Clutter screen: black.