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Intensity control during target approach in echolocating bats; stereotypical sensori-motor behaviour in Daubenton's bats, Myotis daubentonii

Arjan Boonman^* and Gareth Jones

School of Biological Sciences, University of Bristol, Woodland Road, BS8 1UG Bristol, UK

View larger version (19K): [in a new window]   Fig. 1. A sonagram of calls emitted by a Daubenton's bat attacking a target. The vertical arrow indicates the time at which the bat first hits the target. The lower panel shows an enlargement of pulses `a' and `b', representing a pulse emitted far away (±1.5m) and near (±0.1m), respectively, from the target. The consecutive panels show that the pulse repetition rate is increased during the approach, whereas the bandwidth of the first harmonic is decreased during the final stage of target approach. At this stage, energy in higher (>3rd) harmonics can be visible when the bat is recorded at a close distance (upper panel).

View larger version (22K): [in a new window]   Fig. 3. The emitted intensity (rms SPL at 10 cm) by bat individual 4 while attacking the small sphere, measured over two flights, indicated by diamonds and triangles. During the approach the bat reduces intensity over time in a seemingly disorganised fashion (A). However, when plotted against distance between bat and target (B), the same emitted intensities appear to fit the distance to the target with much less variation around the best-fitting regression line. Not all individuals showed a clear decrease in variability in intensity reduction over distance compared with the intensity reduction over time. (A) r2=0.4129, F1,37=25.023, P<0.001. (B) r2=0.7738, F1,37=117.22, P<0.001.

View larger version (20K): [in a new window]   Fig. 2. The intensity compensation behaviour of bat 1 while approaching the big sphere. (A) Emitted sound pressure level (SPL) per halving of distance is reduced by 4 dB (open diamonds); SPL incident on the target increases by 2.0 dB per halving of distance (filled squares). (B) Received SPL per halving of distance increases by 7 dB. The function of received SPL between 0 and 70 cm from target is given by: SPLpeak=-6.9xlog2(d)+41.73, where d = distance (in m).

View larger version (17K): [in a new window]   Fig. 4. Intensity reduction is more dissimilar from the average intensity reduction when plotted against time, than against distance. Equal dissimilarity would result in a slope of 1. Dissimilarity from average intensity reduction over time (x-axis) is plotted against dissimilarity from average intensity reduction over distance (y-axis), and the units of both are % dissimilarity from slope of the average linear regression line of reduction against time and distance, respectively.