Variability in Echolocation Call Intensity in a Community of Horseshoe Bats: A Role for Resource Partitioning or Communication?

Only recently data on bat echolocation call intensities is starting to accumulate. Yet, intensity is an ecologically crucial parameter, as it determines the extent of the bats' perceptual space and, specifically, prey detection distance. Interspecifically, we thus asked whether sympatric, congeneric bat species differ in call intensities and whether differences play a role for niche differentiation. Specifically, we investigated whether R. mehelyi that calls at a frequency clearly above what is predicted by allometry, compensates for frequency-dependent loss in detection distance by using elevated call intensity. Maximum echolocation call intensities might depend on body size or condition and thus be used as an honest signal of quality for intraspecific communication. We for the first time investigated whether a size-intensity relation is present in echolocating bats.We measured maximum call intensities and frequencies for all five European horseshoe bat species. Maximum intensity differed among species largely due to R. euryale. Furthermore, we found no compensation for frequency-dependent loss in detection distance in R. mehelyi. Intraspecifically, there is a negative correlation between forearm lengths and intensity in R. euryale and a trend for a negative correlation between body condition index and intensity in R. ferrumequinum. In R. hipposideros, females had 8 dB higher intensities than males. There were no correlations with body size or sex differences and intensity for the other species.Based on call intensity and frequency measurements, we estimated echolocation ranges for our study community. These suggest that intensity differences result in different prey detection distances and thus likely play some role for resource access. It is interesting and at first glance counter-intuitive that, where a correlation was found, smaller bats called louder than large individuals. Such negative relationship between size or condition and vocal amplitude may indicate an as yet unknown physiological or sexual selection pressure

Spatial echo suppression and echo-acoustic object normalization in echolocating bats

The processing of acoustic cues is critical for all animals in a wide range of behaviours including orientation, predator-prey interactions and social communication. The auditory system can process these sound information with amazing precision. Echolocating bats have developed an extraordinary ability to deal with acoustic cues. Their echo-imaging system has enabled them to detect, pursue and capture tiny prey like insects, to avoid obstacles and to interact with their environment, often in total darkness. Bats heavily rely on the evaluation of echoes for orientation and hunting. The evaluation of external, echolocation- independent sounds also plays an important role for bats, e.g. while localizing prey via prey-generated noise or for social purposes. The current thesis addresses two different aspects of the very complex echo-acoustic situation these extraordinary animals are confronted with in their daily life. The first approach of this thesis is concerned with the question how bats deal with misleading spatial information of echoes. Acoustic orientation most often takes place in echoic environments. Accurate sound localization in natural, echoic environments is a vital task of the auditory system. Many behavioral studies have shown that for accurate sound localization, the auditory system relies only on the spatial information provided by the first wave front and that spatial information of the (delayed) echoes is suppressed (‘precedence effect’). For a bat, this approach is also useful when localizing external, echolocation-independent sound sources, but it is in conflict with the processing of the echoes of self-generated sounds in an echolocation context. In a two-alternative, forced choice paradigm, it is investigated whether and to what extend the echolocating bats Megaderma lyra and Phyllostomus discolor spontaneously suppress the spatial information of either a second echo of their sonar emission or echoes of different external, echolocation-independent sounds. In general, M. lyra and P. discolor did not suppress the spatial information of a second echo independent of the delay. Only one M. lyra showed significant echo suppression. However, this suppression could not be confirmed in an exact repetition of the experiment. Furthermore, it is shown that in the bat M. lyra, spatial echo suppression is restricted to an external sound which carries semantic meaning for the bat, in this case, a typal contact call. Abstract sounds like an acoustic impulse, a time-inverted contact call, or only the first syllable of the contact call do not induce spontaneous echo suppression. The current data indicate that while bats may be able to suppress the spatial information of echoes, this is not their default mode of auditory processing. The reason for this exceptional absence of spatial echo suppression may lie in the shorter time constants of cochlear processing in the ultrasonic frequency range and the strong influence of cognitive components associated with the precedence effect. This study emphasises the contribution of high-level semantic auditory processing to echo suppression. The aim of the second approach was to characterize how echolocating Phyllostomus discolor deals with size-induced variations in echoes due to different-sized ensonified objects. Echolocating bats can identify three-dimensional objects exclusively through the analysis of acoustic echoes of their ultrasonic emissions. However, objects of the same structure can differ in size and the auditory system must achieve a size-invariant, normalized object representation for reliable object recognition. This study describes the behavioral classification of echoes of complex virtual objects that vary in object size. In a phantom-target playback experiment, it is shown that the bat P. discolor spontaneously classified most scaled versions of objects according to trained standards. This psychophysical performance is reflected in electrophysiological responses of a population of cortical units received from a cooperated study, which showed an object-size invariant response. The current results indicate that echolocating bats have indeed a concept of auditory object normalization

Object-Oriented Echo Perception and Cortical Representation in Echolocating Bats

Echolocating bats can identify three-dimensional objects exclusively through the analysis of acoustic echoes of their ultrasonic emissions. However, objects of the same structure can differ in size, and the auditory system must achieve a size-invariant, normalized object representation for reliable object recognition. This study describes both the behavioral classification and the cortical neural representation of echoes of complex virtual objects that vary in object size. In a phantom-target playback experiment, it is shown that the bat Phyllostomus discolor spontaneously classifies most scaled versions of objects according to trained standards. This psychophysical performance is reflected in the electrophysiological responses of a population of cortical units that showed an object-size invariant response (14/109 units, 13%). These units respond preferentially to echoes from objects in which echo duration (encoding object depth) and echo amplitude (encoding object surface area) co-varies in a meaningful manner. These results indicate that at the level of the bat's auditory cortex, an object-oriented rather than a stimulus-parameter–oriented representation of echoes is achieved

Intraspecific call intensity relations.

<p>Call intensities (in dB SPL; calculated for 10cm distance to the bats' nose) for individual <i>R. ferrumequinum</i> (Rf, N = 12), <i>R. mehelyi</i> (Rm, N = 11), <i>R. euryale</i> (Re, N = 12) and <i>R. hipposideros</i> (Rh, N = 10) are plotted against <b>A</b>: forearm length (FA), <b>B</b>: body mass index (BMI) and <b>C</b>: body mass. For statistics, see text. (Rb only used for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012842#pone-0012842-g003" target="_blank">Fig. 3</a> because of missing body size data)</p

Estimated maximum prey detection distances for the five European species of horseshoe bat.

<p>The dark grey bars indicate maximum detection distances for large insects (target strength (TS) = −30 dB) for either an echo perception threshold (thresh) of 0 dB SPL or 20 dB SPL; the light grey bars show maximum detection distances for small insects (target strength (TS) = −60 dB) for either an echo perception threshold of 0 dB SPL or 20 dB SPL. Calculations were based on average maximum call intensities and an average peak echolocation call frequency as measured in this study. Species abbreviations as in the other figures. ‘Rm scaled’ indicated detection distances <i>R. mehelyi</i> would experience if the species called at the intensity we measured, but at a frequency as predicted by allometric scaling (97 kHz instead of 108 kHz).</p

Interspecific call intensity relations.

<p>Maximum call intensities (in dB SPL; calculated for 10cm distance to the bats' nose) for <i>R. ferrumequinum</i> (Rf, N = 12), <i>R. blasii</i> (Rb, N = 6), <i>R. mehelyi</i> (Rm, N = 11), <i>R. euryale</i> (Re, N = 12) and <i>R. hipposideros</i> (Rh, N = 10). Lines and asterisks indicate the significant outcomes from all possible post-hoc pair wise comparisons (t-tests, p-values Bonferroni corrected, ***<0.0001).</p

CF frequencies of sympatric horseshoe bats from Bulgaria are plotted against forearm length, the standard size measure in bats, for <i>Rhinolophus ferrumequinum</i> (Rf, N = 25), <i>R. blasii</i> (Rb, N = 56), <i>R. mehelyi</i> (Rm, N = 85), <i>R. euryale</i> (Re, N = 116) and <i>R. hipposideros</i> (Rh, N = 10).

<p>The peak echolocation call frequencies used by Rm strongly overlapped those of Re and Rh (dotted lines), while the bands used by Rf and Rb were clearly separated.</p

Intraspecific call intensity relations and intra-individual variation.

<p>Averaged call intensities of the six highest intensities of each individual (in dB SPL; calculated for 10cm distance to the bats' nose) for individual <i>R. ferrumequinum</i> (Rf, N = 12), <i>R. blasii</i> (Rb, N = 6), <i>R. mehelyi</i> (Rm, N = 11), <i>R. euryale</i> (Re, N = 12) and <i>R. hipposideros</i> (Rh, N = 10) are plotted against the corresponding averaged peak echolocation call frequencies. Error bars show the corresponding standard deviations.</p