Information Generated by the Moving Pinnae of Rhinolophus rouxi: Tuning of the Morphology at Different Harmonics

Abstract

Bats typically emit multi harmonic calls. Their head morphology shapes the emission and hearing sound fields as a function of frequency. Therefore, the sound fields are markedly different for the various harmonics. As the sound field provides bats with all necessary cues to locate objects in space, different harmonics might provide them with variable amounts of information about the location of objects. Also, the ability to locate objects in different parts of the frontal hemisphere might vary across harmonics. This paper evaluates this hypothesis in R. rouxi, using an information theoretic framework. We estimate the reflector position information transfer in the echolocation system of R. rouxi as a function of frequency. This analysis shows that localization performance reaches a global minimum and a global maximum at the two most energetic frequency components of R. rouxi call indicating tuning of morphology and harmonic structure. Using the fundamental the bat is able to locate objects in a large portion of the frontal hemisphere. In contrast, using the 1 overtone, it can only locate objects, albeit with a slightly higher accuracy, in a small portion of the frontal hemisphere by reducing sensitivity to echoes from outside this region of interest. Hence, different harmonic components provide the bat either with a wide view or a focused view of its environment. We propose these findings can be interpreted in the context of the foraging behaviour of R. rouxi, i.e., hunting in cluttered environments. Indeed, the focused view provided by the 1 overtone suggests that at this frequency its morphology is tuned for clutter rejection and accurate localization in a small region of interest while the finding that overall localization performance is best at the fundamental indicates that the morphology is simultaneously tuned to optimize overall localization performance at this frequency.

The furrows of Rhinolophidae revisited

“…Indeed, we have demonstrated earlier that the noseleaf effects on the emission beam are attenuated when considering the spatial sensitivity of the complete echolocation system [5]. Furthermore, for Rhinolophidae, including the effects of pinnae into the analysis is possibly even more important as Rhinolophidae use ear movements to generate localization cues [6]. Hence, the results we report take into account both the noseleaf and the (moving) pinnae morphologies.…”

“…As the methods employed in this paper have been discussed in detail elsewhere [6,7], we provide only a summary.…”

“…This was performed to allow modelling the structure of the noseleaf in greater detail (see Vanderelst et al [6] for details).…”

“…The effect of the changes in the emission beam pattern on the localization performance was estimated using the model of echolocation in Rhinolophidae proposed by Vanderelst et al [6]. This information theoretic model returns an estimate of the uncertainty (i.e.…”

Adaptive beam-width control of echolocation sounds by CF–FM bats,Rhinolophus ferrumequinum nippon, during prey-capture flight

“…These results imply that the noseleaf is likely to focus the beam pattern of nasal emitters. In addition, numerical computations based on the 3D structure of the emitter portion of bats have demonstrated that the noseleaf affects the directivity pattern of the emitted pulse (Zhuang and Müller, 2006;Zhuang and Müller, 2007;Vanderelst et al, 2010;Vanderelst et al, 2011). In the present study, horizontal beam width of R. ferrumequinum nippon was ±22±5deg, and was expanded to ±36±7deg in the terminal phase.…”