Noseleaf Dynamics during Pulse Emission in Horseshoe Bats

Lin Feng,Rolf Müller,Li Gao,Hongwang Lu,Manuel S. Malmierca

doi:10.1371/journal.pone.0034685

Abstract

Horseshoe bats emit their biosonar pulses nasally and diffract the outgoing ultrasonic waves by conspicuous structures that surrounded the nostrils. Here, we report quantitative experimental data on the motion of a prominent component of these structures, the anterior leaf, using synchronized laser Doppler vibrometry and acoustic recordings in the greater horseshoe bat (Rhinolophus ferrumequinum). The vibrometry data has demonstrated non-random motion patterns in the anterior leaf. In these patterns, the outer rim of the walls of the anterior leaf twitches forward and inwards to decrease the aperture of the noseleaf and increase the curvature of its surfaces. Noseleaf displacements were correlated with the emitted ultrasonic pulses. After their onset, the inward displacements increased monotonically towards their maximum value which was always reached within the duration of the biosonar pulse, typically towards its end. In other words, the anterior leaf’s surfaces were moving inwards during most of the pulse. Non-random motions were not present in all recorded pulse trains, but could apparently be switched on or off. Such switches happened between sequences of consecutive pulses but were never observed between individual pulses within a sequence. The amplitudes of the emitted biosonar pulse and accompanying noseleaf movement were not correlated in the analyzed data set. The measured velocities of the noseleaf surface were too small to induce Doppler shifts of a magnitude with a likely significance. However, the displacement amplitudes were significant in comparison with the overall size of the anterior leaf and the sound wavelengths. These results indicate the possibility that horseshoe bats use dynamic sensing principles on the emission side of their biosonar system. Given the already available evidence that such mechanisms exist for biosonar reception, it may be hypothesized that time-variant mechanisms play a pervasive role in the biosonar sensing of horseshoe bats.

Highlights

Microbats have evolved active biosonar systems that analyze the echoes created by the emission and reflection of ultrasonic pulses
These emission-side structures are common and conspicuous in bat species that emit their ultrasonic pulses through the nostrils, such as the Old-World horseshoe bats (Rhinolophidae) and the NewWorld leaf-nosed bats (Phyllostomidae)
During qualitative observations of these patterns, the outer rim of the anterior leaf could be seen to move away from the head and inwards whereas the rim of the anterior leaf around the nostrils stayed approximately in place. This motion resulted in a non-rigid deformation of the anterior leaf that narrowed its opening aperture and increased the curvature of the leaf surfaces

Summary

Introduction

Microbats have evolved active biosonar systems that analyze the echoes created by the emission and reflection of ultrasonic pulses. In addition to the diffraction of the incoming waves, a significant number of microbat species have evolved specialized structures that diffract the outgoing waves upon emission These emission-side structures are common and conspicuous in bat species that emit their ultrasonic pulses through the nostrils, such as the Old-World horseshoe bats (Rhinolophidae) and the NewWorld leaf-nosed bats (Phyllostomidae). In these bat groups, the baffles that diffract the outgoing waves are referred to as noseleaves

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