Fast-moving bat ears create informative Doppler shifts

Abstract

Many animals have evolved adept sensory systems that enable dexterous mobility in complex environments. Echolocating bats hunting in dense vegetation represent an extreme case of this, where all necessary information about the environment must pass through a parsimonious channel of pulsed, 1D echo signals. We have investigated whether certain bats (rhinolophids and hipposiderids) actively create Doppler shifts with their pinnae to encode additional sensory information. Our results show that the bats' active pinna motions are a source of Doppler shifts that have all attributes required for a functional relevance: (i) the Doppler shifts produced were several times larger than the reported perception threshold; (ii) the motions of the fastest moving pinna portions were oriented to maximize the Doppler shifts for echoes returning from the emission direction, indicating a possible evolutionary optimization; (iii) pinna motions coincided with echo reception; (iv) Doppler-shifted signals from the fast-moving pinna portion entered the ear canal of a biomimetic pinna model; and (v) the time-frequency Doppler shift signatures were found to encode target direction in an orderly fashion. These results indicate that instead of avoiding or suppressing all self-produced Doppler shifts, rhinolophid and hipposiderid bats actively create Doppler shifts with their own pinnae. These bats could hence make use of a previously unknown nonlinear mechanism for the encoding of sensory information, based on Doppler signatures. Such a mechanism could be a source for the discovery of sensing principles not only in sensory physiology but also in the engineering of sensory systems.biosonar | ear motions | Doppler shifts | time-frequency signatures | nonlinear sensing C onspicuous pinna motions (1-5) are an integral part of biosonar behaviors in horseshoe bats and Old World leafnosed bats [families Rhinolophidae and Hipposideridae (6, 7)]. These motions have been demonstrated to enhance sensing and navigation performance (8-10), but the functional role of these dynamic features and the underlying mechanisms have yet to be fully understood (7). In the acoustic domain, source or receiver motions can result in Doppler shifts, i.e., nonlinear scaling of signals in time and frequency. The possibility of pinna-induced Doppler shifts had been mentioned as an aside in early work by Pye and coworkers (2, 3, 11) but has not been considered further-let alone investigated in any depthin the literature since. This complete neglect is regrettable, because ear-generated Doppler shifts could constitute a previously unknown, nonlinear mechanism for the active encoding of sensory information. To test this hypothesis, we have carried out a quantitative experimental investigation of the hypothesis that pinna motions cause functionally relevant Doppler shifts in bats. For pinna-generated Doppler shifts to have a functional role, five necessary conditions must be met: (i) pinna surface speeds must be high enough to produce Doppler shifts that exceed the animal's per...

Biomimetic detection of dynamic signatures in foliage echoes

“…Building on these prior findings, more recent analyses have demonstrated that the timevariant effects can enhance sensory-coding capacity and the ability to determine the direction of a sound source [59,60]. Researchers experimentally replicated the fast ear motions of the horseshoe bat pinna, and recorded CF signals emitted from different azimuth and elevation angles with respect to the receiver [60]. Their results demonstrated direction-dependent Doppler shifts in the received signals, and this finding suggested that the Doppler shifts due to this pinna motion can potentially aid in direction finding [60].…”

“…Building on these prior findings, more recent analyses have demonstrated that the timevariant effects can enhance sensory-coding capacity and the ability to determine the direction of a sound source [59,60]. Researchers experimentally replicated the fast ear motions of the horseshoe bat pinna, and recorded CF signals emitted from different azimuth and elevation angles with respect to the receiver [60].…”

“…Researchers experimentally replicated the fast ear motions of the horseshoe bat pinna, and recorded CF signals emitted from different azimuth and elevation angles with respect to the receiver [60]. Their results demonstrated direction-dependent Doppler shifts in the received signals, and this finding suggested that the Doppler shifts due to this pinna motion can potentially aid in direction finding [60]. Most recently, experimental lab-based studies have sought to analyze the effects of these dynamics within echoes, instead of direct emissions.…”

The extrafoveal preview paradigm as a measure of predictive, active sampling in visual perception

“…Numerous studies have shown that humans and other animals change the rate and force of inhaling based on the presence of odors. Many animals turn their heads, or even alter the position or shape of their ears, in order to use that alteration in order to gain specific information about the incoming sound for localization and recognition (for an example, see Yin & Müller, 2019). A particular example is the star-nosed mole that haptically senses potential prey by a specialized nose organ (Catania, 2011).…”

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“…Different bat species utilize a variety of actions to enhance sonar localization. Bats in the Rhinolophid and Hipposiderid families move the pinnae rapidly in the anterior and posterior directions during call production and reception [ 5 ]. These movements of the pinnae create interaural intensity differences and Doppler shifts in the constant frequency (CF) signals used by these bat species, which help them localize sound sources, particularly in the vertical plane [ 6 ].…”