The receiving beam of freely flying bats

Felix Theo Häfele*

*Corresponding author for this work

Research output: ThesisPh.D. thesis

Abstract

Sound facilitates an abundance of signal types with varying complexity, information density and transmission range and it is an essential modality used by animals across taxa for communication, navigation and survival. Utilization of acoustic information is arguable at its most extreme for echolocating animals. Echolocators sample their environment through sound emission and subsequent analysis of returning echos. Echolocating bats employ a high frequency, high directional beam to probe their environment, which achieves high source levels, reduces clutter echoes, and provides inherent directional information. Bats adapt their calls to their immediate context through modification of frequency, intensity, duration, rate and directionality. These traits likely enable efficient detection, localization, and capture of small prey.

Fast and precise acoustic localization is essential as bats navigate and hunt in complete darkness. Both, emission, and reception dictate the spatial representation of the echo-scene presented to the bat. Similar to bats' emission, highly directional receiving beams have been measured for dead or stationary individuals across a wide range of bat families. However, it is clear that active receiver modification on par with dynamic emission adjustments could exert enhanced control of the acoustic scene for bats, making it essential to measure the receiving beam in freely flying behaving animals. The shape and orientation of the receiving beam affects the monaural and binaural cues which are vital for biosonar-based vertical and horizontal source localization. Presumably bats evaluate frequency specific level differences between both ears to position objects horizontally, so-called Interaural Intensity Differences (IIDs). However, an IID optimized receiving beam orientation sacrifices forward sensitivity, a likely crucial trade-off given the dynamics and complexity of the environment in which bats listen for very weak echoes. In my thesis I employed a specially constructed wind tunnel to study the pinna orientation of freely flying aerial hawking vespertilionid bats, using high speed stereo video and concurrently mapped out the in-flight emission beam using a 21-microphone array. I tracked facial features marker-less using an artificial neural net to reconstruct the 3D head and ear morphology to analyze the auditory focus of bats in flight.

My results reveal that all three investigated bat species utilize a forward-facing ear orientation with strong overlap of left and right hearing beam centered on prey. Due to the large overlap the bats achieve notably weaker IIDs than previously reported, which seriously questions the effectiveness of utilizing in-flight level differences for localization. Nonetheless, the focused hearing, aligned with the emission beam has the benefit of creating a sharp combined beam with strong off-axis attenuation which may allow localization based on directional cues. Moreover, in the terminal phase and highly correlated to the buzz we see dynamic receiver modifications from Myotis nattereri. Through the addition of lower frequencies in the buzz, the receiving beam broadens. Furthermore, M. nattereri move their ears primarily downwards shortly before the capture motion with the interfemoral membrane. We interpret the change of receiver orientation as an anticipatory motion to keep prey in the focused receiving beams and to counteract the off-axis emission beam during the capture motion. Preliminary data from Myotis daubentonii indicates that dynamic receiver motion may occur across all aerial hawking vespertilionids. The striking similarity of receiver orientation across different investigated niche-specialized bats may indicate a general preference for aerial hawking bats favoring sensitivity and sonar range over localization acuity. The loss in binaural acuity may be compensated through directional information gain which might achieve the necessary in-flight localization accuracy through beaming.
Original languageEnglish
Awarding Institution
  • University of Southern Denmark
Supervisors/Advisors
  • Jakobsen, Lasse, Supervisor
Date of defence30. Apr 2024
Publisher
DOIs
Publication statusPublished - 11. Apr 2024

Note re. dissertation

Print copy of the thesis is restricted to reference use in the Library. 

Keywords

  • bat
  • echolocation
  • ear morphology
  • echo reception
  • dynamic control
  • emission beam
  • receiving beam
  • free flight

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