Adaptive behavior for texture discrimination by the free-flying big brown bat, Eptesicus fuscus
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This study examined behavioral strategies for texture discrimination by echolocation in free-flying bats. Big brown bats, Eptesicus fuscus, were trained to discriminate a smooth 16 mm diameter object (S+) from a size-matched textured object (S−), both of which were tethered in random locations in a flight room. The bat’s three-dimensional flight path was reconstructed using stereo images from high-speed video recordings, and the bat’s sonar vocalizations were recorded for each trial and analyzed off-line. A microphone array permitted reconstruction of the sonar beam pattern, allowing us to study the bat’s directional gaze and inspection of the objects. Bats learned the discrimination, but performance varied with S−. In acoustic studies of the objects, the S+ and S− stimuli were ensonified with frequency-modulated sonar pulses. Mean intensity differences between S+ and S− were within 4 dB. Performance data, combined with analyses of echo recordings, suggest that the big brown bat listens to changes in sound spectra from echo to echo to discriminate between objects. Bats adapted their sonar calls as they inspected the stimuli, and their sonar behavior resembled that of animals foraging for insects. Analysis of sonar beam-directing behavior in certain trials clearly showed that the bat sequentially inspected S+ and S−.
KeywordsTexture discrimination Echolocation Bat Adaptive sonar Frequency-modulated sonar signals
This research was supported by the NSF grant, “Active Sensing for Three-Dimensional Auditory Localization” to CFM, an NSF-REU award to BF and an HHMI Undergraduate Research Fellowship and Senior Summer Scholars awards to TW. Data were collected under a research protocol approved by the University of Maryland Institutional Animal Care and Use Committee. We would also like to thank Ray Gracon, Amaya Perez, Wei Xian and Kaushik Ghose for their assistance.
Supplementary material 1 Video animation of trial presented in Fig. 5a. Video and sound data were slowed down by a factor of 10. The upper-right panel shows the high-speed video recording from each trial. The upper-left panel shows a top-down view of the flight room (note: view rotated in relation to Fig. 5 to match the high-speed video view). The smooth object (S+) is displayed as a white circle, and the textured S- is displayed as a red square. The bat is shown in brown (note: cartoon head aim not corrected for the direction of the sonar beam but instead shows the direction of flight path). The flight path is shown in blue and the vocalization beam directions are indicated by black lines for the first pass and gray lines for the second pass. In this trial, the bat first inspects S- and then immediately goes on to hit S+, as indicated by the beam directions calculated by the 16 microphone array. There is a decrease in the durations of the vocalizations and the pulse interval between vocalizations leading up the inspection of S-. The last vocalization of inspection of S- was highlighted in pink. Before the bat passes S-, the bat makes a vocalization beyond S- (beam directions have been scaled to the minimum overlap zone), indicating a shifting of gaze beyond S- (green). During the final approach and hit of S+, the bat decreases pulse interval and duration of its vocalizations and locks its beam direction to the target (yellow) (AVI 1501 kb)
Supplementary material 2 Video animation of trial presented in Fig. 5b. In this trial, the bat begins with an inspection of S-. The bat makes a loop around the room and ultimately hits S+. The second pass around the targets is indicated in gray. Note the decrease in pulse interval and duration as the bat inspects the targets and also when it hits S+ (AVI 2714 kb)
Supplementary material 3 Video animation of trial presented in Fig. 5c. This trial is similar to SM1 in that the bat first inspects S- and then immediately hits S+. There is a decrease in pulse interval and slight decrease in duration as the bat inspects S- (AVI 1195 kb)
Supplementary material 4 Video animation of trial presented in Fig. 5d. In this trial, the bat makes two passes at the objects. In the first pass, the bat inspects S- and then it flies beneath it. In the second pass, the bat inspects S+ before ultimately hitting S+. Due to the close spacing between S+ and S- in this trial, we were unable to determine if the bat inspected each object sequentially or if it inspected both objects at the same time (AVI 2801 kb)
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