Jamming Avoidance Strategies Employed by Bats and Auditory Scene Analysis

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Hanging bats

    Echolocating bats generate high frequency echolocation calls and listen to the returning echoes to track prey, avoid obstacles and orient in space. It seems like a simply thing to do when only one bat is echolocating. However, it turns into a more complicated problem for the bat when there are several bats in the vicinity. Each individual bat need to segregate its own calls and echoes from a complex acoustic scene caused by neighboring batsí echolocation.

    In order to test how the echolocating bat uses its echolocation under complex acoustic scene, we flew paired big brown bats (Eptesicus fuscus) in a large laboratory flight room and let them compete for a single prey item. Echolocation calls were recorded to investigate how the bat adjusts its vocal flight behavior to avoid signal jamming with other bats. We found a new strategy that the bat employed to avoid signal interference, namely silence. Silent behavior is defined as at least one bat in a pair ceasing vocalization for more than 0.2 second), occurred as much as 76% of the time (mean of 40% across seven pairs) when their separation was shorter than 1 m, but only 0.08% when a single bat flew alone. Spatial separation, heading direction, and similarity in call design of paired bats were related to the prevalence of this silent behavior. Bats with similar call design showed more silent behavior than those with dissimilar call design. Our data suggest that the bat uses silence as a strategy to avoid interference from sonar vocalizations of its neighbor, while listening to conspecific-generated acoustic signals to guide orientation.

    The other strategy, which is commonly reported in previous research, is that individual bats adjust their echolocation call design, such as call frequency, duration or sweep rate, to avoid signal jamming with conspecifics. Bats adapt their echolocation call design to cope with dynamic changes in the acoustic environment, including habitat change or the presence of nearby conspecifics/heterospecifics. Our data showed that differences in five call parameters, start/end frequencies, duration, bandwidth and sweep rate, significantly increased in the two-bat condition, compared with the baseline data. In addition, the magnitude of spectral separation of calls was negatively correlated with the baseline call design differences in individual bats. Similar to what we found in the silent behavior, bats with small baseline call frequency differences showed larger increases in call frequency separation when paired than those with large baseline frequency differences, suggesting that bats must actively change their sonar call structure if pre-existing differences in call design are small.

    This study reports on adaptive echolocation of free-flying big brown bats when flying in pairs. Free-flying bats provide the chance to observe their nearly natural behavior, but stimuli in an environment is harder to control than experiments with the bat restrained in one place. Research findings from this study motivate further investigation. Psychoacoustic experiments are necessary to determine the batís ability to discriminate different signals, determine the 3-D position of an object, recognize meaningful sounds from background noise, etc.