Influence of signal direction on sonar interference

Abstract

How bats mitigate mutual interference is a long-standing question that has ecological and technological implications as biosonar systems continue to outperform artificial sonar systems in noisy, cluttered environments. Echolocating bats display a mutual suppression response, slowing their pulse emission rates when flying in groups to gain a net improvement in sonar performance. However, flight paths, directional effects, plus contextual and attentional processes influence the levels of mutual interference, causing some noises to be more deleterious than others. To tease apart the impact of these factors, we used playback experiments with flying Mexican free-tailed bats, Tadarida brasiliensis , to test whether the relative directional orientation of an interfering signal versus its returning echoes had a greater impact on a bat's sonar performance. We tested two competing hypotheses: (1) the intensity hypothesis predicts that a signal that is emitted towards a signalling bat is louder and will mask returning echoes more than a signal that is emitted in the same direction as a bat's emitted signal; (2) the similarity hypothesis predicts that a signal that is emitted in the same direction as a bat's signal will create echoes similar in amplitude and timing to those of the signalling bat's own echoes , which should cause greater interference than a loud, less similar signal. We analysed emission rates of bats navigating in both open and cluttered spaces while we varied the directional orientation of an artificial stimulus relative to the bat's flight trajectory. Both emission rates and navigational performance decreased significantly when playback was directed in the same heading as the flying bat, indicating that competing echo wave-fronts are more disruptive to a flying bat than the much louder source signal that produced them. This observation is important because it potentially reveals the impact of attentional processes on sonar performance. This study provides new information on how echolocating bats experience interference and the behavioural strategies they use to overcome jamming. • Bat biosonar is directionally sensitive to the source of acoustic interference. • Pulses arriving from behind bats had the greatest impact on biosonar performance. • Low-intensity echo streams may be more distracting than the louder source signal. • The results have important implications for how bats space themselves in groups.