Response Of Bat Echolocation Behavior To Ambient Noise

Acoustic communication plays a crucial role in the survival and reproduction of many animals.However,ambient noise has an impact on animal acoustic communication.Many species have evolved short-term adaptations to mitigate interference from background noise.A diverse range of animals alter the acoustic structure of their vocalizations in the presence of background noise,such as the increase in amplitude(the Lombard effect),the adjustment in signal duration,spectral structure of signals and repetition/redundancy.Although noise-induced vocal adjustments have been reported in many animals,whether and why animals increase signal duration in noise remain controversial.Echolocating bats serve as a mammalian model for studies of vocal control.The great leaf-nosed bat(Hipposideros armiger),a constant-frequency(CF)bat,was used to study the response of echolocation call duration and other acoustic parameters to ambient noise.A multi-channel microphone array was used to record 203 856 echolocation calls of 22 adult bats under silence and broadband white noise conditions.By analyzing the statistical differences of various acoustic parameters of echolocation modification at both call and sub-call levels under silence and noise conditions,we addressed the questions of(1)how do constant frequency bats adjust echolocation call in response to ambient noise and whether bats would increase their echolocation call duration as generic vocal responses to noise,and(2)whether CF bats in noise would change their signal duration differently at the call level and the sub-call level.The main results were showed as follows:The amplitude of echolocation call,CF component and FM component in the noise were significantly increased(all P < 0.05),but there were amplitude compensation differences at the sub-call level(P < 0.05).At the same time,under the noise condition,the duration and bandwidth of FM component were significantly increased(all P <0.05),and the end frequency of FM component was significantly decreased(P < 0.05).However,the call duration(P = 0.35),the peak frequency the CF component(P = 0.71)and the duration of the CF component(P = 0.93)were not changed.At the overall level,the amplitude and duration of the echolocation calls,the amplitude and duration of CF component,and the amplitude and duration of FM component were significantly positively correlated under noise and silence conditions(all P < 0.05).At the individual level,the correlation between these acoustic parameters showed similar results(all P < 0.05).The duration of the FM component was correlated with the bandwidth of the FM component when controlling for the amplitude of the FM component(P < 0.05).The study showed that the great leaf-nosed bats are able to respond to noise by adjusting the amplitude,spectral structure and duration of their echolocation calls.Bats increase their echolocation call amplitude as generic vocal response to noise.The modification of signal duration has significant individual differences.The duration of FM component may increase with the increase of the amplitude and bandwidth of FM component,and this association exists not only at the whole level but also at the individual level.Animals using long vocalizations do not necessarily increase the duration of long acoustic signals in noise.It is important to analyze vocalizations at both the call and sub-call levels and considering the biomechanical linkages between vocal modifications when studying animals’ vocal response to interfering noise.