Echolocating Daubenton's bats call louder, but show no spectral jamming avoidance in response to bands of masking noise during a landing task.

Echolocating bats listen for weak echoes to navigate and hunt, which makes them prone to masking from background noise and jamming from other bats and prey. As for electrical fish that display clear spectral jamming avoidance responses (JAR), bats have been reported to mitigate the effects of jamming by shifting the spectral contents of their calls, thereby reducing acoustic interference to improve echo-to-noise ratio (ENR). Here, we tested the hypothesis that frequency-modulating bats (FM bats) employ a spectral JAR in response to six masking noise bands ranging from 15 to 90 kHz, by measuring the -3 dB endpoints and peak frequency of echolocation calls from five male Daubenton's bats (Myotis daubentonii) during a landing task. The bats were trained to land on a noise-generating spherical transducer surrounded by a star-shaped microphone array, allowing for acoustic localization and source parameter quantification of on-axis calls. We show that the bats did not employ spectral JAR as the peak frequency during jamming remained unaltered compared with that of silent controls (all P>0.05, 60.73±0.96 kHz, mean±s.e.m.), and -3 dB endpoints decreased in noise irrespective of treatment type. Instead, Daubenton's bats responded to acoustic jamming by increasing call amplitude via a Lombard response that was bandwidth dependent, ranging from a mean of 0.05 dB/dB (95% confidence interval 0.04-0.06 dB/dB) noise for the most narrowband noise (15-30 kHz) to 0.17 dB/dB (0.16-0.18 dB/dB) noise for the most broadband noise (30-90 kHz). We conclude that Daubenton's bats, despite having the vocal flexibility to do so, do not employ a spectral JAR, but defend ENRs via a bandwidth-dependent Lombard response.