Many species of animals rely on acoustic communication for critical life activities. The efficacy of acoustic communication, however, can be seriously hindered by ambient noise. Although it has been widely reported that signallers modify the structure of their acoustic signals in noise, whether such signal modulations contribute to reducing noise interference is often unclear. Signal duration is a perceptually important parameter that can directly affect signal detection. Here, we address whether signal lengthening is an adaptive strategy for improving acoustic communication in noise, combing results from a meta-analysis, a noise playback experiment, and numerical simulations. The meta-analysis showed that animals from arthropods to mammals showed a mixed pattern of signal duration modifications in response to noise, covering all three possibilities of an increase, a decrease, and no change. The noise playback experiment revealed that noise-induced signal duration modulation was characterized by strong inter-individual variations and correlated positively with signal amplitude, challenging the functional role of signal lengthening in reducing noise interference. Lastly, numerical simulations suggest that experimentally observed signal duration increases in noise disappear when the effect size is considered. We conclude that signal lengthening is not a strategy for animals to mitigate noise interference, despite its wide occurrence. 

We obtained echolocation calls of the great leaf-nosed bats in silence and a noise condition of 50 dB SPL. We analysed a total of 20,6667 echolocation calls from 23 bats, with a median of 9240 calls per bat (range, 1366–15828 calls). 

Echolocation calls of the bat were recorded with an array of 16 ultrasonic microphones (NEUmic, Ultra Sound Advice, London, UK) on the floor of a recording room. The echolocation calls recorded by the microphones were high-passed filtered at 20 kHz by an analog filter (VBF40, Kemo Ltd, Dartford, UK) and digitized by a sound-recording system (PXIe 8840, with a data acquisition card PXIe 6358, National Instruments, Austin, USA) at a sampling rate of 250 kHz.