Signal lengthening is not a strategy for animals to mitigate noise interference

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. 

Methods
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.

诸多动物物种依赖声学通讯（acoustic communication）完成关键生命活动。
然而，声学通讯的效能会受到环境噪声的严重干扰。尽管已有大量研究报道信号发送者会在噪声环境下调整其声学信号的结构，但这类信号调制是否有助于降低噪声干扰，目前仍不明确。
信号时长是一项具有重要感知意义的参数，可直接影响信号的检测效果。本研究结合荟萃分析（meta-analysis）、噪声回放实验（noise playback experiment）与数值模拟（numerical simulations）的结果，探讨信号时长延长是否为提升噪声环境下声学通讯效能的适应性策略。
荟萃分析结果显示，从节肢动物（arthropods）到哺乳类的各类动物，在噪声下的信号时长调整呈现混合模式，涵盖延长、缩短以及无变化三种情况。
噪声回放实验表明，噪声诱导的信号时长调制存在显著的个体间差异（inter-individual variations），且与信号振幅呈正相关，这对信号时长延长以降低噪声干扰的功能作用提出了挑战。
最后，数值模拟结果显示，若考虑效应量（effect size），实验中观察到的噪声环境下信号时长延长现象会不复存在。综上，尽管信号时长延长现象广泛存在，但并非动物用以缓解噪声干扰的适应性策略。

研究方法
我们获取了大蹄蝠（great leaf-nosed bats）在静音环境与50 dB SPL噪声环境下的回声定位声波（echolocation calls）。我们共分析了23只蝙蝠的20,6667个回声定位声波，每只蝙蝠的声波样本中位数为9240个（范围：1366–15828个）。
本研究在录音室地面布置了由16个超声麦克风（ultrasonic microphones）组成的阵列（NEUmic, Ultra Sound Advice, London, UK），用于记录蝙蝠的回声定位声波。麦克风记录到的回声定位声波先通过模拟滤波器（VBF40, Kemo Ltd, Dartford, UK）进行20 kHz高通滤波，随后由录音系统（PXIe 8840，配数据采集卡PXIe 6358, National Instruments, Austin, USA）以250 kHz的采样率进行数字化处理。