Males increase call frequency, not intensity, in response to noise, revealing no Lombard effect in the little torrent frog.

Longhui Zhao,Jianghong Ran,Yezhong Tang,Jianguo Cui,Yue Yang,Qinghua Chen,Jichao Wang,Steven E Brauth,Xiaoqian Sun

doi:10.1002/ece3.4625

Abstract

Noise is one of the main factors that can influence the processes of sound communication across a wide range of animal groups. Although the effects of ambient noise on animal communication, including anthropogenic noise, have received increasing attention, few studies have examined changes in the fine structure of acoustic signals produced by vocalizing species in constantly noisy environments. Here, we used natural recordings to determine the associations between stream noise and call parameters in the little torrent frog (Amolops torrentis). We also used playbacks of stream noise recorded in natural habitats and playbacks of white noise to examine how male vocal signals change with increasing noise levels. The results show that noise intensity has a significant effect on male call frequency, but not on call amplitude or other call characteristics. Based on this evidence, we suggest that in streamside species stream noise drives males to alter call frequency and call as loudly as possible in order to improve discriminability. These findings provide insights into the role played by ecological selection in the evolution of noise‐dependent anuran vocal plasticity.

Highlights

Acoustic signals are used by many animal species to find mating part‐ ners, integrate spatiotemporal information, deter rivals, and defend against predators (Bradbury & Vehrencamp, 2011)
A sound level meter and directional microphone connected to a digital recorder were used to measure the SPLs of calls and ambient noise and to record calls and am‐ bient noise, respectively, at a point 50 cm from the frog aligned with the snout‐vent orientation of the subject
The maximum frequency and bandwidth increased significantly as noise levels increased while the call amplitude did not exhibit any obvious changes

Summary

| INTRODUCTION

Acoustic signals are used by many animal species to find mating part‐ ners, integrate spatiotemporal information, deter rivals, and defend against predators (Bradbury & Vehrencamp, 2011). While noise‐dependent short‐term vocal plasticity has been well examined, less is known about adaptive changes in con‐ stantly natural noisy environments (Brumm & Slabbekoorn, 2005; Röhr, Paterno, Camurugi, Juncá, & Garda, 2015) These adap‐ tive changes may predict long‐term evolutionary processes asso‐ ciated with fluctuating ambient noise conditions such as increased anthropogenic noise. Anurans are one of main taxa that rely heavily on acous‐ tic signals for communication, and only a limited number of taxa have been used in previous studies concerning the influence of environ‐ mental selection on anuran call characteristics (Boeckle, Preininger, & Hödl, 2009; Penna & Hamilton‐West, 2007; Penna, Pottstock, & Velasquez, 2005) For this reason, more work on the effects of noise on the acoustics of multiple signal characteristics is needed. We verified some of our findings by estimating the effect of the density of calling neighbors on vocal behavior

| MATERIALS AND METHODS

Findings

| DISCUSSION