Ultrasonic frogs call at a higher pitch in noisier ambiance

Abstract

The ultrasonic communication in Concave-eared torrent frogs Odorrana tormota is believed to be an adaptation to avoid masking by the intense low-frequency noise of the rushing stream in their habitat. The acoustic adaptation hypothesis for ultrasonic origin predicts that some organisms subjecting to persistent acoustic interference from broadband, low-frequency environmental noise, might shift their signal frequency upward into frequency bands with lower noise energy. In other words, low-frequency environmental noise might cause upward shifts of species’ vocalization frequencies making their signals more conspicuous. Presently, it is unclear whether male O. tormota adjust their signal features in response to a change in the ambient noise level. We tested the prediction of the acoustic adaptation hypothesis by recording the vocalizations of male O. tormota inhabiting two streams with different background noise levels in Huangshan in central China and comparing their call features including the fundamental frequency (F0). Results showed that the spectrotemporal characteristics of the vocal signals of males in the two habitats were indifferent, except the duration of the call harmonic segments and three parameters related to the call fundamental frequency (F0). In terms of the F0, the pooled and individual frog data showed that frogs inhabiting the noisier habitat tended to emit calls having higher F0. The higher F0 increases the signal-to-noise ratio, thus benefiting the detection of vocalization. Thus, similar to several anuran species, concave-eared torrent frogs also display noise-dependent adjustment of vocal pitch in their vocalizations for making them more audible.

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

“…The amplitude of the environmental noise level experienced by the concave‐eared torrent frog is comparable to that of the little torrent frog (Zhang et al, ); however, call amplitude in the concave‐eared torrent frog can increase from 70–75 to 80–85 dB when exposed to 53–83 dB noise (Shen & Xu, ). In a previous study, ambient noise and call characteristics were compared across three streamside species (Zhao, Wang, et al, ).…”

“…The acoustic adaptation hypothesis regards this shift as an adaptive adjustment to avoid the low frequencies most susceptible to masking by environmental noise (Morton, ). Alternatively, the Lombard hypothesis proposes that the upward shift in frequency may be a consequence of increased call amplitude in noise (Nemeth, Zollinger, & Brumm, ; Zhang, Chen, Chen, & Zhao, ). In this study, the maximum frequency and bandwidth increased significantly as noise levels increased while the call amplitude did not exhibit any obvious changes.…”

“…Some frog species have been reported to call at higher frequencies in noisier environmental conditions (Parris et al, ; Vargas‐Salinas & Amezquita, ; Zhang et al, ). In other species, however, such changes have not been found (Halfwerk, Lea, Guerra, Page, & Ryan, ; Jang et al, ).…”

“…In other species, however, such changes have not been found (Halfwerk, Lea, Guerra, Page, & Ryan, ; Jang et al, ). Unlike the concave‐eared torrent frog ( Odorrana tormota ) and some birds that increase the minimum frequency resulting in bandwidth narrowing (Francis, Ortega, & Cruz, ; Montague, Danek‐Gontard, & Kunc, ; Slabbekoorn & Peet, ; Zhang et al, ), little torrent frogs increase the maximum frequency and bandwidth as noise levels increase. Stream noise energy is mainly concentrated in low‐frequency bands, so the upward shift in the highest frequencies of the calls may be more adaptive than increasing the lowest frequencies in torrent frogs.…”

Vocalizations of female frogs contain nonlinear characteristics and individual signatures

“…The temperature and humidity indoors ranged from 16 to 19°C and 80 to 100%, respectively (measured during the sound recording sessions). The noise level indoors was measured using an SPL meter (TES 1357; Tianjin Sheng Xing Tai Technology Co., Tianjin, China) and it ranged between 38 and 43 dB SPL RMS–this was substantially lower than that of the ambient noise level outdoors along Fu Creek (range: 62–78 dB SPL; [ 20 ]).…”