The effects of background noise and behavioral context on the acoustic characteristics of coo calls in Japanese macaques.

Anthropogenic noise may affect the spectral and temporal structures of contact calls in wild non-human primates. However, most studies have compared groups living at varying distances from noise sources, while few have focused on the relationship between specific noise types and vocalizations at the individual level, which is necessary to examine vocal plasticity. We aimed to clarify the effects of background noise on the acoustic characteristics of coo calls of Japanese macaques on an individual basis; thus, we examined the effects of noise levels, behavioral context, and number of individuals in proximity on the duration and spectral measure of calls. We conducted focal observations of adult female Japanese macaques, and recorded coo calls and background noise levels. We analyzed 128 calls from nine subjects and found that the start and mean frequencies of coo calls were higher at higher noise levels. The call duration and frequency modulations were higher when the number of individuals in proximity was lower and the end frequency was higher during foraging and moving. These findings suggest that different factors affect the acoustic characteristics of the coo calls of Japanese macaques simultaneously. The most intense background noise sources were typically low-frequency sources that overlapped the frequency bands of the calls. Hence, they exerted a profound influence on call acoustics, which is consistent with the results of previous studies. Within-individual adjustments to ambient noise indicate vocal plasticity, enabling efficient signal transmission to the receiver.