Silencing as an antipredator tactic: perceived risk inhibits rutting calls in sika deer

Fear of predators, including humans, often provokes behavioural changes in wild animals. Here, we investigated the influence of perceived risk on the vocal activity of large terrestrial mammals by evaluating the vocal rates of antipredator alert calls and other loud calls unrelated to antipredator behaviour. For this purpose, we targeted sika deer, Cervus nippon , a species that uses vocal communication frequently during the rutting season, across 21 monitoring sites in mainland Japan. We evaluated the vocal rates of alert barks and two types of rutting calls at each site during the autumn months from 2021 to 2024 using passive acoustic monitoring. We also assessed antipredator vigilance levels as an indicator of perceived risk using camera traps. In total, we collected bioacoustic data covering 1960 monitoring days, including 45 139 target calls and 651 video clips, to evaluate vocal and vigilance behaviours, respectively. Based on these data, we constructed Bayesian generalized linear models. The results indicated that increased vigilance significantly reduced the vocal rates of both types of rutting calls but had little effect on alert barks. This study is the first to suggest that silencing (i.e. reduced vocal activity) may represent an antipredator behaviour associated with heightened perceived risk. Unlike rutting calls, alert barks were emitted regardless of perceived risk. This may be explained by their inconspicuous acoustic properties, which could make the caller more difficult for a predator to locate.