Role of Emitter and Severity of Aggression Influence the Agonistic Vocalizations of Geoffroy's Spider Monkeys (Ateles geoffroyi)

摘要

Natural selection has resulted in the acoustic convergence of many animal vocalizations. During agonistic interactions vocalizations may vary depending on the role an individual plays in the interaction and on the severity of the attack. Motivation-structural rules describe how aggressors are thought to have evolved to use low-frequency vocalizations, whereas victims often use high-frequency vocalizations. This is because call frequency (Hz) is negatively related to body size across species. Motivational theory predicts that during more severe attacks, vocal structure will also change in response to increased arousal, leading to the production of noisy (high-entropy) vocalizations. Little is known about the acoustic characteristics of vocalizations produced during agonistic encounters in primates, and the limited available data are highly biased toward Old World species. Here, we evaluated the effect of the role of the emitter (aggressor or victim) and the severity of the behavior (with or without contact) on the acoustic parameters of 185 agonistic vocalizations emitted by 15 spider monkeys (Ateles geoffroyi) in two captive groups. Our results supported the predictions of both the motivation-structural rules and motivational theory: Call frequency (Hz) was lower in aggressors than in victims and lower during high-severity encounters than low-severity encounters. Further, average entropy was higher during high-severity encounters. These findings suggest that the agonistic vocalizations of spider monkeys convey information about both the role of the emitter and the severity of the interaction.