Chimpanzees modify recruitment screams as a function of audience composition

Abstract

Wild chimpanzees produce acoustically distinct scream vocalizations depending on their social role during agonistic interactions with other group members. Here, we show that victims during such agonistic interactions alter the acoustic structure of their screams depending on the severity of aggression experienced, providing nearby listeners with important cues about the nature of the attack. However, we also found that victims of severe attacks produced screams that significantly exaggerated the true level of aggression experienced, but they did so only if there was at least one listener in the audience who matched or surpassed the aggressor in rank. Our results are consistent with the more general hypothesis that chimpanzees possess sophisticated understanding of third-party relationships, so-called triadic awareness, and that this knowledge influences their vocal production.

Fig. 1.

Time–frequency spectrograms of screams given by four individuals to severe and mild aggression. Screams were produced by BH, a subadult female (A); KT, a subadult male (B); FL, an adult female (C); and BB, an adult male (D). The darker the image, the higher was the amplitude.

Fig. 2.

Scatter-plots illustrating the relationship between the type of aggression experienced by the caller and nine acoustic measurements of the screams produced. The x axis of each scatter-plot shows the four types of aggression the victim experienced. Contact and directed aggression were classified as severe, whereas nondirected and posture threat aggression were classified as mild. All further analyses reported in the text are based on these two aggression categories (severe and mild). Each point is the median value from the first three calls in each bout of screaming (n = 84).

Fig. 3.

Changes in the vocal structure of victim screams as a function of the level of aggression experienced and whether or not an individual equal to or outranking the aggressor was in the audience. The mean and standard error values for these six variables are shown. Error bars represent one standard error. Sample sizes are as follows: for occurrences of severe aggression with a high-ranking bystander present, n = 12, and with a high-ranking bystander absent, n = 9; for occurrences of mild aggression with a high-ranking bystander present, n = 10, and with a high-ranking bystander absent, n = 11.