Emotions mediate nonlinear phenomena production in the vocalizations of two ape species.

Nonlinear phenomena (NLP) are widely observed in mammal vocalizations. One prominent, albeit rarely empirically tested, theory suggests that NLP serve to communicate individual emotional states. Here, we test this 'emotional hypothesis' by assessing NLP production in the vocalizations of chimpanzees and bonobos across various social contexts. These two species are relevant to test this hypothesis since bonobos are more socially opportunistic than chimpanzees. We found that both species produced, albeit at different frequencies, the same five distinct NLP types. Contextual valence influenced NLP production in both species with negative valence being associated with more frequent NLP production than positive and neutral valence. In contrast, using aggression severity and caller role as proxies for arousal, we found that in bonobos, but not in chimpanzees, vocalizations uttered during contact aggression or from victims and females contained more NLP. In contrast, the type of NLP produced was neither influenced by valence nor arousal in either species. Our study supports the emotional hypothesis regarding the occurrence of NLP production in mammals, particularly in opportunistics such as bonobos. This reinforces the hypothesis of an adaptative role of NLP in animal communication and prompts further investigations into their communicative functions.This article is part of the theme issue 'Nonlinear phenomena in vertebrate vocalizations: mechanisms and communicative functions'.