Multimodal coordination of group departures in western gorillas

Group departures in social animals require coordination in time and space. Movement cues are important, but many species vocalize in relation to departure, indicating that the relevant information can be multimodal. Here, the use of vocalizations in relation to visual cues was investigated in three groups of western gorillas, Gorilla gorilla , in the Central African Republic. Grunt and grumbles were the common call types. Grumbles were mostly produced by initiators typically just after a departure was initiated. By contrast, grunts were equally produced by initiators and followers, but here the peak was observed just before departure. Grunts were produced by individuals adopting different body postures. Individuals producing grunts while lying on the ground were slower to move than those sitting or standing, indicating that call–posture combinations provided information about an individual's likelihood to move. Finally, initiations were more likely to succeed when more individuals vocalized, which is consistent with shared decision-making. The results of this study indicate that gorilla departure decisions rest on multimodal information, showing the mechanism by which social animals may solve collective action problems without language.