Vocal behavior and risk assessment in wild chimpanzees

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If, as theory predicts, animal communication is designed to manipulate the behavior of others to personal advantage, then there will be certain contexts in which vocal behavior is profitable and other cases where silence is favored. Studies conducted in Kibale National Park, Uganda investigated whether chimpanzees modified their vocal behavior according to different levels of risk from intergroup aggression, including relative numerical strength and location in range. Playback experiments tested numerical assessment, and observations of chimpanzees throughout their range tested whether they called less frequently to avoid detection in border areas. Chimpanzees were more likely to call to playback of a stranger’s call if they greatly outnumbered the stranger. Chimpanzees tended to reduce calling in border areas, but not in all locations. Chimpanzees most consistently remained silent when raiding crops: they almost never gave loud pant-hoot calls when raiding banana plantations outside the park, even though they normally give many pant-hoots on arrival at high-quality food resources. These findings indicate that chimpanzees have the capacity to reduce loud call production when appropriate, but that additional factors, such as advertising territory ownership, contribute to the costs and benefits of calling in border zones.