Generating alarm calls in response to the presence of predators is a phenomenon that has been widely researched however what causes calls to diversify in less urgent contexts is little examined. This study examines the diversity in the quiet ‘hoos’ produced by chimpanzees in three settings – travel, rest and alert and the situations leading to this diversity.
For this experiment, researchers studied wild-living, habituated chimpanzees living in the forests of Uganda (14 males and 15 females). The hoos were recorded and classified according to their context of production. Any call of adequate quality that was produced in the three settings mentioned earlier, was subjected to acoustic analysis. It was found out that all three hoo types are connected with group cohesion, but none of them seemed to generate a different response from receivers. Alert hoos were emitted at lower rates compared to travel hoos, while rest hoos were almost exclusively emitted as single calls also the interval between calls was longer in alert than travel hoos.
The researchers found out that since chimpanzees are highly subjected to predation, this may have shaped the use of more quiet hoos, especially when individuals are travelling in small groups. Also in order to maintain cohesion, receivers must respond differently to signallers in each setting because visual or vocal signals are likely to be unreliable in maintaining cohesion. This leads to the conclusion that an increase in the demand of cooperative activities is one factor driving the diversification of alarm calls.
Crockford, C., Gruber, T., & Zuberbühler, K. (2018). Chimpanzee quiet hoo variants differ according to context. Royal Society open science, 5(5), 172066. doi:10.1098/rsos.172066
