Vocal communication in wild chimpanzees: a call rate study

Background. Patterns of vocal communication have implications for species conserva-tion: a change in calling behaviour can, for instance, reflect a disturbed habitat. More importantly, call rate is a parameter that allows conservation planners to convert call density into animal density, when detecting calls with a passive acoustic monitoring system (PAM). Methods. We investigated chimpanzee (Pan troglodytes schweinfurthii) call rate during the late dry season in the Issa Valley, western Tanzania by conducting focal follows. We examined the socio-ecological factors that influence call production rate of savanna woodland chimpanzees. Results. We found that sex, proportion of time spent in a vegetation type, proportion of time spent travelling, time of the day, party size and swollen parous female presence had a significant effect on the call rate. Call rate differed among the different demographic classes with subadult and adult males vocalising twice as often as the subadult and adult females and three times as often as the juveniles. Applications. The use of PAM and recent statistical developments to estimate animal density is promising but relies on our knowing individual call rate, often not available for many species. With the improvement in automatic call detection, we anticipate that PAM will increasingly be broadly applied to primates but also across taxa, for conservation.

Automated summary

The study identified factors such as sex, time spent in different vegetation types, group size, and the presence of swollen parous females that significantly influence chimpanzee call rate. Call rate varied significantly among different demographic classes, with subadult and adult males vocalizing more frequently than females and juveniles. The use of passive acoustic monitoring systems (PAM) and advancements in statistical methods show promise for estimating animal density, although individual call rates for many species remain unknown.