Ontogeny of dialects across chimpanzee vocalisations.

Dialects are a vocal signature of population membership. Early in human ontogeny, vocal behaviour is relatively similar across populations from an acoustic perspective. As ontogeny progresses, the difference between populations becomes larger. While research in recent decades has shown chimpanzees show population-specific vocal signatures, debates regarding the sources of this variation, namely whether it results from social, genetic, or ecological factors, continue. By studying the ontogeny of vocal dialects, we can gain new insights into the processes generating cross-population variation in vocal acoustics. In this study, we measured the acoustic characteristics of infant (N = 21) and juvenile (N = 10) chimpanzee (Pan troglodytes schweinfurthii) laughs (N = 4733), grunts (N = 3694), and whimpers (N = 2798) at Chimfunshi Wildlife Orphanage and Gombe National Park. Using permuted discriminant function analysis, we found that we could discriminate between sites for all call types, indicating the presence of dialects across the chimpanzee vocal repertoire. This pattern held for juveniles, but infant whimpers did not show site-specific vocal signatures. Furthermore, we could discriminate between sites significantly better for juveniles compared to infants. We discuss the different processes through which the presence of population-specific vocal signatures across the vocal repertoire from infancy may emerge and argue that the development of increasingly recognisable vocal dialects in ontogeny suggests a role for social and ecological factors in shaping chimpanzee vocal acoustics.