Flexibility of gestural production in captive groups of chimpanzees (Pan troglodytes), Sumatran orangutans (Pongo abelii) and siamangs (Symphalangus syndactylus)

Abstract

Abstract Flexibility is an important hallmark of complex communication systems, yet few studies have systematically assessed it in non-human species. In this study, we investigated different aspects of flexibility in the gestural communication systems of 7 groups of captive apes (N = 53), including 19 chimpanzees (Pan troglodytes), 16 Sumatran orangutans (Pongo abelii) and 18 siamangs (Symphalangus syndactylus). We operationalized flexibility in four different ways: (i) the use of one gesture type across several contexts (i.e. contextual flexibility), (ii) the use of many gesture types in the same context, (iii) morphological variation with which individual gesture types can be produced (i.e. morphological flexibility), and (iv) the ability to combine signals into different sequences. Comparisons across individuals and species revealed variation in contextual and morphological flexibility. In particular, contextual flexibility increased through age, and it was overall higher in chimpanzees than in siamangs. However, morphological flexibility was highest in siamangs and lowest in orangutans. Overall, variation could be largely explained in terms of changes in social experience and differences in the socio-ecological characteristics of the species. Our findings suggest that contextual and morphological flexibility might be good candidates to study inter-individual and inter-specific variation in primate gestural production.