Flexibility of Gestural Production in Captive Groups of Chimpanzees (Pan troglodytes), Sumatran Orangutans (Pongo abelii), and Siamangs (Symphalangus syndactylus)

Abstract Across species, communication systems may differ in their levels of flexibility, but comparisons are challenging, because flexibility is not operationalized in a consistent way. 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 (i.e., pragmatic flexibility), (iii) the production of one gesture type with different limbs and/or toward different target locations (i.e., morphological flexibility), and (iv) the ability to combine different signals into a sequence in different orders (i.e., combinational flexibility). Comparisons across individuals and species revealed variation in contextual and morphological flexibility. In particular, contextual flexibility increased with age and was overall higher in chimpanzees than siamangs. Moreover, morphological flexibility increased with social integration and was highest in siamangs and lowest in orangutans. Overall, variation largely reflected differences in social experience and possibly in the socioecological characteristics of the species. Our findings suggest that contextual and morphological flexibility might be good candidates to study interindividual and interspecific variation in primate gestural production.