Wild-captive contrasts in non-vocal communicative repertoires and functional specificity in orang-utans
The creation of novel communicative acts is an essential element of human language. Although some research suggests the presence of this ability in great apes, this claim remains controversial. Here, we use orang-utans (Pongo spp.) to systematically assess the effect of the wild-captive contrast on the repertoire size of communicative acts. We find that individual communicative repertoires are significantly larger in captive compared to wild settings, irrespective of species, age-sex class or sampling effort. Twenty percent of the orang-utan repertoire in captivity were not observed in the wild. In Sumatran orang-utans, the more sociable species, functional specificity was also higher in captive versus wild settings. We thus conclude that orang-utans, when exposed to a more sociable and terrestrial lifestyle, have the behavioural plasticity to invent new communicative behaviours that are highly functionally specific. This productive capacity by great apes is a major prerequisite for the evolution of language and seems to be ancestral in the hominid lineage.