Culture has long been assumed to be uniquely human but recent studies, in particular on great apes, have
suggested that cultures also occur in non-human primates. The most apparent cultural behaviours in great apes involve tools in the subsistence context where they are clearly functional to obtain valued food. On the other hand, tool-use to modify acoustic communication has been reported only once and its function has not been investigated. Thus, the question whether this is an adaptive behaviour remains
open, even though evidence indicates that it is socially transmitted (i.e. cultural). Here we report on wild
orang-utans using tools to modulate the maximum frequency of one of their sounds, the kiss squeak, emitted in distress. In this variant, orang-utans strip leaves off a twig and hold them to their mouth while producing a kiss squeak.