Field playback of loud calls to gibbons ( Hylobates lar): territorial, sex-specific and species-specific responses

Abstract

We present the results of an experimental study of territoriality among monogamous lar gibbons ( Hylobates lar), scoring vocal and movement responses to field playback of their loud calls. Playback of neighbouring groups' male-female duets on territory borders, and of these duets and music controls in territory centres, indicated a general territorial response. Responses to playback of a strange male's solo and of a strange female's solo in territory centres indicated that resident males would evict deeply intruding males and pairs but not necessarily females, and that resident females would evict deeply intruding females but probably not males or pairs. Vocal responses also implied that, although males carry the burden of routine territory defence in border disputes involving chases and special calls, the female part of the daily duet also contributes to routine exclusion of outside females. Responses of lar females to playback of the solo of a female capped gibbon ( H. pileatus) suggested that the females of one species cannot sex the calls of females of the other species. This could partly account for the existence of bigamous mixed-species groups at the interface between the two species. This result also implies that female calls contribute to routine exclusion of conspecific females, and thus to the maintenance of monogamy.