Adult male Lar gibbon sings the female great-call: A case study of inter-sex song production in a non-human primate

Gibbons are known as one of the most vocal non-human primates. They vocally advertise and reinforce their pair-bonds by singing complex duets, in which both the male and female perform a predetermined sex-specific set of song phrases, including the loud and elongated great-calls. Only females and sub-adult male gibbons have been previously observed performing great-calls. Once a male gibbon matures, he normally stops great-call production completely but continues singing other and less high-pitched song phrases. This case study describes a fully adult, castrated male lar gibbon (Hylobates lar, 32 years old, zoo-housed) who performs both male phrases of the duet, including the female great-call. The male regularly produced great-calls despite being in a, relatively weak, pair bond with a female conspecific. His great-calls adhered to the general structure of typical female great-calls but were shorter and had a lower maximum frequency. Notably, he produced these great-calls predominantly when the female was absent, especially when she was in their inside enclosure whilst he was outside. Behavioural observations indicate that the occurrence of great-calls by the male cannot be predicted based on pre-song behaviour or his interaction with the female. The recurrence of sub-adult singing behaviour in a fully grown adult is most likely due to his reduced testosterone levels. This study sheds light on the intricate dynamics of duetting and the unique occurrence of cross-sex song production in gibbons, emphasizing the complexity of pair-bond communication in this species.