Singing behavior of white-handed gibbons (Hylobates lar) of Khao Yai National Park, Thailand

This thesis manuscript focusses on the remarkable vocal abilities of white-handed gibbons (Hylobates lar) of Khao Yai National Park, Thailand. To understand the mechanisms underlying the acoustic communication system of these gibbons, I conducted my research project in Khao Yai National Park in Thailand, on 13 wild groups. White-handed gibbons, also known as lar gibbons, part of the Hylobatidae family, are the evolutionarily most distant relatives of humans within the hominoid lineage and are of interest regarding their complex vocal communication system. Their large vocal repertoire, lengthy songs and abilities to produce different song types - resultant from multiple note assemblages - make them an exceptional study system to address scientific questions, also about the origin of human vocal communication. A first well-known feature is that the mated pair sings stereotyped, loud and long songs on a daily basis and in a synchronised fashion; the duet songs. These songs are thought to function primarily in pair-bonding and mate and territorial defence, but these hypotheses have never been investigated in much depth. Another well-known feature is that gibbon groups also sing in response to predators and from previous research it became apparent that predator songs are assembled in different ways compared to duet songs. Here also, not much has been done beyond these basic findings. A first goal of this research was therefore to investigate the intergroup singing interactions and explore the role of duet songs as a complex territorial communication network. Duet songs often spread to neighbouring groups, resulting in song waves. In my research I have been able to show that duet responses can either non-aggressive turn-taking exchanges, where groups delay their song onset so that the preceding gibbon group can end their duet without interruption; alternatively, duet responses can be competitive, counter-singing exchanges, in which case the song of the preceding group is temporally overlapped by the subsequent duet song. To investigate this phenomenon, I examined the mechanisms underlying the duet song production of two subsequent groups. Results showed that intergroup distance, group composition and, most importantly, genetic relatedness had a significant impact on whether gibbons opted for competitive or non-aggressive duet exchanges with their neighbours. The study thus shows that kinship plays a role in non-aggressive interactions between groups. The second part of my research focusses on gibbon songs produced in response to predators, particularly how the same basic song units are assembled in context-specific ways. I was interested in how gibbons use their elaborate songs to encode external events and what inferences listeners can make from these vocalizations. Previous observations have shown that neighbouring groups respond to songs with the matching type, i.e. neighbours’ duets by their own duets, but also predator songs by their own predator songs. This happens in the absence of visual contact, suggesting that gibbons can decode and understand the meaning of each other’s songs, similar to how humans use language to address and understand each other. To investigate these coding and inference abilities in gibbons, I conducted playback and predator model experiments. In a first study, I broadcasted predator and duet songs to different groups and recorded vocal and behavioural responses. In a second study, I presented two predator types, the clouded leopard and the reticulated python, and again recorded subjects’ vocal and behavioural responses. Regarding the playback experiments, I found that gibbons could discriminate and infer the meaning of different song types (i.e. duet song and predator song) by displaying different vocal and behavioural responses. Regarding predator model presentations, a previous study showed that the leopard model reliably elicited strong vocal and anti-predatory behavioural responses, unlike the python model. This time, the life-sized python model also elicited strong singing and anti-predator responses, in contrast to previous research. More importantly, gibbons arranged song elements differently and songs could be distinguished regarding several acoustic components and sequential arrangements according to the type of predator encountered, a basic requirement for referential alarm calling behaviour. The basal position of gibbons in the Hominoïdae family combined with their usually complex vocal abilities make white-handed gibbons a particularly promising species to address questions of how nonhuman primates code external events in their vocal signals and what inferences they can make from these vocalizations. This kind of data will be relevant to better understand the conditions that might have led to the emergence and current complexity of human vocal communication, as seen in spoken language.