Kloss gibbons (Hylobates klossii) in the Peleonan Forest: group size and distribution, song function, and predation avoidance

Helen Marie Dooley

    Research output: ThesisDoctoral Thesis

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    Kloss gibbons (Hylobates klossii) are endemic to the Mentawai Islands off the west coast of Sumatra in Indonesia. This is the first study of Kloss gibbon behaviour since the 1980s and the first detailed study of the Peleonan forest population in the north of Siberut Island. This study provides territory and group size estimates for the population, examines the sex-specific functions of singing, and assesses a suite of traits that appear to function in human predation avoidance.
    Kloss gibbons were observed at the Siberut Conservation Programme field site in the Peleonan forest over 18 months in 2007 and 2008. Behavioural and GPS data were collected for one habituated group of six individuals and for four other groups. Average group size in the Peleonan (5.8 individuals) exceeded the group sizes reported for the previously studied Paitan River (3.4) and Sirimuri River (4.1) populations. Delayed dispersal may explain this difference as no groups included more than one reproductive female but three groups included a sub-adult. The average home range size was 38.8 hectares, slightly larger than the Paitan population (31-35 ha) but much larger than the Sirimuri (11 ha). Low levels of aggression within and between groups in both the Peleonan and Paitan populations suggests that competition for space and resources is much lower than in Sirimuri where inter-group encounters are more frequent and more aggressive. Parents may tolerate mature offspring residing on their territory because resource competition is low and because sub-adults may provide a benefit in the protection of younger immatures. Groups in the Peleonan were observed fissioning during daily activity. I suggest that this is related to predation avoidance through crypsis rather than a response to feeding competition because no intra-group aggression was observed and groups foraged both as a whole and in sub-groups.
    Kloss gibbons are one of only two gibbon species in which mated pairs do not duet, thus allowing independent analyses of the functions of singing for each sex. The presence or absence of male and female singing was recorded for the population (198 days) and for a focal group (47 days) along with factors that may influence singing. Rainfall inhibits singing in both males and females. Male and female songs likely function in intra-sexual resource defence because singing is associated with singing by same-sex neighbours, and same-sex choruses are more likely to occur after one or more days of silence (by that sex). Female songs may also function to defend resources from male competitors because females were more likely to sing after dawn and earlier on morning when males had sung pre-dawn, and the number of groups singing in female choruses was positively correlated with the number of males that had sung earlier. Singing was not coordinated within a mated pair, thus vocal coordination of the pair has been lost in Kloss gibbons and their songs do not convey information to neighbours about the strength of the pair bond.
    Humans are the primary predator of Kloss gibbons (there are no felids or raptors in the Mentawai Islands). Notes were taken on the behaviour of unhabituated Kloss gibbons during 120 encounters with humans. The risk of predation associated with singing (the most conspicuous behaviour) is lowered by singing in same-sex choruses, singing less often during daylight hours and by males singing their post-dawn songs away from their group. Group fission, and infrequent play and grooming, in combination with their monochromatic black pelage, makes Kloss gibbons particularly cryptic and lowers their risk of detection by hunters. When unhabituated gibbon groups encountered humans, they sometimes (31% of 96 encounters with groups) engaged in a coordinated display wherein one adult individual acted as a ‘decoy’ by approaching and distracting human observers, while other group members fled silently in multiple directions. ‘Decoy’ individuals were always adult-sized and never females with infants, suggesting that ‘decoy’ individuals put themselves at risk to increase the survival of related immatures (and adult females with infants) who have a greater risk of predation. I argue that multiple aspects of Kloss gibbon behaviour have evolved in response to a long history of predation by humans and that adult males and sub-adults play an important role in the protection of younger group members.
    Original languageEnglish
    QualificationDoctor of Philosophy
    • Judge, Debra, Supervisor
    • Schmitt, Linc, Supervisor
    Publication statusUnpublished - 2015


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