Subsidiënt: Jorian Hendriks / Wageningen University
Subsidie: 23.3.6.2
Natural hybridization can have a dramatic impact on evolutionary processess through promoting speciation, facilitating adaptation, and genetic diversity; with implications for primate biodiversity. However, natural hybridization in Gibbons (Hylobatidae) is poorly understood as gibbons are hard to survey and monitor. This is because gibbons are strictly arboreal, crpytic, inconspicuous, and typically inhabit remote rainforests locations; however, their loud calls allow them to heard despite rarely being seen. There is an imperative need to develop a cost-efficient and effective approach to study gibbon taxonomic relationships and spatial population structure considering the challenges and difficulty of gibbon morphological determination and procuring gibbon genetic samples. Acoustic analysis of gibbon song structure could be a promising approach to study gibbons, as song structure is genetically expressed and species-specific. In this study, I aim to determine whether acoustic analysis of gibbon song structure can be used as a tool for species discrimination and infer spatial structural variability in a gibbon population inhabiting a natural hybrid gibbon zone in Central Borneo, Indonesia. Using passive and active bioacoustics, I recorded gibbon vocalizations from six replicate arrays distributed across a 80 km2 stretch of primary dipterocarp rainforest. Gibbon song structure was described by 10 quantitative structural parameters and was analyzsed using (un) supervised dimension reduction, cluster, and classification analytical techniques. Based on acoustic analysis of gibbon song structure, the natural hybrid zone supports two distinct parental populations representing H. albibarbis and H. muelleri, and a Bornean hybrid gibbon population (H. albibarbis x H. muelleri) based on intermediate ‘hybrid’ call types found. Two vocal parameters - duration of climax phase and average climax inter-note interval - were the best predictors in discriminating great calls between co-occurring populations. Spatial structural variability in gibbon song structure was present across small-scale geographic gradients (~ca 10 kilometers); with spatial structural variability in parental great calls reflected in inferences made based on mapping of modelled probabilities of Bornean hybrid song structure as being H. albibarbis-like or H. muelleri-like. Acoustic analysis of gibbon song structure carries promise as a robust monitoring tool for species discrimination in a natural hybrid gibbon zone. Topographic barriers and gibbon ranging behaviour resulted in isolation pockets across small spatial scales which led to the spatial structural variability in gibbon population song structure found.
