Searching For The Hybrid

Barito Ulu in Central Kalimantan province is the only region in Borneo that harbours a hybrid gibbon population. Located at the fork where the Rekut River breaks from the Busang River, Rekut Camp is a historic site for ecological research in Barito Ulu, first established in 1989. Tucked away in pristine dipterocarp rainforest over 600 kilometres from the provincial capital of Palangka Raya, Rekut Camp lies remote and isolated amongst steep cliffs and pounding waterfalls.

At Rekut, the ranges of two gibbon species overlap: the Bornean white-bearded gibbon (Hylobates albibarbis) and Müller’s gibbon (Hylobates muelleri). Natural hybridisation between these two species has led to the creation of a Bornean hybrid gibbon, independent of human disturbance or habitat fragmentation. This gibbon is one of the least-studied ‘subtypes’ within the gibbon family

Gibbon vocalisations are strongly genetically determined, particularly the great call of female gibbons, which is the most genetically conserved phrase of a gibbon’s vocal repertoire. Thus, if variations in the female great call are detected (i.e., different from the mother’s), it is often indicative of some sort of mixing. These unique vocal signatures were what first led researchers to investigate the Barito Ulu hybridisation in 1976 (Marshall and Sugarjito, 1976).

The hybrid gibbon was discovered almost 50 years ago but, since its discovery, only two in-depth studies have been conducted on its vocal structure and characteristics: the first in 1992 and the second between 2004-2006. Neither study has been published. Consequently, I intend to re-visit the topic through my own research, examining the vocal structure of the only known hybrid gibbon population in Central Borneo.

My name is Jorian Hendriks, and I am a master’s student at Wageningen University (WUR) in the Netherlands. I am conducting an acoustic study on the hybrid gibbon population in collaboration with Universitas Gadjah Mada (UGM), based in Indonesian Borneo. To do this, I am using three acoustic methods – passive bioacoustics, active bioacoustics, and triangulation surveys. I have chosen to use bioacoustics, as I would like to study and compare the vocal structure and characteristics of the female great call between the co-occurring gibbon species in and around Rekut Camp. I will use data collected from triangulation to estimate gibbon population density and understand the habitat use patterns of gibbons in Central Borneo. In doing so, I intend to build on previous research, whilst laying the foundations for future studies to be conducted on the hybrid gibbon population.

Acoustic methods (BNF – Bioacoustics) are rapidly growing in popularity for vociferous primates like gibbons due to their effectiveness in sampling large areas at a relatively low cost. This is especially useful in areas experiencing rapid rates of deforestation and fragmentation like Borneo. Building on the 1992 study, with the help of bioacoustics, I can sample an area as large as 9000 ha in a span of just six weeks. Over much of this area, gibbon surveys (and, indeed, any ecological research projects) are being conducted for the first time. The sample design I have selected gives rise to all sorts of interesting hypotheses that can be answered; however, this study means to do the following:

1. Understand and compare the vocal structure of the hybrid gibbon in relation to H. albibarbis and H. muelleri;
2. Identify key vocal characteristics that would allow for this differentiation;
3. Build our understanding of the spatial extent of the hybrid zone; and
4. Understand if there is any association between gibbon density and habitat types.

In completing my master’s dissertation, I hope to provide a comprehensive report with up-to-date maps and information on the hybrid gibbon population, facilitating future research and monitoring of the hybrid gibbon for its long-term protection. I also hope that this will raise awareness of the biodiversity conservation potential of Central Borneo.

I would like to give recognition and acknowledgement to the numerous parties involved, as this research is a collaborative endeavour and would not have been possible without the support of these key contributors:

• Dr. Muhammad Ali Imrom, my Indonesian supervisor and research counterpart from Universitas Gadjah Mada;
• Team Gibbon: Bang Azis, Bang Unyil, Bang Hendri, Bang Ahmat, Mas Ebrry and Mas Alan, for helping me with data collection;
• The Borneo Nature Foundation for on-the-ground logistical support, research expertise, and planning; photography credits: Joan Prahara (BNF)
• Project funders, which include Apenheul Natuurbehoud Fonds, Stichting Lucie Burgers, the Hendrik Muller Fonds, Fona Conservation, and project sponsors Idea Wild, Wildlife Acoustics, and Cornell Lab of Ornithology.

Written by Jorian Hendriks