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Interspecific relations between two gibbon species (H. lar and H. pileatus) at Khao Yai National Park, Thailand

U. Suwanvecho and W.Y. Brockelman, Center for Conservation Biology, ISTRD, Mahidol University, Salaya, Nakhon, Pathom 73170, Thailand

     A small contact zone between H. lar and H. pilatus occurs in Khao Yai National Park, Thailand. The size of the zone is about 176 km2. A transition from 90% H. lar phenotypes to 90% H. pileatus at the zone occurs within a distance of only 11 km. Hybridization, mixed species groups, and groups with one male and two females from each species (mixed trios) occur in the area (Brockelman & Gittins 1984). However, population survey, by listening from hills throughout the area, during 1980$BCT(J indicates very strong positive mate assortment of both species. Of the 199 surveyed groups, only 3 groups were found to be heterospecific. Five groups were found to be mixed trios. The number of adult hybrids paired in territorial groups was only 16, 11 of which were mated with lar and 4 with pileatus. Data from the initial survey indicate that H. lar has the tendency to encroach on the east back of the Takhong River. In 1999 we established Khlong Sai gibbon study site for ecological and behavioral study of the two closely related gibbon species within the narrow zone of sympatry. One of the objectives is to predict the long-term trends in interspecific relations in the zone. This study site encompasses 5 study groups: two of which are lar and three pileatus; and 2 neighboring groups: one lar and one pileatus. Group composition of the seven gibbon groups indicate the replacement of mixed species groups that occurred in the area previously in the initial survey, by homospecific groups. This indicates the instability of mixed-species groups and the significance of homospecific pairing which might be the main factor restricting gene flow between the two species. Our observations show that interspecific territoriality exists between H. lar and H. pileatus. The existence of interspecific territorialiy would also be an important factor slowing diffusion of genes across the zone.

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