Vietnam is home to six gibbon species, which are all either “Endangered” or “Critically Endangered” (IUCN, 2010). Suitable techniques to rehabilitate, release and monitor gibbons successfully have to be developed now, if we want to have them available to save the most “Critically Endangered” species, and prevent that more common species ever reach the “Endangered” state.
From 2010 to 2013 eight adult “Endangered” southern yellow-cheeked gibbons (Nomascus gabriellae) were rehabilitated and fitted with VHF or VHF/GPS-GSM radio-transmitters attached to Biothane collars, and released into secondary rain forest in South Vietnam. The aim was to assess the suitability of various collars for gibbons and determine the fix success rate (FSR) on arboreal primates in this type of habitat. We collected valuable data on methods to monitor reintroduced gibbons in cases where human presence needs to be limited. Including the radio-transmitters the collars weighed between 47 g (VHF) and 230 g (VHF/GPS-GSM) and were fitted on gibbons weighing between 5 kg and 6.5 kg; thus collar weight ranged from 0.7 to 3.8 % of the gibbons’ body weight. For the first six collars we created weak links, while the last two collars had a drop-off buckle with a programmed timer. Battery life for all collars was estimated at a minimum of 5-6 months, but was considerably longer in practice. Collars remained in place for up to 13 months and whilst no collars caused damage to the skin, hair loss was observed with the GPS collars. The VHF collars had antennas up to 20 cm long, which were the only part the gibbons tried to manipulate, usually during the first hours after fitting, after which they were ignored. Gibbons wearing the GPS collars were not observed singing, otherwise there were no behavioural changes observed. VHF transmission reached up to a maximum of 700 m. FSR of the GPS collars was 60 % or more. Our data shows that collaring gibbons with GPS collars is a suitable method to monitor released gibbons in secondary rain forest and allows collecting valuable data after release.
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