The release of northeast Bornean orangutans to Tabin Wildlife Reserve, Sabah, Malaysia

Summary

The Bornean orangutan (Pongo pygmaeus), Endangered from habitat loss and hunting, has produced hundreds of rescued orphans passing through Sabah's Sepilok Orangutan Rehabilitation Centre since the 1960s, many historically released into Tabin Wildlife Reserve with no systematic tracking of outcomes. Orangutan Appeal UK and the Sabah Wildlife Department launched a dedicated post-release monitoring program, selecting a release site in regenerating forest based on food-tree density, distance from settlements, and access for researchers, and screening candidates for behavioral readiness and freedom from transmissible diseases before release at a minimum age of six. Because orangutans' arboreal anatomy precludes conventional radio collars, the project worked with Vienna's Research Institute of Wildlife Ecology to develop and surgically implant a novel subcutaneous telemetry device, alongside intensive behavioral sampling and GPS ranging data. Three orangutans were hard-released in 2010 with no supplementary food, followed by five more from 2012 released in small groups under a soft-release protocol offering ad-hoc food; of the initial cohort, one dispersed by month six, and two died by months 10 and 12, while all released animals integrated adequately with wild orangutans and one released female successfully gave birth to a healthy infant. Difficulties included tracking fast-moving animals over difficult terrain, unreliable telemetry range, persistent begging behavior from prior dependence on humans, and balancing welfare-driven food supplementation against building independent foraging skills. The project rates itself partially successful, crediting its pioneering implanted-telemetry technology and the volume of new behavioral data collected, even as mortality among early candidates tempers the outcome.