Post-release behaviour of reintroduced Orangutans (Pongo pygmaeus wurmbii) in Bukit Baka Bukit Raya National Park and Bukit Batikap Conservation Forest, Central Kalimantan, Indonesia.

Abstract

Rescue, rehabilitation and reintroduction programmes are a vital conservation tool used to protect individuals under immediate threat, and then re-establish viable populations in protected habitats. The critically endangered orangutan (Pongo spp.) has experienced significant population decline, primarily due to deforestation and hunting, therefore a considerable number of individuals are rescued from fragmented forests or the illegal pet trade. Whilst the protection of the remaining wild populations remains paramount, the substantial number of individuals currently residing in rescue centres means reintroduction is a crucial strategy for reducing the threat of extinction. Previously, many reintroductions have been ineffective, primarily attributed to inadequate project management, the introduction of unregulated tourism, or due to the individuals’ failure to acquire vital survival skills. Furthermore, the lack of post-release monitoring often inhibits analyses of the post-release progress, survival rates, and the long-term effects of human-facilitated rehabilitation. Therefore, the reintroduction of Bornean orangutans (Pongo pygmaeus wurmbii) into two protected forests, Bukit Baka Bukit Raya National Park and Bukit Batikap Conservation Forest, Central Kalimantan, provided vital opportunities for understanding how early life experiences and rehabilitation shapes post-release behaviour, ultimately aiding the improvement of reintroduction protocols. Mixed model analyses of data from consistent post-release monitoring revealed that across the first three months post-release, orangutans displayed behavioural irregularities when compared to their fully-wild counterparts. Temporarily, significantly elevated levels of resting and travelling counteracted reduced levels of feeding, attributed to recovery from transportation and stress from reintroduction into a novel habitat. After approximately three months post-release, behaviour stabilised and closely resembled that of wild orangutans, revealing the robust behavioural flexibility of the species. Evidence is also provided for distinct dispersal strategies following reintroduction, most notably flanged males were more likely to disperse out of the study site, whilst females often displayed site fidelity and established their home range surrounding their release location. Reintroduced individuals often displayed larger home ranges, when compared to wild individuals, likely in response to substantial overlaps between individuals around release locations, and subsequent resource competition. Overall, reintroductions were deemed successful, however, differences in dietary composition were discovered, most notably, lower fruit consumption, and a subsequent increased reliance on fall-back foods. Dietary differences are likely attributed to insufficient foraging skills, therefore improvements to rehabilitation protocols are provided, vital for increasing post-release success. Ultimately, the success of this reintroduction programme, and the establishment of new populations reinforces the value of reintroduction as an effective tool for conservation.