Research Article: Spatial behavior in rehabilitated orangutans in Sumatra: Where do they go?

Date Published: May 1, 2019

Publisher: Public Library of Science

Author(s): Dominik Fechter, Simone Ciuti, Doris Kelle, Peter Pratje, Carsten F. Dormann, Ilse Storch, Matthew S. Crowther.


Wildlife restoration is one of the key components of conservation strategies, and this includes the rehabilitation and release of animals confiscated from wildlife traffickers. When primates are re-introduced, most individuals need a pre-release training to acquire the skills needed to survive in the wild. Pre-release training may either negatively or positively affect primate post-release behavior and survival. Post-release behavior, however, has rarely been monitored even though it is the only means to assess the survival of released individuals. Here, we present a thorough analysis of data from a 3-year radio tracking study on 32 orangutans (Pongo abelii) released in Sumatra after their rehabilitation. We investigated whether and how the age at release, the duration of the pre-release rehabilitation and training, and the release location affected the post-release individual spatial behavior. Orangutans released at older age exhibited post-release habitat selection patterns that were more comparable to that shown by wild conspecifics, i.e., they chose areas closer to rivers and at lower elevations (150–250 meters a.s.l.) where previous research had documented greater food availability. In contrast, individuals released at younger age showed a stronger spatial dependency on the rehabilitation station and exhibited disrupted habitat selection patterns; although after several months after the release all individuals tended to decrease their spatial reliance on the rehabilitation facility. This study indicates that the rehabilitation of individuals for a longer period and their release further from the rehabilitation station have facilitated the subsequent development of more natural spatial behavior, i.e. driven by food availability rather than by the dependence on care-giving human facility. Our study provides indications on how to improve the rehabilitation and release of confiscated orangutans, highlighting the importance of the age at release, the length of the rehabilitation program, and the location of the release site.

Partial Text

Population restorations are an important tool used to foster viable populations of threatened species worldwide [1]. These can be either the reintroduction of individuals into an area formerly occupied by the species, or a reinforcement, which implies that individuals are released into an area already occupied by conspecifics [1–3]. Before engaging in a population restoration, several criteria must be met. The reasons causing the decline or even the extinction of the former population, for example, have to be correctly identified, removed or sufficiently reduced [1]. The area selected for the restoration must go through a habitat suitability assessment to ensure that sufficient resources are available to the target species [1].

The step AIC model selection procedure allowed us to simplify the structure of the starting full GLM (AIC = 3333.4), which resulted in the final version reported in Table 2 (AIC = 3323.7, pseudo-R2 = 0.53). The final model, explaining more than 50% of the variance in habitat selection and post-release spatial behavior in rehabilitated orangutans, retained most of the interaction terms; the sex of orangutans, however, was not retained (Table 2). The parameters estimated by the best GLM were plugged in the RSF (Eq 1) to depict orangutan selection patterns: interaction terms were all significant and were portrayed in Figs 2–5 (for detailed model predictions depicting inter-individual variability as well as model uncertainty see supplementary information, S9–S13 Figs).

We made use of radio tracking data to determine how key variables drove the post-release spatial behavior and habitat selection of rehabilitated orangutans in central-eastern Sumatra, Indonesia. We showed that the age at release, the duration of pre-release rehabilitation training, the time since release and the location of the release site positively affected the likelihood of released orangutans to assume food-availability-driven spatial behavior in the wild.

The variables we selected (i.e. age at release, the duration of pre-release rehabilitation training, the time since release and the location of the release site) explained more than fifty percent of the variance in habitat selection and post release spatial behavior in rehabilitated orangutans. However, other factors, such as semi-solitary lifestyle [91], age-dependent avoidance behavior [34,83] or the relatively high orangutan density in close proximity to release stations [54] should be taken into account in future studies in order to deepen our understanding of post-release spatial behavior in rehabilitated orangutans.