Program and ethics information

Our study was conducted in proximity of the Frankfurt Zoological Society (FZS) station close to the border of the Bukit Tigapuluh National Park. The FZS station is part of the Sumatran Orangutan Conservation Project (SOCP) [53]. The SOCP, initiated in 1999, coordinates re-introductions of Sumatran orangutans and is based on a memorandum of understanding signed by the PanEco Foundation, the Yayasan Ekosistem Lestari (YEL), the Frankfurt Zoological Society, and the Indonesian Ministry of Forestry Directorate General of Forest Protection and Nature Conservation. The aim of the project is to provide a comprehensive conservation approach including species and ecosystem management with each partner focusing on one specific aspect. More than 360 confiscated orangutans have gone through quarantine and the related rehabilitation process so far, and more than 270 individuals have been reintroduced into the wild (Pratje, pers. communication). In 2010, the program has been evaluated and assessed as successful by the IUCN [19].The SOCP, the work of Frankfurt Zoological Society (FZS) station, as well as all procedures used in this study were approved by the Sumatran Ministry of Research, Technology and Higher Education (Kementerian Riset dan Teknologi; No.: 228/SIP/FRP/SM/VIII2014) and endorsed by the Indonesian Nature Conservation Agency (Balali Konservasi Sumber Daya Alam; LoE, 14.10.2010). All procedures also followed the IUCN guidelines for reintroduction [1,2] and major Indonesian animal welfare legislation.

Orangutan study area

Our study was conducted in the 50 km² area surrounding the Frankfurt Zoological Society (FZS) station, located in the Bukit Tigapuluh ecosystem of the Jambi and Riau provinces in Central Sumatra, Indonesia (Fig 1). The FZS station is located close to the border of the Bukit Tigapuluh National Park (-1°16’N, 102°56’E) and is part of the Sumatran Orangutan Conservation Program (SOCP) [15,53]. The Bukit Tigapuluh ecosystem is a lowland dipterocarp rainforest with a very rugged topography highly suitable for orangutans [56]. A dense network of small rivers and streams runs through the steep valleys. The elevation ranges between 60 and 843 meters above sea level (a.s.l.); slopes of at least 40% gradient cover more than 75% of the total area [57].

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Fig 1. Orangutan study area located at the edge of the Bukit Tigapuluh National Park.

Black dots around the Frankfurt Zoological Society (FZS) station (-1°16’N, 102°56’E) represent orangutan relocations collected using Very High Frequency radio tracking from 2011 to 2013 (n = 32 orangutans; n = 1,020 VHF relocations). Relocations were used to estimate the population-level home range (Kernel method, 99% isopleth). Grey dots represent locations where orangutans have been released. Individual orangutan maps are presented in the supporting information (S1–S8 Figs).

https://doi.org/10.1371/journal.pone.0215284.g001

Orangutan rehabilitation, release and radio-tracking

Since 2003, the Frankfurt Zoological Society (FZS) has released more than 160 ex-captive orangutans in this area of Sumatra following the IUCN guidelines for reintroduction [2,16]. The guidelines require that after confiscation from private households or pet markets, all orangutans have to be quarantined for a minimum of 30 days and undergo a health check, including an assessment of their physical and mental condition before entering a rehabilitation facility and having contact with other orangutans [56]. The orangutans cared for by the SOCP were transferred to the rehabilitation facility after the quarantine period. Upon arrival, individual records were created for each orangutan containing all information for identification purposes [56], such as photos of face and dentition, fingerprints, age estimate, and body measurements. Age estimates were based on dentition, bone length, body weight and behavior [58] (Ghassani, pers. communication). All orangutans had to undergo a medical check including tests for hepatitis A, B and C via blood samples and tests of fecal samples for parasites [56]. After all health checks were completed, the orangutans were transferred to socialization cages [56], where they were held with conspecifics to help them develop affiliative relationships, which sometimes continued to persist after release [14]. Socialization cages were built following IUCN guidelines: concrete floor for hygienic removal of droppings and located two meters above the ground. The cages were made of stable metal pipes with food hoist baskets and metal crates for nest building. The cages were sized approximately 8 m x 8 m x 4 m and divided into three sections, all connected by locks. Each individual went through an assessment of survival skills, such as nest-building, foraging, predator avoidance behavior and climbing skills to determine which skills were needed to be taught prior to be ready for the release [15] (Pratje, pers. communication).

During the rehabilitation process orangutans went through three main steps of the pre-release process. Firstly, they were subjected to behavioral enrichment to stimulate them mentally and physically. They practiced nest building, handling forest fruits and enhancing their motor skills, including food in hoist baskets. These baskets were provided to stimulate their interest and abilities in foraging activities by searching for food and getting it out of the object. This kind of training was performed daily. Secondly, orangutans went through forest training, where they spent the day in the forest under supervision of trainers to build up strength and practice climbing, nest building and foraging. Each orangutan underwent forest training every second day. Thirdly, orangutans had the chance to watch each other closely and develop relationships on a daily basis. This was important, since social learning plays an important role in the development of immature orangutans’ survival skills. Wild immatures normally learn through their mother [29,59], whereas rehabilitants may show the best progress when they learn from one another [15].

Orangutans were released when they showed sufficient survival capabilities. To assess survival capabilities, the orangutans were closely monitored by orangutan trainers during their forest training and in the socialization cages. In addition to being able to build a nest, foraging activity needed to reach a stable 40% of overall daily activity (Julius Paolo Siregar, personal communication). Steady movement through the trees, including the ability to move between trees and search for food independently had to reach 30% of their daily activity (Julius Paolo Siregar, personal communication). If the continuous monitoring of these individuals showed a significant increase of these capabilities for three months in a row, then an individual was deemed ready for release. This was done in the attempt to minimize the development of a strong human habituation. Table 1 reports details of the time spent in rehabilitation by the orangutans monitored in this study.

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Table 1. Sex, estimated age at release, pre-release and release information on the 32 orangutans (OUs, 13 females, 19 males) released and monitored in central sumatra from 2011 to 2013 by means of very high frequency radio tracking.

Individual spatial VHF relocations were used to build the population-level Resource Selection Function (RSF), which estimates OUs’ resource selection in a presence-available design. ‘Used’ locations (i.e., presence) correspond to sites where OUs have been relocated, whereas ‘available’ locations are random points drawn within the population home range (kernel method, 99% isopleth), which define resource availability.

https://doi.org/10.1371/journal.pone.0215284.t001

The FZS followed a soft release regime including behavioral post release monitoring, with no feeding platforms, but supplemental food that could be provided daily within the first week after release. All animals were eventually released during the rainy season when food availability was the highest (October—March) at individual release sites that were scattered around the FZS station at different distances (Table 1). All release sites had enough resources in the surroundings, such as abundant fruit trees, water, suitable forest structure for climbing, nest-building and locomotion.

Prior to their release, between 2011 and 2013, the 32 orangutans monitored in this study (13 females, 19 males, age 5–21 years old at time of release, see Table 1) were surgically implanted with a very high frequency (VHF) transmitter, developed by the Research Institute of Wildlife Ecology, Vienna. The transmitters were implanted subcutaneously in the thick connective fat tissue of the dorsal neck area. The physical dimensions of the standard implants were 28 mm in diameter, 11 mm in height and weigh 16 g. The transmitters were made of medicinal ceramic [60] and, if the orangutan was still in proximity of the research station, were removed after two years before battery expiration (Pratje, personal communication). The transmitter emitted a VHF-signal for 8 h every day, between 07:00 and 15:00. The telemetry team performed a systematic search of radio-tagged orangutans within a 10 km radius around the release site on a weekly basis. The center of the FZS station is also the center of a path network aligned like a spider web, divided into five sectors. Once a week the projects telemetry-team followed the paths in each sector and checked for a VHF-signal every 500 meters at fixed signaling points. When an orangutan was located via triangulation, the signal was followed until the orangutan was visible [61,62] and the coordinates were determined using a GPS device (Garmin 60CSx). Over a period of 36 months the projects telemetry-team collected 1020 relocations from 32 orangutans (Table 1).

Resource availability in relation to geographical data

We analyzed orangutan spatial behavior in relation to three different geographical datasets which have been identified as proxies for resource availability in a previous study at the same study site [54]: the location of the FZS station, the digital elevation model, and the location of rivers.

The FZS station provided released orangutans with social contact to other orangutans waiting to be released and with staff involved in the pre-release training and other activities. The FZS station provided supplementary fruit provisions for returning orangutans which appeared to be undernourished or rejected by wild orangutans. Note that orangutans were not released at the FZS station, but at individual release sites scattered at varying distances around the station (see Fig 1, S1–S8 Figs and below for full details).

Elevation was a good proxy for food availability since the number and abundance of fruit tree species increased with decreasing elevation, with the highest abundance being around 150–250 m a.s.l. [54]. Raster information on elevation (digital elevation model) was derived from the NASA Shuttle Radar Topographic Mission (SRTM) at 90 m resolution and was resampled to a spatial resolution of 30 m using regularized splines with tension interpolation [63].

The distance to the nearest river was another proxy for food availability since the highest number and abundance of fruit tree species were located in riparian forest areas close to rivers [54]. The most prominent family of fruit trees was Moraceae, which has short and frequent fructification cycles [64,65], thus providing a year-round accessible stable food resource [54]. Locations of the rivers in our study area were provided by the Indonesian Forestry Ministry.

Orangutan population-level resource selection

We modeled orangutan population-level resource selection by fitting a Resource Selection Function (RSF) [55]. A RSF is defined as any statistical model deployed to estimate the relative probability of selecting a resource unit versus alternative possible resource units. Resource selection functions are particularly suited to presence-availability designs, where used resources are sampled at locations where animals are relocated (presence data), whereas resources sampled at random locations within the area the animals could potentially use are used to characterize resource availability [55]. In our study case we followed Manly’s design II [55], meaning that the resources used by monitored orangutans were sampled at the individual level—i.e., radio tracking relocations—whereas available resources were sampled at the population level—i.e. random points within the population home range. Population level home range was calculated using the Kernel method (99% isopleth) [66]. Manly’s design II is recommended when the number of relocations for some of the monitored individuals is low, i.e., when the computation of availability at the individual home range level (e.g. Manly’s design III) is not suitable because of insufficient number of points needed to define it. A sensitivity analysis showed that at least seven random points associated to each orangutan relocation were needed to obtain stable model estimates. Therefore, we opted to draw ten random points per used location to obtain robust parameter estimates [67] (Table 1).

For each ‘used’ and ‘available’ orangutan location, we computed the distance to the FZS station, the distance to the nearest river and the elevation using ArcGIS 10.2 [68]. Each “used” location and related available random locations were associated with orangutan name, age, sex, the time spent in rehabilitation (in days), the time after release (in days), and the distance of the release site to the FZS station (Table 1). We thus created a dataset with ‘used’ (1s) and ‘available’ (0s) as a binary response variable. Predictors were screened to exclude collinearity issues (Pearson correlation coefficient |r_p| < 0.7; variance inflation factor VIF < 3; [69,70]).

A RSF approach implies two main steps. First, we need to estimate selection coefficients (i.e., beta estimates) using a logistic model. Second, we have to plug the coefficients estimated by the logistic model into a resource selection function [71–73]. We thus fitted a Generalized Linear Model (GLM) with a binomial distribution of error (step 1). We hypothesized the selection for the environmental predictors—i.e., distance to the FZS station, distance to river, elevation, including quadratic terms to account for nonlinear effects—to vary depending on orangutan age, time spent in rehabilitation, and time after release. We thus included these interaction terms in our starting model. To test whether animals released closer to the FZS station were more bonded to it, we added the interaction term between the distance to the FZS station and the distance of the release site to the FZS station. We finally included orangutan sex and orangutan identity (name) as fixed factors in the model to account for sex differences in habitat selection and for pseudoreplication of data, respectively. We chose to run a GLM with orangutan name as fixed effect rather than a mixed effect model (GLMM: [69,74]) with orangutan identity as random intercept. Both model classes properly account for pseudoreplication; however, because we could not meet the main assumption of mixed models, i.e., normally distributed random intercepts, we opted for the more robust GLM. We eventually simplified the structure of our starting GLM using the stepAIC function of the MASS package in R [75] to find the most parsimonious model based on the Akaike Information Criterium [76].

Parameters estimated of the best GLM were used to depict RSF predictions for orangutan selection patterns (step 2). The RSF was assumed to take the exponential form [55] as follows: (Eq 1) where β₁ to β_n are coefficients estimated by the GLM, which are associated with environmental variables x₁ to x_n, respectively. All statistical analyses were performed in R version 3.2.3 [75].

Age at release plays a key role in post-release spatial behavior

We found that younger orangutans were more likely to range closer to the FZS station, while older individuals were generally more likely to exhibit spatial behavior similar to wild counterparts. The strong spatial bond to the FZS station in younger orangutans was most likely related to the need for social contacts and food [14,77,78]. Given that young wild orangutans still strongly depend on their mother and spend most of the time in close proximity [29], it is plausible that the FZS station and the people there are assuming this function. Although extended effort was put into teaching the orangutans the needed survival skills, e.g. nest building, foraging, predator avoidance behavior and climbing, they still have a disadvantage to their wild conspecifics. The spatial bond to the FZS station decreased with increasing age at release. This is in accordance with the natural emancipation process, which implies that orangutans become independent from their mother and try to establish a home range of their own [29]. Previous research showed that orangutans that stayed at greater distance from the release station displayed spatial behavior that was more similar to that shown by wild conspecifics of similar age [15]. Although actual behavior of the orangutans was not measured, the recorded spatial behavior in this study indicates a similar pattern (see below for further discussion). In contrast, orangutans remaining in the vicinity of the release station may still be underdeveloped [15,79,80].

Whereas habitat selection in younger orangutans was mainly driven by their attraction to the FZS station, older orangutans showed strong selection for areas closer to the rivers with riparian forest. In addition to the highest number and abundance of fruit tree species [54], these areas have the highest frequency of large-diameter trees, and provide the most suitable forest structure for locomotion [81], indicating it is the best suitable habitat. With increasing age, selection for elevation around 200 m a.s.l. also became more pronounced. Compared to the maximum elevation in our study area (843 m a.s.l.), areas located at 200 m a.s.l. are expected to be inhabited by several fruit tree species and provide abundant fruit availability [54]. However, these patterns may be also related to older orangutans becoming increasingly dominant [82,83] and thus possibly occupying the better suitable habitat, forcing the younger orangutans to move to less suitable areas. The current release practice favors releasing orangutans at a young age, i.e. as juveniles or adolescents, because this stage of semi-independence is believed to be the best period for learning and integration [14,84]. Based on our findings, however, we suggest releasing orangutans at an older age.

Duration of pre-release rehabilitation and training affects post-release spatial behavior

We found that orangutans benefit from longer pre-release rehabilitation and training. Most ex-captive orangutans have limited capabilities to survive on their own [18]. Therefore, it is imperative to utilize their high learning abilities [14,29,85,86] and provide sufficient pre-release training [14,16]. The benefits of longer training are a higher chance to acquire needed survival skills, including the possibility to learn from more experienced conspecifics while held in socializing cages [14,16,87]. Therefore, the SOCP rehabilitation program has had to walk a fine line between providing sufficient pre-release training and releasing orangutans as soon as possible to minimize the chance of habituation. Social contact and care of apes by humans creates strong affiliation to humans [78], which is believed to have a negative influence on the adaptation process of rehabilitated orangutans [15]. In contrast to that, our results showed that the orangutans with longer pre-release training showed stronger selection for areas with higher food availability and a reduced bond to the release station, mimicking behavior expected from wild conspecifics. This indicates that pre-release training outweighs the possible negative effects of the developments of strong affiliations to humans.

Spatial behavior changes with increasing time after release

Our data showed a significant change in habitat selection over time after release. Other primate reintroduction and release programs report that the greatest behavioral change happens in the first year after release [88]. Irrespective of age, with increasing time after release orangutans in our study showed selection for habitat at greater distance from the FZS station and their selection for elevation became more pronounced. This is especially important for young orangutans. A recent study of the same population [89] found that survival rates of young rehabilitated orangutans are lower than those of young wild conspecifics [25,90], which is most likely related to immature foraging skills, narrowly based habitat selection (this study), and poor predator avoidance behavior [15]. The increasing distance to the release station and the more pronounced selection for elevation with increasing time after release is evidence of adaptation to more natural ranging behavior [15,89]. With more time after release, orangutans become more experienced and confident [29], and start exploring areas further away from the release station, thus eventually finding other areas with stable food supplies.

Location of the release site influences post-release spatial behavior

Our data suggest that orangutans released at greater distances from the FZS station were less likely to go back to it. Therefore, if habitat suitability is ensured [16], releasing orangutans as far away as possible from human presence (in this case the FZS station), might be desirable to foster emancipation. However, it is possible that geographical barriers and the difficulty traversing the terrain contribute to the finding that orangutans released near the FZS station were more likely to stay in its vicinity. Also, orangutans released near to the release station have a greater chance of being in hearing distance and therefore might be drawn back to it. With increasing distance of the release site from the FZS station new challenges arise, including logistics and increased transportation stress for the orangutans [16]. In addition, the inaccessibility might lead to low-intensity post release monitoring. Therefore, only orangutans with a very high level of survival skills should be released far away from the release station. A closely monitored release near the release station might be preferable for less experienced orangutans.

Limitations of this study

The unknown pre-confiscation history of the rehabilitated orangutans remains a challenge in explaining individual differences in behavior and abilities during pre- and post-release. Depending on the age at capture the orangutans may or may not have learned basic survival skills from their mothers [18,80]. For most orangutans the treatment during captivity is unknown [11]. Some orangutans are held as pets and might have developed a strong affiliation to humans [15], others were abused during captivity and might have developed an aversion against humans, which might influence their post-release spatial behavior. In long-term projects such as release programs for orangutans, data continuity is a big problem and data quality often suffers from constant staff rotation. To prevent data loss standardized data sheets should be developed and used, e.g. for the initial health assessment or individual daily records during the rehabilitation process. Projects with geographically separated facilities for quarantine, rehabilitation and release should also have standardized hand-over procedures and documentation for all relevant data. Only then we will be able to increase our fundamental understanding of future behavioral development of rehabilitated orangutans.