The Bornean orang-utan Pongo pygmaeus is one of the most iconic species, yet formal studies of its terrestriality have been conducted only recently (Loken et al., Reference Loken, Spehar and Rayadin2013; Ancrenaz et al., Reference Ancrenaz, Sollmann, Meijaard, Hearn, Ross and Samejima2014). Understanding the factors that influence orang-utan terrestriality has important implications for the conservation of this Endangered great ape (Loken et al., Reference Loken, Spehar and Rayadin2013; Ancrenaz et al., Reference Ancrenaz, Sollmann, Meijaard, Hearn, Ross and Samejima2014) but the extent and context of orang-utan terrestriality remain poorly understood (Ancrenaz et al., Reference Ancrenaz, Marshall, Goossens, van Schaik, Sugardjito, Gumal and Wich2008).

Camera trapping has revolutionized the study of wildlife (Hance, Reference Hance2012) and provided a method for studying cryptic behaviour such as terrestriality in orang-utans. Loken et al. (Reference Loken, Spehar and Rayadin2013) reported that orang-utans in Wehea Forest, East Kalimantan, Indonesia, were almost as terrestrial as the pig-tailed macaque Macaca nemestrina (Caldecott, Reference Caldecott1986) and that there was no clear relationship between canopy connectivity and terrestriality. Ancrenaz et al. (Reference Ancrenaz, Sollmann, Meijaard, Hearn, Ross and Samejima2014) conducted a large-scale analysis of camera trap data from across Borneo and Sumatra and concluded that human disturbance does not appear to be the main driver of terrestriality. These studies indicate that orang-utan terrestriality may be more common than previously thought, and that orang-utans may be capable of using landscapes that necessitate terrestriality.

We report a 2.5-year camera-trapping study of terrestriality in the north-east Bornean orang-utan P. pygmaeus morio in three forest types (secondary, recently logged and primary) in and near Wehea Forest (38,000 ha), which comprises mostly undisturbed forest bordered by active logging concessions (Fig. 1). A number of studies have investigated the impact of logging on orang-utans (Hardus et al., Reference Hardus, Lameira, Menken and Wich2012). Most of these studies have focused on the effects of logging on populations (e.g. density), however, and there remains a gap in our understanding of how individuals and various species and subspecies react to logging.

Fig 1. Locations of camera trap stations in areas of secondary, recently logged and primary forest in and near Wehea Forest, East Kalimantan, Indonesia. The rectangle on the inset shows the location of the main map on the island of Borneo.

In 2012 we established 41 camera trap stations, covering c. 110 km², along old logging roads in the area of secondary forest, which was last logged in 1996. Data collected at two mineral licks (sepans) were not included in the analysis. These results were reported in Loken et al. (Reference Loken, Spehar and Rayadin2013) and have been modified slightly to standardize comparisons across the three forest types.

In October 2012 we set camera traps in an active logging concession adjacent to Wehea Forest. Twenty-two 100-ha compartments were logged, but not intensively, during April 2012–April 2014. Previously this logging block had contained primary forest and had no logging roads except for an access road used to enter Wehea Forest. We set 36 camera trap stations along newly constructed secondary logging roads in 13 compartments, immediately following the cessation of logging activities there. All stations remained in place until April 2014. In July 2013 we set 20 camera trap stations in primary forest in the heart of Wehea Forest (Fig. 1). All stations were set along ridgelines that were part of a middle transect developed in Wehea Forest for research purposes. These stations remained in place until June 2014. In each area we used Bushnell Trophy Cam camera traps (Bushnell, Cody, USA), which were fixed to trees, c. 50 cm from the ground, and fitted with a plastic cover above and a bed of leaves below to protect against rain and mud. Each camera was set to take three pictures per trigger, with a reset time of 1 s.

Our study comprised 7,661 trap nights in secondary forest, 15,775 trap nights in recently logged forest and 5,049 trap nights in primary forest. Photographs from each area were sorted by species, and the time and date of each independent capture (photographs taken > 1 hour apart) were recorded. Amongst c. 300,000 photographs we recorded 63 of orang-utans in secondary forest, 189 in recently logged forest and 44 in primary forest.

We used two parameters as estimates of orang-utan terrestriality across study sites. The first parameter, relative abundance index, was used to compare orang-utan-trapping success (Ancrenaz et al., Reference Ancrenaz, Hearn, Ross, Sollmann and Wilting2012) and is a good indicator of the amount of effort (total captures per trap night) required to obtain photographs of orang-utans. The second parameter was ground-use score, which was calculated as the ratio of independent photographs of orang-utans to the total number of independent photographs of all species at each study site, multiplied by the percentage of stations that recorded orang-utans. Together, these parameters are a good indicator of the level of orang-utan terrestriality at each of our study sites (Table 1).

Table 1 Capture history and level of terrestriality of orang-utans Pongo pygmaeus morio in and near Wehea Forest, East Kalimantan, Indonesia (Fig. 1), with forest type, number of trap nights, total number of captures, percentage of camera trap stations with captures, relative abundance index and ground-use score.

* Data from secondary forest were originally reported in Loken et al. (Reference Loken, Spehar and Rayadin2013).

Orang-utans were more terrestrial in the recently logged forest (relative abundance index 1.20, ground-use score 1.87), which is consistent with Ancrenaz et al. (Reference Ancrenaz, Sollmann, Meijaard, Hearn, Ross and Samejima2014) and indicates that anthropogenic canopy disturbances may increase orang-utan terrestriality (Rijksen & Meijaard, Reference Rijksen and Meijaard1999). However, our results also indicate a high level of terrestriality in both primary (relative abundance index 0.87, ground-use score 0.87) and secondary forest areas (relative abundance index 0.82, ground-use score 0.79), indicating that anthropogenic canopy disturbances are not the only driver of terrestriality. Loken et al. (Reference Loken, Spehar and Rayadin2013) demonstrated that orang-utans were also terrestrial in areas with high canopy connectivity. This is consistent with our findings: orang-utans also demonstrated terrestrial behaviour in the primary forest, which had a closed canopy and ample opportunity for moving through trees.

Possible explanations for the varying levels of terrestriality are differences in abundance and visibility of orang-utans across the study sites. However, we believe these are not the primary causes of differences between our estimates. Firstly, encounter rates with orang-utans were highest in the secondary forest and lowest in the recently logged forest. If used as a naive estimate of relative abundance (Loken et al., Reference Loken, Spehar and Rayadin2013), we would expect encounter rates to be highest in the recently logged forest. Secondly, visibility was the same in each forest type, as cameras were set along similar types of features (e.g. roads, ridge lines, trails).

Previously, terrestrial behaviour in orang-utans was considered uncommon and used only as a means of acquiring resources (MacKinnon, Reference MacKinnon1974). Our results indicate that terrestriality may be a regular strategy, employed almost equally by males and females as a means of locomotion (Table 2). In the recently logged forest, where orang-utans demonstrated the highest degree of terrestriality, most photographs of orang-utans appeared to show the primates walking along the road rather than across it, which would be the case if they were forced to the ground by a break in the canopy. This is consistent with photographs from the other forest types, and indicates that orang-utans may be taking advantage of both anthropogenic (e.g. roads) and natural (e.g. ridgelines) features in their environment, thus demonstrating opportunistic and resilient behaviour.

Table 2 Number of records of orang-utans in each age/sex class, from secondary, recently logged and primary forest in and near Wehea Forest (Fig. 1).

Orang-utans may be using newly created roads opportunistically but the level of disturbance they can tolerate is still unknown. Orang-utan populations can be maintained in sustainably logged forests (Knop et al., Reference Knop, Ward and Wich2004; Husson et al., Reference Husson, Wich, Marshall, Dennis, Ancrenaz, Brassey, Wich, Atmoko, Setia and van Schaik2009; Ancrenaz et al., Reference Ancrenaz, Ambu, Sunjoto, Ahmad, Manokaran, Meijaard and Lackman2010) and these forests should be incorporated into orang-utan conservation strategies (Meijaard et al., Reference Meijaard, Albar, Nardiyono, Rayadin, Ancrenaz and Spehar2010; Wich et al., Reference Wich, Gaveau, Abram, Ancrenaz, Baccini and Brend2012; Wilson et al., Reference Wilson, Meijaard, Venter, Ancrenaz and Possingham2014). We must be careful, however, not to reinforce the notion that orang-utans can survive in any human-altered landscape. Orang-utans still need trees, and lots of them, and protection of Borneo's remaining forests should continue to be of highest priority for both Indonesia and the global community.