Coming down from the trees: Is terrestrial activity in Bornean orangutans natural or disturbance driven?

Abstract

The orangutan is the world's largest arboreal mammal, and images of the red ape moving through the tropical forest canopy symbolise its typical arboreal behaviour. Records of terrestrial behaviour are scarce and often associated with habitat disturbance. We conducted a large-scale species-level analysis of ground-based camera-trapping data to evaluate the extent to which Bornean orangutans Pongo pygmaeus come down from the trees to travel terrestrially, and whether they are indeed forced to the ground primarily by anthropogenic forest disturbances. Although the degree of forest disturbance and canopy gap size influenced terrestriality, orangutans were recorded on the ground as frequently in heavily degraded habitats as in primary forests. Furthermore, all age-sex classes were recorded on the ground (flanged males more often). This suggests that terrestrial locomotion is part of the Bornean orangutan's natural behavioural repertoire to a much greater extent than previously thought, and is only modified by habitat disturbance. The capacity of orangutans to come down from the trees may increase their ability to cope with at least smaller-scale forest fragmentation, and to cross moderately open spaces in mosaic landscapes, although the extent of this versatility remains to be investigated.

Canopy structure drives orangutan habitat selection in disturbed Bornean forests

“…However, recent work has found orangutans to be much more flexible in their behavior, and more resilient to anthropogenic disturbance than previously thought (12,13). For example, contrary to previously held views, orangutans travel terrestrially in all forest types but display increased terrestrial activity in both heavily disturbed and primary forest, suggesting that not only can they adapt their behavior but that anthropogenic disturbance is not necessarily the dominant driver of such behavioral adaptation (14).…”

“…There were no significant differences in the way age and sex classes used canopies (Kruskal-Wallis test, P > 0.05 for all structural metrics) (Fig. 2), suggesting that structural drivers influence orangutans in similar ways despite pronounced sexual dimorphism and adult male bimaturism, as well as observed age and sex differences in terrestrial activity and branch use (14,24; but see refs. 22, 23, where no differences in locomotion between age and sex classes were detected).…”

“…Remaining in the upper canopy is therefore a more astute strategy, which is further aided by uniform canopy height. When canopies are irregular or structurally complex, lateral canopy movement can become even more energetically expensive than terrestrial locomotion because of the required increase in vertical movement (14). In contrast, uniform canopies enable orangutans to remain at the same height, thereby enhancing lateral movement.…”

“…Tall trees (including emergent crowns) are also important nesting locations (26, 27) and concentrated fruiting sites (11,28), especially in disturbed forests (29), likely contributing to their selection. Although orangutans can and do descend to the ground to cross gaps (14), this activity is energetically expensive because it requires descending and reascending the canopy (25), and also exposes orangutans to increased pathogens and predation risk (14). Remaining in the upper canopy is therefore a more astute strategy, which is further aided by uniform canopy height.…”

A Short Cut to Mushrooms – Red Langur (Presbytis rubicunda) Consumption of Terrestrial Fungus

“…Table 1 shows the breakdown in behaviour from direct observations and camera traps. Direct focal animal observations are more effective to observe details about feeding but camera traps provide insight into behaviours which may not be easily observed or underperformed when humans are present [Ancrenaz et al, 2014;Cheyne et al, 2018]. Red langurs spent 20% of time on the ground feeding (it is not always possible to see from the photos what is being consumed).…”