Abstract: Forest structure influences the spatial patterns of ecological communities in important ways. Tree-dwelling animals such as primates are especially impacted by forest structure characteristics, as many primates have specific structural requirements for effective movement through arboreal habitats. Most primates are arboreal, however, there is mounting evidence of arboreal primates using the ground more often than previously assumed due to an increase in the use of camera trap surveys. Yet, we still have a limited understanding of the ecological factors that influence terrestriality in these primates. The main goal of this study is to identify the relative influence of forest structure on the terrestrial behavior of arboreal primates.
We compiled ecological data concerning forest structure, weather, fruit availability, and predator presence at Gunung Palung National Park (GPNP), West Kalimantan, Indonesia. We used five years of camera trap videos (207 sampling locations, >53,000 trap days) which captured occurrences of primates on the ground and the presence of clouded leopards, a known primate predator. To measure forest structure, we used a ground-based LiDAR system to scan 120 camera trap locations, using a 50 m radius around each location to create a detailed 3D image of the surrounding forest. We derived multiple forest structure metrics from each 3D image, including forest height, canopy connectivity, and diameter of trees (dbh). We used occupancy models to statistically assess the relative influence of ecological factors on the terrestriality of Bornean orangutans, red leaf monkeys, long-tailed macaques, and pig-tailed macaques at GPNP.
We found that the seven contiguous habitat types at GPNP differed substantially in forest structure metrics like canopy height and average dbh. Primate terrestrial behaviors were highly habitat-dependent for some taxa and not importantly driven by weather or fruit availability. We also found clear evidence of predator avoidance –primate terrestrial observations were relatively uncommon or absent in places or at times when clouded leopards were active. These results help elucidate the intricacies of a relatively rare behavior among arboreal primates, highlight the importance of forest structure in primate habitat use and terrestrial behavior, and advance understanding of primate behavioral responses to various ecological pressures.
