Terrestrial Positional Behavior of Wild <i>Pongo pygmaeus</i>

OBJECTIVES: As a predominantly arboreal animal in the wild, the terrestrial positional behavior of Pongo pygmaeus is poorly understood, having been studied almost exclusively in captive settings. This study uses camera-trap footage to provide the first assessment of wild orangutan terrestrial locomotor and postural behavior on natural substrates. MATERIALS AND METHODS: Video footage of orangutans from motion-activated cameras in the Gunung Palung National Park, West Kalimantan, Indonesia was collected over a five-year period. The resulting 100 instances of orangutan terrestriality were analyzed to document incidences of positional behavior, including hand and foot postures as well as overstride pattern during locomotion and qualitative assessment of hindlimb joint excursions during orthograde behaviors. RESULTS: When locomoting terrestrially, Pongo pygmaeus primarily engaged in quadrupedal fist-walking with heel-strike. Wrist position and overstride pattern during quadrupedal walking were variable both within and between individuals. For posture, individuals were captured standing upright more often than pronograde, both monopedally and bipedally, and almost always with full extension of the hip and knee. DISCUSSION: These observations of wild orangutan terrestrial positional behavior address prior ambiguities related to hand and foot positioning during locomotion. Maintained full extension of the hindlimb in the absence of substrate compliance indicates the form of bipedalism orangutans use in the trees translates to natural terrestrial substrates. Finally, the high proportion of observations with asymmetrical movement and posture may indicate a lateral decoupling of limbs in addition to the expected fore- and hindlimb independence of apes, suggesting orangutan positional adaptability is intrinsic regardless of substrate.