Gait parameters in vertical climbing of captive, rehabilitant and wild Sumatran orang-utans (Pongo pygmaeus abelii)

Vertical climbing is central to the locomotor and foraging strategies of the great apes and, indeed, to theories about the evolution of locomotor specialisations of hominoid primates. Nevertheless, its kinematics have yet to be fully evaluated. Here, we present spatio-temporal parameters of 80 climbing sequences containing 560 limb cycles obtained from video recordings of captive, rehabilitant and wild Sumatran orang-utans (Pongo pygmaeus abelii). Gait parameters such as cycle duration, duty factor, laterality of footfalls, relative stride length and normalised speed were analysed to identify the influence of body size (through an intraspecific comparison of age/sex classes), the influence of the environment (through an intraspecific comparison of animals living in different habitats) and the influence of morphology (through an interspecific comparison with the climbing characteristics of other primates) on climbing ability in orang-utans. Results show that there are only few differences between the climbing of wild, rehabilitant and captive individuals. Cycle duration is longer and speed is lower for the wild individuals than for other groups due to the complexity of their environment and lack of familiarity with each substrate, which results in a more cautious gait. Sex/age-related differences in climbing were found to be small, although juveniles generally exhibited a shorter cycle duration and lower duty factor than other groups. The spatio-temporal gait parameters of adult females carrying infants were not found to differ significantly from those for females without an infant, which suggests that climbing kinematics are not affected by the presence of a clinging infant. Extended-elbow vertical climbing is primarily characterised by a higher duty factor than flexed-elbow climbing, indicating that the former is an energetically more demanding form of locomotion. In comparison with other primates, orang-utans exhibit a longer cycle duration, longer strides but lower climbing speed, reflecting a compromise between the demands of a large body mass and extreme joint mobility.