Gait characteristics of vertical climbing in mountain gorillas and chimpanzees

Biomechanical analyses of arboreal locomotion in great apes in their natural environment are scarce and thus attempts to correlate behavioural and habitat differences with variations in morphology are limited. The aim of this study was to investigate the gait characteristics of vertical climbing in mountain gorillas (Gorilla beringei beringei) and chimpanzees (Pan troglodytes) in a natural environment to assess differences in the climbing styles that may relate to variation in body size. We investigated temporal variables (i.e., cycle duration, duty factors, and stride frequency) and footfall sequences (i.e., diagonal vs. lateral sequence gaits) during vertical climbing (both ascent and descent) in 11 wild mountain gorillas and compared these data to those of eight semi-free-ranging chimpanzees, using video records ad libitum. Comparisons of temporal gait parameters revealed that largebodied mountain gorillas exhibited a longer cycle duration, lower stride frequency and generally a higher duty factor than smaller-bodied chimpanzees. While both apes were similarly versatile in their vertical climbing performance in the natural environment, mountain gorillas most often engaged in diagonal sequence/diagonal couplet gaits and chimpanzees most often used lateral sequence/diagonal couplet gaits. This study revealed that mountain gorillas adapt their climbing strategy to accommodate their large body mass in a similar manner previously found in captive western lowland gorillas, and that chimpanzees are less variable in their climbing strategy than has been documented in captive bonobos.