Comparison of Hand Use and Forelimb Mechanics of Vertical Climbing in Wild Mountain Gorillas and Free-Ranging Chimpanzees

Biomechanical analyses of great ape arboreal locomotion in a natural environment are scarce, thus limiting attempts to correlate behavioural and habitat differences with variation in skeletal morphology. Vertical climbing is a crucial locomotor and foraging strategy of great apes and the hands are critically important to maintaining stability on irregular, arboreal substrates. However, little is known about arboreal grips and hand postures, or how these might vary with forelimb posture during vertical climbing on natural substrates of different sizes. This is particularly true of mountain gorillas, which are considered the least arboreal of all African apes and for which the characteristics of vertical climbing have not yet been studied. The aim of this study was to compare temporal kinematics of hand and forelimb use during vertical climbing in wild, habituated mountain gorillas (Gorilla beringei beringei) of the Bwindi Impenetrable Forest (Uganda) and sanctuary chimpanzees (Pan troglodytes) of the Chimfunshi Wildlife Orphanage Trust (Zambia) to assess differences in climbing styles that may relate to variation in hand or forelimb morphology and body mass. We investigated hand and forelimb posture coupled with temporal gait parameters during vertical climbing (both ascent and descent) in 15 mountain gorillas and eight chimpanzees, using video records ad libitum. In both apes, forelimb posture was correlated with substrate size during both ascent and descent climbing. Both apes used power grips and a diagonal power grip, involving three different thumb postures. Gorillas showed greater ulnar deviation of the wrist during climbing than chimpanzees, and the thumb played an important supportive role when vertically descending compliant substrates in gorillas. Comparisons of temporal gait parameters indicated that large-bodied gorillas exhibited significant longer cycle duration, lower stride frequency and generally a higher duty factor than chimpanzees. This study revealed that wild mountain gorillas adapt their climbing strategy to accommodate their large body mass in a similar manner found in captive western lowland gorillas, but that our sanctuary chimpanzees showed less variation in their climbing strategy within a natural environment than has been documented in captive bonobos