Hand use and posture during manipulative behaviours and arboreal locomotion in African apes

The skill with which primates use their hands to explore and interact with the environment sets them apart from most other mammals. The non-human primate hand serves an important functional role during not only terrestrial and arboreal locomotion, but also enhanced grasping and manipulative behaviours. Understanding how living primates use their hands for these various functions is fundamental for understanding the order Primates and the evolution of humans within this order. While bipedalism and the extraordinary manipulative abilities of our human hand for manufacturing stone tools are considered to be unique, their origins remain controversial. Understanding this evolutionary shift in human hand use from locomotion to manipulation requires comparative studies of hand use in our closest living relatives, the African apes (chimpanzees, bonobos and gorillas). To date, however, little research has been done on African ape daily hand use, including both locomotor and manipulative behaviours, especially in natural environments. This dissertation will address this gap by conducting detailed studies of hand use and posture during two complex manipulative behaviours (i.e., plant-processing, nut-cracking) and arboreal locomotion (i.e., vertical climbing) in the natural environment of African apes.

I conducted the first comprehensive analysis of bonobo palm oil nut-cracking in a natural environment at the Lola ya Bonobo sanctuary, Democratic Republic of the Congo. All eighteen bonobos showed exclusive laterality for using the hammerstone and there was a significant group-level right-hand bias. The study revealed 15 hand grips for holding differently-sized and -weighted hammerstones, 10 of which had not been previously described in the literature. The findings also demonstrated that bonobos select the most effective hammerstones when nut-cracking and that bonobos, despite rarely using tools in the wild, can be efficient nut-crackers with a skill level that is similar to palm oil nut-cracking chimpanzees of Bossou, Guinea.

I further provided the first insights into the manual skills of Bwindi mountain gorillas by examining hand-use strategies, hand grips, and hand-preference (i.e., laterality) during the processing of three different plants. Two of these plants are woody-stemmed plants for which the food is more challenging to access in comparison to leaves, lacking physical defenses that are relatively simple to process. Bwindi gorillas used the greatest number of hand actions to process the most complex plant food (i.e., peel-processing) similar to complex thistle feeding by Virunga mountain gorillas. The manipulative actions were ordered in several key stages organised hierarchically. The demands of manipulating natural foods elicited 19 different hand grips and variable thumb postures, of which three grips were new and 16 grips have either been previously reported or show clear similarities to grips used by other wild and captive African apes and humans. A higher degree of lateralisation was elicited for the most complex behaviour of peel-processing but the strength of laterality was only moderate, suggesting that peel-processing is not as complex as thistle leaf-processing by Virunga gorillas.

Finally, I examined for the first time hand use, forelimb posture and gait chacteristics during vertical climbing in wild, habituated mountain gorillas (Gorilla beringei) of the Bwindi Impenetrable National Park, Uganda, and semi-free-ranging chimpanzees (Pan troglodytes) of the Chimfunshi Wildlife Orphanage Trust, Zambia, both within a natural environment. This research revealed that 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 showed that large-bodied gorillas exhibited significant longer cycle duration, lower stride frequency and generally a higher duty factor than chimpanzees. This quantitative analysis revealed that mountain gorillas adapt their climbing strategy to accommodate their large body mass in a similar manner found in captive western lowland gorillas, and that chimpanzees showed less variation in their climbing strategy than has been documented in captive bonobos. In summary, this study demonstrates the importance of forceful hand grips and the variable use of the thumb relative to substrate size in both ape species, and particularly in large-bodied mountain gorillas as they face more biomechanical challenges during vertical climbing than smaller-bodied chimpanzees.

Together, this dissertation provides new insights into the functional link between hand morphology and behaviour in African apes in their natural environments that may ultimately generate more informed reconstructions of fossil hominin locomotor and manipulative behaviours. Furthermore, this research shows that the suite of "unique human grips" or "unique human manipulative abilities" that have typically defined humans is getting much smaller the more we learn about African apes, particularly in their complex natural environment where the hand has to adjust to varying foods and arboreal substrates, and where individuals have ample opportunity to learn and develop high manipulative skills.