Feeding skills and the effect of injury on wild chimpanzees
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While gorillas and orangutans have been shown to display considerable manual skill in obtaining certain plant foods, complex feeding skills in chimpanzees have only been described in the restricted context of tool use. This thesis provides the first study of plant-processing skills in a non-tool using community of chimpanzees in Budongo Forest, Uganda. Furthermore, this community contains over 20% of individuals with upper or lower limb injuries. The strategies used by injured individuals in compensating for injury were investigated through a comparison of feeding skill between the able-bodied and injured population. A cognitive approach to feeding behaviour in chimpanzees was adopted, with respect to the implications this may have for overcoming the effects of injury. Chimpanzees were found to employ a broad range of skills in feeding, reflecting variation in their environment and in their diet. Three food types were examined, each illustrating a particular aspect of feeding skill. In processing leaves of Broussonettia papyrifera, chimpanzees use complex multi-stage feeding techniques, employ bimanual co-ordination at several stages and elicit behaviour that is hierarchical in overall organisation. Able-bodied individuals show considerable standardisation in their feeding with a preference for two techniques. In contrast, when feeding on figs, chimpanzees rely upon simple processing techniques but at the same time employ strategies that serve to minimize the effects of feeding competition. In the case of Ficus mucuso chimpanzees co-ordinate several handfuls of food simultaneously between limbs, and with Ficus sur, chimpanzees display a range of dynamic feeding postures and positions in order to access food patches and increase relative food availability. No significant hand preferences were found in any of the three feeding tasks. Even the most severe of injuries does not result in a decline in feeding efficiency, and the possible mechanisms contributing to this were addressed. Injured individuals were found not to invent novel solutions to familiar tasks, but instead to modify their existing repertoire in order to work around their injuries, thus sharing the program-level organisation observed in able-bodied individuals and compensating at the level of individual actions. However, the physical limitations imposed by the injured limb considerably reduce bimanual coordination and manoeuvrability in the tree, which may have long-term negative implications.
Thesis, PhD Doctor of Philosophy
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