Effects of manual disability on feeding skills in gorillas and chimpanzees

Foraging gorillas and chimpanzees employ skilful bimanual techniques to process food plants, and there are wild populations in which 10-20% of individuals have severe hand injuries. We examined the feeding skills of one injured gorilla and two injured chimpanzees, while they dealt with plants for which intact peers employ complex techniques. Feeding efficiency was only slightly compromised in the disabled subjects, and none of them had acquired novel techniques specific to their remaining capacity. Instead, all three subjects used techniques like those of able-bodied individuals, though when several optional methods were available, their balance of preference was different. They adapted the techniques to their disabilities by the flexible substitution of a range of alternative means to achieve each step of the process, including unusual grips, use of one hand instead of two or vice versa, and use of the mouth or a foot. Compensation was at the level of detailed execution rather than overall technique and depended on transferring motor organization to novel effector organs. Our findings have implications for theories of the acquisition of complex manual skills in great apes and for the flexibility of great ape mental skills.