Hand preferences in the skilled gathering tasks of mountain gorillas (Gorilla g. berengei).

Manual dexterity of 44 wild gorillas of all ages and both sexes was investigated on six naturally acquired feeding tasks of varied logical structure, which included multi-stage and bimanually coordinated procedures. At least 400 min observation of feeding per animal, and analysis at the level of bouts rather than individual actions, can be expected to produce statistically robust results; 22 years background data allowed effects of genealogy and injury to be investigated. Five tasks elicited very strong hand preferences in most animals (weakest on the simplest task), while one was usually performed with a strategy in which left and right hands were used symmetrically; the preferences were fully established at 3 years old. Preferences were highly correlated within each of two sets of tasks, but between the two sets there was no correlation across animals in direction or strength of preference. No population trends towards left or right handers were found in either set of tasks, or both sets pooled; distributions were U-shaped and approximately symmetric, with a slight bias towards right-handed fine manipulation in one set of tasks. Processing efficiency was only slightly greater with the preferred hand, and similar in left- and right-handed animals. Neither direction nor strength of hand preference showed a tendency to run in families, but females showed greater strength of preference. Major injury masked strong hand preference, but injuries could not account for the overall distribution of preference; instead this may reflect inbreeding. Feedback acting on initially random hand choices, and imitation at the "program" level rather than slavish copying of acts, are proposed to account for the results.