HANDEDNESS IN WILD CHIMPANZEES

The debate over nonhuman primate precursors to human handedness is unsettled mainly due to lack of data, particularly on apes. Handedness in wild chimpanzees at the Tai National Park, Cote d'Ivoire, has been monitored in four tasks. For the simple unimanual ones, reaching and grooming, adults use both hands equally (ambidextrous), while for the more complex unimanual wadge-dipping and the complex bimanual nut-cracking, adults are highly lateralized. These results support the hypothesis that lateralization increases with the complexity of the task. The lateralization is constant for years for each task but may vary in an individual with respect to different tasks. For nut-cracking, females are more lateralized than males. The ontogeny of handedness for nut-cracking shows many variations in the tendency to use one hand and in the side preferred, until at about 10 years of age, the individual achieves her adult handedness. No population bias toward one side exists in Tai chimpanzees. No heritability of handedness between mother and offspring was observed. Human and chimpanzees handedness are compared.