Left-hand Preference for a Complex Manual Task in a Population of Wild Siamangs (<Emphasis Type="BoldItalic">Symphalangus syndactylus</Emphasis>)

Many hypotheses have been proposed linking the emergence of lateralized activities—such as handedness—in primates, with hemispheric specialization and the evolution of complex communication such as human language. Although data to test these ideas are rapidly accumulating for many primate taxa, some species are still largely unexplored, especially under natural conditions. I present the first data on a population of wild siamangs (Symphalangus syndactylus), highly arboreal small apes. Preference for the left hand is shown both at the individual and population levels for a complex manual task: collecting and drinking water from tree holes. There was no difference in hand preference between males and females, and immature individuals showed more variable patterns than adults. These results are consistent with the postural origins theory, allow a new interpretation of the findings of comparable studies, and indicate a useful behavior for future investigations of laterality in wild primates.