<Note> Wild Chimpanzees at Mahale are not Manually Lateralised for Throwing

Abstract

INTRODUCTION The importance of throwing has figured consistently and prominently in scenarios about the evolutionary origins of human behaviour1,2 The utility of imparting force to airborne projectile weapons, launched ballistically, is obvious, whether to deter, punish or subdue predators, prey, or competitors. Other functions of throwing are less obvious but could be equally useful, such as bringing down hanging fruit, gaining others’ attention, shattering large objects into fragments, etc. Scenarios have stressed a number of important variables, such as posture (especially bipedality), sensorimotor skill (especially hand-eye coordination), cerebral asymmetry, etc. Calvin1 linked all of these to the evolutionary origins of language. However, few of these ideas have been tested empirically. One way to tackle evolutionary aspects of human throwing is to look at throwing performance in our nearest living relations. Although throwing has been known in both wild3 and captive4 chimpanzees for almost a century, quantitative studies are few; often they are subsumed in broader studies of manual behaviour, e.g. ref. 5. Furthermore, studies often do not distinguish between aimed (targeted) throwing versus unaimed (perhaps better termed hurling) throwing, underarm versus overarm delivery, or one-handed versus twohanded throwing. Several studies of manual laterality in throwing by captive chimpanzees have been published, and all have reported population-level, right-sided bias6–8. Others have incorporated these reported findings into comprehensive accounts of the origin of handedness, e.g. ref. 9 We can find no published quantitative data on manual laterality in throwing from any non-human primate species in nature, much less from chimpanzees. One obvious reason for this absence is that, unlike in the artificial conditions of captivity, wild primates throw only rarely. Here we report such a dataset, collected ethologically from a population of chimpanzees in nature, over an extended period, in an effort to balance the picture.