Male-biased stone tool use by wild white-faced capuchins (Cebus capucinus imitator)

Abstract

Tool-using primates often show sex differences in both the frequency and efficiency of tool use. In species with sex-biased dispersal, such within-group variation likely shapes patterns of cultural transmission of tool-use traditions between groups. On the Panamanian islands of Jicaron and Coiba, a population of white-faced capuchins (Cebus capucinus imitator)--some of which engage in habitual stone tool use--provide an opportunity to test hypotheses about why such sex-biases arise. On Jicaron, we have only observed males engaging in stone tool use, whereas on Coiba, both sexes are known to use tools. Using 5 years of camera trap data, we show that this variation reflects a true sex difference in tool use rather than a sampling artefact, and then test hypotheses about the factors driving this pattern. Differences in physical ability or risk-aversion, and competition over access to anvils do not account for the sex-differences in tool-use we observe. Our data show that females are physically capable of stone tool use: females on Coiba and juveniles on Jicaron as small and smaller than adult females regularly engage in tool use. Females also have ample opportunity to use tools: the sexes are equally terrestrial, and competition over anvils is low. Finally, females rarely scrounge on left-over food items either during or after tool-using events, suggesting they are not being provisioned by males. Although it remains unclear why white-faced capuchin females on Jicaron do not use stone-tools, our results illustrate that such sex biases in socially learned behaviors can arise even in the absence of obvious physical, environmental and social constraints. This suggests that a much more nuanced understanding of the differences in social structure, diet and dispersal patterns are needed to explain why sex-biases in tool use arise in some populations but not in others.