Development of tool-use behavior in a social context
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The habitual use of tools by wild capuchin monkeys presents a unique opportunity to study the maintenance and transmission of tradition. Here we study the development of tool-use in wild bearded capuchin monkeys (Sapajus libidinosus), and the social influence on this behavior. We followed a wild, habituated group in which most adults habitually crack nuts using anvils and hammer stones. We followed 16 infants and juveniles in five collection periods during 26 months, recording their behavior and the behavior of other individuals around them. Manipulation of nuts started at the age of few months, as single-object manipulation and object-to-surface actions. Combinatory actions appeared later, shortly after weaning. The juveniles adopted more efficient strategy for nut-cracking with age, and the rate of non-efficient actions decreased. Overall, infants and juveniles sustain a high level of interest in nuts and stones long before they can efficiently crack. We suggest that this interest is maintained with the help of social influence. We show that the juveniles match the behavior of the adults in both time and space. The rate of manipulation of nuts by the juveniles was higher when others in the group cracked and ate nuts, and also when the juveniles themselves were near the anvils, where nut-cracking takes place. We suggest a model for the social influence on nut-cracking skill development that is based on two related processes: social facilitation from observing groupmates engaged in nut-cracking, and the opportunity for practice provided by the anvils, hammer stones and nut shells on and around the anvils. Finally, we show that the rate of manipulation of nuts by infants and juveniles declines exponentially in the minutes following the end of nut-cracking activity around them, as does the time infants and juveniles spent near an anvil. The dynamics of social facilitation changed with the age of the juveniles, with the older juveniles showing a longer half-life for nut-related behavior. This is the first study to provide such a broad and quantitative approach to the development of tool use, or indeed, any behavior thought to be traditional, in a wild population.