Maternal influence on the development of nut-cracking skills in the chimpanzees of the Taï forest, Côte d'Ivoire (Pan troglodytes verus).

Chimpanzees' (Pan troglodytes) nut-cracking behavior represents one of the most complex forms of tool-use known among nonhuman animals. Given the close phylogenetic relationship between these apes and humans, investigating how such complex behavior develops in immatures can reveal the evolutionary roots of the cognitive processes that enabled the evolution of outstanding technological skills in our lineage. In this study, we investigated whether maternal behavior directly enhanced nut-cracking skills in immature individuals. We analyzed the behavior of 11 immatures and their mothers (N = 8) during nut-cracking activity, spanning over three consecutive nut-cracking seasons in the Taï National Park, Côte d'Ivoire. We used generalized linear mixed models to (a) obtain values of maternal scaffolding (defined as provision of learning opportunities) and active nut-sharing behavior of each mother according to the age of their offspring, and their average nut-cracking efficiency; (b) to test whether these variables enhanced immatures' nut-cracking skills; and (c) to test whether immatures' features (age, sex, and begging behavior) influenced maternal behavior as observed in our videos. Although the predicted values of maternal scaffolding and active nut-sharing did not obviously affect immatures' skills, they were positively influenced by the average maternal efficiency and by sharing hammers with their mothers. In addition, our observations showed that mothers were more likely to share nuts with their sons than with their daughters, and the more their offspring begged. Concurrently, male immatures were also found to beg more often than females. Our results add evidence on the ontogenetic pathway leading to the full acquisition of nut-cracking in wild chimpanzees and on the effect that maternal behavior can have in promoting the acquisition of this complex tool-use behavior. Moreover, our study strengthens the importance of naturalistic observations to understand complex skill acquisition. Finally, we suggest future avenues for investigating the maternal influence on learning.