Capuchin monkeys (genus Cebus ) have evolutionarily converged with humans and chimpanzees in a number of ways, including large brain size, omnivory and extractive foraging, extensive cooperation and coalitionary behaviour and a reliance on social learning. Recent research has documented a richer repertoire of group-specific social conventions in the coalition-prone Cebus capucinus than in any other non-human primate species; these social rituals appear designed to test the strength of social bonds. Such diverse social conventions have not yet been noted in Cebus apella , despite extensive observation at multiple sites. The more robust and widely distributed C. apella is notable for the diversity of its tool-use repertoire, particularly in marginal habitats. Although C. capucinus does not often use tools, white-faced capuchins do specialize in foods requiring multi-step processing, and there are often multiple techniques used by different individuals within the same social group. Immatures preferentially observe foragers who are eating rare foods and hard-to-process foods. Young foragers, especially females, tend to adopt the same foraging techniques as their close associates.

The type and variety of learning strategies used by individuals to acquire behaviours in the wild are poorly understood, despite the presence of behavioural traditions in diverse taxa. Social learning strategies such as conformity can be broadly adaptive, but may also retard the spread of adaptive innovations. Strategies like pay-off-biased learning, by contrast, are effective at diffusing new behaviour but may perform poorly when adaptive behaviour is common. We present a field experiment in a wild primate, Cebus capucinus , that introduced a novel food item and documented the innovation and diffusion of successful extraction techniques. We develop a multilevel, Bayesian statistical analysis that allows us to quantify individual-level evidence for different social and individual learning strategies. We find that pay-off-biased and age-biased social learning are primarily responsible for the diffusion of new techniques. We find no evidence of conformity; instead rare techniques receive slightly increased attention. We also find substantial and important variation in individual learning strategies that is patterned by age, with younger individuals being more influenced by both social information and their own individual experience. The aggregate cultural dynamics in turn depend upon the variation in learning strategies and the age structure of the wild population.

Researchers of “culture” have long been interested in the role of social learning in establishing patterns of behavioral variation in wild animals, but very few studies examine this issue using a developmental approach. This 7-year study examines the acquisition of techniques used to process Luehea candida fruits in a wild population of white-faced capuchin monkeys, Cebus capucinus, residing in and near Lomas Barbudal Biological Reserve, Costa Rica. The two techniques for extracting seeds (pounding or scrubbing) were approximately equal in efficiency, and subjects experimented with both techniques before settling on one technique—typically the one they most frequently observed. In a sample of 106 subjects that had already settled on a preferred technique, the females adopted the maternal technique significantly more often than expected by chance, but the males did not. Using a longitudinal approach, I examined the acquisition of Luehea processing techniques during the first 5 years of life. Regression analysis revealed that the technique most frequently observed (measured as proportion of Luehea processing bouts observed that used pounding as opposed to scrubbing) significantly predicted the technique adopted by female observers, particularly in the second year of life; the amount of impact of the observed technique on the practiced technique was somewhat less significant for male observers. These results held true for (a) observations of maternal technique only, (b) observations of technique used by all individuals other than the mother, and (c) observations of maternal and non-maternal techniques combined.

Significance: Extensive care of offspring by mothers is a fundamental trait of all mammals, including humans, and the loss of a mother can be catastrophic for offspring. Here, we identify previously undocumented ways in which the death of a mother affects her offspring, using long-term, longitudinal data from seven primate species. First, females that experience early maternal loss are often less able, as adults, to produce offspring that survive to maturity. Second, in some species young offspring are more likely to die if their mother is facing impending death (death in the next few years), even while she is still alive. This work has important implications for our understanding of the evolution of the long life spans that primates exhibit.

An important extension to our understanding of evolutionary processes has been the discovery of the roles that individual and social learning play in creating recurring phenotypes on which selection can act. Cultural change occurs chiefly through invention of new behavioral variants combined with social transmission of the novel behaviors to new practitioners. Therefore, understanding what makes some individuals more likely to innovate and/or transmit new behaviors is critical for creating realistic models of culture change. The difficulty in identifying what behaviors qualify as new in wild animal populations has inhibited researchers from understanding the characteristics of behavioral innovations and innovators. Here, we present the findings of a long-term, systematic study of innovation (10 y, 10 groups, and 234 individuals) in wild capuchin monkeys (Cebus capucinus) in Lomas Barbudal, Costa Rica. Our methodology explicitly seeks novel behaviors, requiring their absence during the first 5 y of the study to qualify as novel in the second 5 y of the study. Only about 20% of 187 innovations identified were retained in innovators’ individual behavioral repertoires, and 22% were subsequently seen in other group members. Older, more social monkeys were more likely to invent new forms of social interaction, whereas younger monkeys were more likely to innovate in other behavioral domains (foraging, investigative, and self-directed behaviors). Sex and rank had little effect on innovative tendencies. Relative to apes, capuchins devote more of their innovations repertoire to investigative behaviors and social bonding behaviors and less to foraging and comfort behaviors.