Sex differences in early experience and the development of aggression in wild chimpanzees

Abstract

Sex differences in physical aggression occur across human cultures and are thought to be influenced by active sex role reinforcement. However, sex differences in aggression also exist in our close evolutionary relatives, chimpanzees, who do not engage in active teaching, but do exhibit long juvenile periods and complex social systems that allow differential experience to shape behavior. Here we ask whether early life exposure to aggression is sexually dimorphic in wild chimpanzees and, if so, whether other aspects of early sociality contribute to this difference. Using 13 y of all-occurrence aggression data collected from the Kanyawara community of chimpanzees (2005 to 2017), we determined that young male chimpanzees were victims of aggression more often than females by between 4 and 5 (i.e., early in juvenility). Combining long-term aggression data with data from a targeted study of social development (2015 to 2017), we found that two potential risk factors for aggression—time spent near adult males and time spent away from mothers—did not differ between young males and females. Instead, the major risk factor for receiving aggression was the amount of aggression that young chimpanzees displayed, which was higher for males than females throughout the juvenile period. In multivariate models, sex did not mediate this relationship, suggesting that other chimpanzees did not target young males specifically, but instead responded to individual behavior that differed by sex. Thus, social experience differed by sex even in the absence of explicit gender socialization, but experiential differences were shaped by early-emerging sex differences in behavior.

Self-protection as an adaptive female strategy

“…The sex difference in direct aggression occurred across diverse cultures by age 2 years and continued through adolescence (Maccoby & Jacklin, 1974). In non-human primates as well, females typically engaged in less intense physical aggression than males (Sabbi et al, 2021;Smuts, 1987).…”

Non-genetic maternal effects shape individual differences in cortisol phenotypes in wild chimpanzees

“…As offspring associate almost permanently with their mothers until around the age of 12 years (Reddy and Sandel, 2020), maternal social phenotype is the key determinant of the social environment of immature offspring. As some mothers are more consistently gregarious than others (Tkaczynski et al, 2020b), and social settings likely impact rates of exposure to social stressors for offspring (Sabbi et al, 2021; Tkaczynski et al, 2020a), maternal effects on average cortisol levels are perhaps not surprising in immature individuals. However, the maternal effects observed in our study apply for individuals of all age classes, suggesting that they endure beyond the immature phase.…”

Puberty initiates a unique stage of social learning and development prior to adulthood: Insights from studies of adolescence in wild chimpanzees

“…Juvenility is preceded by adrenarche, which occurs in late infancy (∼2–3 years of age) and has been suggested to be associated with sex variation in aggressive behavior; Fig. 2 ; Sabbi et al, 2020 , Sabbi et al, 2021 . The beginning of the juvenile stage is marked primarily by the cessation of (a) nursing, (b) being carried by mothers during travel, and (c) sleeping together with mothers in a single night nest rather than constructing one’s own beside mothers ( Box 1 ; Fig.…”