Biased cultural transmission of a social custom in chimpanzees

Abstract

Cultural transmission studies in animals have predominantly focused on identifying between-group variation in tool-use techniques, while immaterial cultures remain understudied despite their potential for highlighting similarities between human and animal culture. Here, using long-term data from two chimpanzee communities, we tested whether one of chimpanzees' most enigmatic social customs-the grooming handclasp-is culturally transmitted by investigating the influence of well-documented human transmission biases on their variational preferences. After identifying differences in style preferences between the communities, we show that older and dominant individuals exert more influence over their partners' handclasp styles. Mothers were equally likely to influence their offspring's preferences as nonkin, indicating that styles are transmitted both vertically and obliquely. Last, individuals gradually converged on the group style, suggesting that conformity guides chimpanzees' handclasp preferences. Our findings show that chimpanzees' social lives are influenced by cultural transmission biases that hitherto were thought to be uniquely human.

Fig. 1.. Chimpanzees at the Chimfunshi Wildlife Orphanage Trust (Zambia) engaging in grooming handclasps.

The chimpanzees use different styles of handclasping, for instance, (A) palm-to-palm style and (B) palm-to-wrist style.

Fig. 2.. The styles by which the chimpanzees engaged in the grooming handclasp could be linked to sociodemographic features.

The estimated influence on handclasp style preference depicted (A) across the rank continuum and (B) across the observed age range. The sizes of the circles (raw data) are proportional to the number of observations, the solid lines represent the averaged regressions, and the transparent lines depict all the posterior samples. Red, males; black, females.

Fig. 3.. The chimpanzees have group-specific handclasp preferences, with younger chimpanzees showing more variation than older chimpanzees.

The estimated preferences for each option (green, wrist; black, elbow; yellow, forearm; blue, other; purple, palm) for (A) group 1 and (B) group 2. Dashed lines show the estimated group preferences μ_jk, whereas points represent individual preferences. Open circles are the handclasp style preferences for individuals >8 years old, and filled squares are the preferences for individuals ≤8 years old. Lines connect the same individuals as they age. The points for younger individuals are more widely scattered around the group means than those for older individuals.