Kin-based cultural transmission of tool use in wild chimpanzees

Abstract

Current research on animal culture has focused strongly on cataloging the diversity of socially transmitted behaviors and on the social learning mechanisms that sustain their spread. Comparably less is known about the persistence of cultural behavior following innovation in groups of wild animals. We present observational data and a field experiment designed to address this question in a wild chimpanzee community, capitalizing on a novel tool behavior, moss-sponging, which appeared naturally in the community in 2011. We found that, 3 years later, moss-sponging was still present in the individuals that acquired the behavior shortly after its emergence and that it had spread further, to other community members. Our field experiment suggests that this secondary radiation and consolidation of moss-sponging is the result of transmission through matrilines, in contrast to the previously documented association-based spread among the initial cohort. We conclude that the spread of cultural behavior in wild chimpanzees follows a sequential structure of initial proximity-based horizontal transmission followed by kin-based vertical transmission.

Keywords: Pan troglodytes; chimpanzees; kin transmission; social transmission; tool use.

Fig. 1. Social transmission of cultural behavior in wild chimpanzees.

(A) Probability of moss-sponging by subjects depending on whether they have at least one moss-sponger in the matriline: parameter estimates with SEs. (B) Probability of moss-sponging by subjects depending on their association history with other moss- or leaf-spongers in the group, calculated using the half-weight index (HWI): model estimates with SEs (shaded areas). The numerical results are provided in table S1.

Fig. 2. Proportion of trials with subjects using moss to manufacture a sponge.

The size of the circles corresponds to the total number of trials observed (that is, the larger the circle is, the more trials the subject had). The red lines represent the means. Note that the figure represents raw data. Results of the generalized linear mixed model are provided in fig. S1 and table S1.

Fig. 3. Model of the two-dimensional transmission pattern.

Circles represent all individuals seen moss-sponging at least once between 2011 and 2014. The eight original moss-spongers from Hobaiter et al.’s study (2) are represented in gray. The arrows represent the social transmission found by Hobaiter et al. (2). The rectangles represent matrilines in which vertical social learning took place, the main transmission mechanism involved in the spread. The dashed lines represent the possible learning path of the cases (n = 7) in which individuals acquired the behavior in the absence of vertical transmission because individuals did not have moss-sponging kin, suggesting learning from individuals that they were highly associated with (linked with the individual with the highest association index).