Traditions in spider monkeys are biased towards the social domain

Abstract

Cross-site comparison studies of behavioral variation can provide evidence for traditions in wild species once ecological and genetic factors are excluded as causes for cross-site differences. These studies ensure behavior variants are considered within the context of a species' ecology and evolutionary adaptations. We examined wide-scale geographic variation in the behavior of spider monkeys (Ateles geoffroyi) across five long-term field sites in Central America using a well established ethnographic cross-site survey method. Spider monkeys possess a relatively rare social system with a high degree of fission-fusion dynamics, also typical of chimpanzees (Pan troglodytes) and humans (Homo sapiens). From the initial 62 behaviors surveyed 65% failed to meet the necessary criteria for traditions. The remaining 22 behaviors showed cross-site variation in occurrence ranging from absent through to customary, representing to our knowledge, the first documented cases of traditions in this taxon and only the second case of multiple traditions in a New World monkey species. Of the 22 behavioral variants recorded across all sites, on average 57% occurred in the social domain, 19% in food-related domains and 24% in other domains. This social bias contrasts with the food-related bias reported in great ape cross-site comparison studies and has implications for the evolution of human culture. No pattern of geographical radiation was found in relation to distance across sites. Our findings promote A. geoffroyi as a model species to investigate traditions with field and captive based experiments and emphasize the importance of the social domain for the study of animal traditions.

Figure 1. Distribution of traditions observed across the five study sites.

The photograph of a behavior indicates its presence at the customary or habitual level at the field site. A faded photograph of a behavior indicates its absence or occurrence only at the present level at the field site. A missing photograph with an ‘E’ indicates the absence of the behavior at a field site due to ecological reasons (Photographs by Claire J. Santorelli and Luisa Rebecchini and drawing by Claire J. Santorelli).

Figure 2. Percentage of behaviors, which met criteria for traditions, belonging to the three domain categories (social, food-related and other) at each site.

See Table S1 for the division of the 22 traditions into the three domains and Figure 1 for the traditions at each site.

Figure 3. Percentage of behaviors showing evidence of tradition at each site by sub-domain category.

See Table S1 for the division of behaviors into sub-domains and Figure 1 for the traditions at each site. Note that the only behavior in the sub-domain ‘Other’ was wind catching at the Santa Rosa site.

Figure 4. Association between the number of customary/habitual behaviors or absence of behavior each pair of study sites shared and the distance between each pair of sites.