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The evolution of food sharing in primates

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Abstract

The aim of this study is to explain the occurrence of food sharing across primates. Defined as the unresisted transfer of food, evolutionary hypotheses have to explain why possessors should relinquish food rather than keep it. While sharing with offspring can be explained by kin selection, explanations for sharing among unrelated adults are more controversial. Here we test the hypothesis that sharing occurs with social partners that have leverage over food possessors due to the opportunity for partner choice in other contexts. Thus, we predict that possessors should relinquish food to potential mates or allies, who could provide or withhold matings or coalitionary support in the future. We used phylogenetic analyses based on both maximum likelihood and Bayesian approaches in a sample of 68 primate species to test these predictions. The analyses strongly indicate that (1) sharing with offspring is predicted by the relative processing difficulty of the diet, as measured by the degree of extractive foraging, but not overall diet quality, (2) food sharing among adults only evolved in species already sharing with offspring, regardless of diet, and (3) male–female sharing co-evolved with the opportunity for female mate choice and sharing within the sexes with coalition formation. These results provide comparative support for the hypothesis that sharing is “traded” for matings and coalitionary support in the sense that these services are statistically associated and can thus be selected for. Based on this, we predict that sharing should occur in any species with opportunities for partner choice.

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Notes

  1. Other hypotheses explaining sharing in humans or other animals such as costly signaling are not discussed here because there is no convincing evidence supporting them in primates.

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Acknowledgements

The authors would like to thank Michael Gurven, Karin Isler, Charles Nunn, Gabrielle Russo, Maria van Noordwijk, Janneke van Woerden, the UCSB’s Human Behavioral Ecology Lab, and several anonymous reviewers for discussions and many helpful comments on earlier versions of the manuscript. We also thank Charles Nunn for sharing parts of his forthcoming book “The comparative method in evolutionary anthropology and biology”, which was highly informative for the methods used in this study. Finally, we are very grateful to the Swiss National Science Foundation (grant PBZHP3-133433), the Cogito Foundation (grant S-106/06), and the A.H. Schultz Foundation for financial support to AJ. The authors declare no conflicts of interest.

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Authors and Affiliations

  1. Anthropological Institute and Museum, University of Zurich, Winterthurerstr. 190, 8057, Zurich, Switzerland

    Adrian V. Jaeggi & Carel P. Van Schaik

  2. Research Priority Program in Ethics, University of Zurich, Zollikerstr. 117, 8008, Zurich, Switzerland

    Adrian V. Jaeggi

  3. Integrative Anthropological Sciences, University of California Santa Barbara, Santa Barbara, CA, 93106-3210, USA

    Adrian V. Jaeggi

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  1. Adrian V. Jaeggi
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Correspondence to Adrian V. Jaeggi.

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Online resource 1

Following Pagel and Meade (2006), we plotted the posterior distributions of rate coefficients, i.e., estimated probabilities (q) for evolutionary transitions between states (see Fig. 3). Rate pairs, i.e., the probabilities of gains or losses of one trait in the presence or absence of the other trait, were arranged vertically for easy comparison. Differences between rate pairs provide evidence for correlated evolution, e.g., pairs q13 and q24, which correspond to gains of food sharing with and without another trait of interest. The graphs represent (a) sharing among adults, sharing with infants, (b) sharing from males to females, multi–male groups, (c) sharing among males, male–male coalitions, (d) sharing among unrelated males, male–male coalitions, (e) sharing among females, female–female coalitions, and (f) sharing among unrelated females, female–female coalitions. The written values are the mean ± SD values of q as well as the percentage of models that estimated q to zero (“zero bin”) and are based on six runs for each model (PDF 602 kb)

Online resource 2

This table provides an overview of the harmonic means and the resulting Bayes factors for each analysis across different settings of the rate deviation parameter. The settings that were reported, based on the recommended range of acceptance (0.2–0.4) and visual inspection of the plotted Markov chains to confirm convergence (available on request), are indicated in bold. Each analysis was run six times with each setting and the reported values are means and standard deviations of these six runs. Mean harmonic means for the same model never differ by more than 1 across different parameter settings, differences tend to follow the same direction for dependent and independent models, and the resulting Bayes factors are always, and most often substantially, greater than 2 and thus consistently provide support for dependent evolution despite some variation within and across parameter settings (PDF 82 kb)

Online resource 3

These graphs present the mean harmonic mean of six runs of dependent and independent models, respectively, plotted against the number of iterations of the model. Furthermore, we included histograms of harmonic means in the posterior distribution. In each graph, the lower chain and the histogram to the left, both in red, represent the independent models, whereas the upper chain and the histogram to the right, both in green, represent the dependent models. The x-axis for the histograms has the same range as the y-axis for the Markov chains. The graphs represent (a) sharing among adults, sharing with infants, (b) sharing from males to females, multi-male groups, (c) sharing among males, male–male coalitions, (d) sharing among unrelated males, male–male coalitions, (e) sharing among unrelated females, female–female coalitions, and (f) sharing among females, female–female coalitions. The graphs show that the Markov chains converged and that the differences between the harmonic means of dependent and independent models used to infer evidence for dependent evolution of the two traits were stable along the runs (PDF 189 kb)

Online resource 4

Figure a is a boxplot of the bibliographic frequency of female mate choice showing the significant difference between single-male (0) and multi–male (1) species. Figure b shows the significant correlation of the bibliographic frequencies of female mate choice and male–female food sharing within multi–male groups (N = 23) (PDF 117 kb)

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Jaeggi, A.V., Van Schaik, C.P. The evolution of food sharing in primates. Behav Ecol Sociobiol 65, 2125–2140 (2011). https://doi.org/10.1007/s00265-011-1221-3

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