Egalitarian despots: hierarchy steepness, reciprocity and the grooming-trade model in wild chimpanzees, Pan troglodytes

Stefano S.K Kaburu,Nicholas E Newton-Fisher

doi:10.1016/j.anbehav.2014.10.018

Abstract

Biological market theory models the action of natural selection as a marketplace in which animals are viewed as traders with commodities to offer and exchange. Studies of female Old World monkeys have suggested that grooming might be employed as a commodity to be reciprocated or traded for alternative services, yet previous tests of this grooming-trade model in wild adult male chimpanzees have yielded mixed results. Here we provide the strongest test of the model to date for male chimpanzees: we use data drawn from two social groups (communities) of chimpanzees from different populations and give explicit consideration to variation in dominance hierarchy steepness, as such variation results in differing conditions for biological markets. First, analysis of data from published accounts of other chimpanzee communities, together with our own data, showed that hierarchy steepness varied considerably within and across communities and that the number of adult males in a community aged 20-30 years predicted hierarchy steepness. The two communities in which we tested predictions of the grooming-trade model lay at opposite extremes of this distribution. Second, in accord with the grooming-trade model, we found evidence that male chimpanzees trade grooming for agonistic support where hierarchies are steep (despotic) and consequent effective support is a rank-related commodity, but not where hierarchies are shallow (egalitarian). However, we also found that grooming was reciprocated regardless of hierarchy steepness. Our findings also hint at the possibility of agonistic competition, or at least exclusion, in relation to grooming opportunities compromising the free market envisioned by biological market theory. Our results build on previous findings across chimpanzee communities to emphasize the importance of reciprocal grooming exchanges among adult male chimpanzees, which can be understood in a biological markets framework if grooming by or with particular individuals is a valuable commodity.

Highlights

Biological market theory models the action of natural selection as a marketplace in which animals are viewed as traders with commodities to offer and exchange
In agreement with the grooming-trade model, adult male chimpanzees of the Ngogo community (Kibale National Park, Uganda) tended to direct grooming up the social dominance hierarchy, with most reciprocity occurring between individuals with similar social ranks (Watts, 2000b), while data collected from males of the significant relation between aggression performed and rank (Sonso) community (Budongo Forest Reserve, Uganda) in 1994/1995 revealed that males directed grooming up the hierarchy, but grooming reciprocity was not stronger among males holding similar ranks (Newton-Fisher & Lee, 2011)
What is the relationship between hierarchy steepness and grooming reciprocity?—We found no statistically significant difference between communities in the degree to which male chimpanzees reciprocated grooming effort, M-group males were more reciprocal in the frequency of bouts exchanged during the stable period

Summary

Introduction

Biological market theory models the action of natural selection as a marketplace in which animals are viewed as traders with commodities to offer and exchange. Under the model, when individuals are not able to offer alternative services (commodities), such as when rank relationships are shallow (egalitarian hierarchies: van Schaik, 1989; Vehrencamp, 1983) or access to resources cannot be controlled (e.g. when food is scattered), grooming is exchanged for itself (i.e. reciprocated: Barrett et al, 1999). Commodities such as agonistic support are likely to be rank-related rather than dominance-restricted as most individuals should be able to offer the commodity, but its quality (e.g. the effectiveness of agonistic support) will depend on the trader’s rank. In agreement with the grooming-trade model, adult male chimpanzees of the Ngogo community (Kibale National Park, Uganda) tended to direct grooming up the social dominance hierarchy, with most reciprocity occurring between individuals with similar social ranks (Watts, 2000b), while data collected from males of the Sonso community (Budongo Forest Reserve, Uganda) in 1994/1995 revealed that males directed grooming up the hierarchy, but grooming reciprocity was not stronger among males holding similar ranks (Newton-Fisher & Lee, 2011)

Methods

Results

Conclusion