We identify evolutionary advantages that may incentivize tolerance toward extra‐group individuals in humans & nonhuman primates, including enhanced benefits in the domains of transfer, mating, & food acquisition

The evolution of intergroup tolerance in nonhuman primates and humans. Anne C. Pisor, Martin Surbeck. Evolutionary Anthropology: Issues, News, and Reviews, August 6 2019. https://doi.org/10.1002/evan.21793

Abstract: Primate individuals use a variety of strategies in intergroup encounters, from aggression to tolerance; however, recent focus on the evolution of either warfare or peace has come at the cost of characterizing this variability. We identify evolutionary advantages that may incentivize tolerance toward extra‐group individuals in humans and nonhuman primates, including enhanced benefits in the domains of transfer, mating, and food acquisition. We highlight the role these factors play in the flexibility of gorilla, chimpanzee, bonobo, and human behavior. Given humans have an especially broad range of intergroup behavior, we explore how the human foraging ecology, especially large spatial and temporal fluctuations in resource availability, may have selected for a greater reliance on tolerant between‐community relationships—relationships reinforced by status acquisition and cultural institutions. We conclude by urging careful, theoretically motivated study of behavioral flexibility in intergroup encounters in humans and the nonhuman great apes.

1 INTRODUCTION

Attempting to explain the prevalence of intergroup aggression in primates, especially in humans (Homo sapiens sapiens), evolutionary anthropologists have focused extensively on intergroup contest and warfare. In response, other evolutionary anthropologists have focused extensively on peace systems in primates, especially in humans. Focusing on these two ends of the spectrum—war or peacefulness—has come at the cost of fully characterizing within‐species variation in individuals' behavioral strategies in intergroup encounters (e.g., Refs. 1-4; see also, Ref. 5: table 22‐1). Furthermore, both of these approaches emphasize selection pressures that favor or disfavor intergroup aggression; less researched are the selection pressures that, given disincentives for intergroup aggression, favor tolerant encounters and the prolongment of tolerant encounters in intergroup association.

In the present review, our goal is to call for explicit theorization about the individual‐level selection pressures that favored flexible behavior in intergroup encounters in humans and nonhuman primates, especially the often‐overlooked pressures that may favor tolerant encounters and association given disincentives for aggression. We review how tolerant behavior toward extra‐group conspecifics in specific domains—such as food access, mating, and reconnaissance before transfer—may have been favored by natural selection in nonhuman primates. In the course of this review, we pay special attention to the group‐living, nonhuman great apes, but not because these species are necessarily the best analogies for intergroup behavior in humans. We focus on these species for two reasons: first, due to our common ancestry, humans and the extant nonhuman great apes share a number of traits derived within the Primate order, suggesting that there is (at least some) insight to be gained by drawing comparisons between these species; and second, to highlight how little we still know about intergroup encounters in the nonhuman great apes, especially in gorillas and bonobos.

Given what has been observed of intergroup behavior in nonhuman primates, we assess whether consideration of the potential selective benefits favoring intergroup encounter and association in these species provides insight into human behavior. Our review of the literature suggests that the particularly high prevalence of intergroup tolerant encounter and association in humans may be derived, even within the great apes; we hypothesize that this high prevalence reflects human reliance on resources that vary extensively in their availability across space and time. Given that our field has invested much energy into studying the selection pressures favoring or disfavoring intergroup aggression, we conclude by urging evolutionary anthropologists to explicitly theorize about individual‐level selection pressures that may favor intergroup tolerant encounters, and even prolonged intergroup association, so that we can better understand the variation in intergroup behavior within and between species.


4 DISCUSSION

In evolutionary anthropology and in disciplines influenced by it, a common current assumption made by researchers is a “strong human universal toward parochial altruism”—in‐group favoritism at out‐group cost.86 Research focus on chimpanzees as a referential model for human behavior34 tends to promote this perspective. However, evidence suggests that individual behavior in intergroup encounters is actually quite flexible, both in humans (e.g., per the study from which the preceding quote was drawn86) and in the group‐living great apes generally. Disincentives for intergroup aggression have been thoroughly discussed by other reviews; however, these disincentives provide insight only into when selection could favor individual tolerance toward extra‐group members, but not why it does under these circumstances. Here, drawing on existing observations of nonhuman primates, we assembled potential fitness benefits that may favor intergroup tolerant encounter and association (Table 1). Though scientists know comparatively little about intergroup encounters in bonobos and gorillas relative to chimpanzees—a situation that should be remedied—the fitness benefits we identified seem to account for at least some of the observed variability in intergroup behavior in bonobos and gorillas.

Our review of the literature suggests that the benefits favoring intergroup tolerant encounter and association in nonhuman primates can account for some, but not all, of the flexibility of intergroup tolerance in humans. In both humans and nonhuman primates, mating and transfer, as facilitated by visitation, and opportunities for social learning are potential benefits to be gained from intergroup tolerant encounter and association. Likewise, across the Primate order, kinship and partner preferences can further amplify the benefits and minimize the costs of encounter. However, humans have a much higher prevalence of intergroup tolerant encounter and association than do nonhuman primates—at least, as observed to date. Evidence from anthropology and across the social sciences suggests that humans' reliance on resources with extensive spatial and temporal variability has necessitated flexible interest in between‐community relationships as a means of managing the risks of resource shortfalls and ensuring access to nonlocally available resources. When and where the benefits of between‐community resource access have been high, cultural institutions and social status have also enhanced and reinforced these benefits. This is not to say that humans do not engage in intergroup aggression—the ethnographic, archaeological, and contemporary records provide ample evidence of parochialism and warfare—but rather that human intergroup behavior can be both more tolerant and more aggressive than what we have observed in our closest relatives and that this flexibility in intergroup behavior is functional.

We advance the hypotheses outlined in this review for testing by the evolutionary anthropological community. Similar ideas with respect to the importance of between‐community resource access have been outlined by functionalist anthropologists, archaeologists, and human behavioral ecologists previously—although usually without treatment of why between‐community resource access is of particular importance in humans. We hope that by amalgamating these perspectives and building upon them, the present paper inspires newfound interest in the flexibility of human and nonhuman great ape intergroup behavior, moving our discipline beyond its current focus on parochialism. In addition to our larger hypothesis with respect to the human foraging ecology, we wish to highlight other related questions to be addressed by future work. (1) The higher the frequency of shortfalls, the more likely that individuals will recall these shortfalls (whether via their own memories or even via oral traditions) and maintain between‐community relationships accordingly50, 51—but how frequent must they be? Is once every several generations enough? (2) Will the connections we drew between status acquisition, cultural institutions, and the relative importance of between‐community resource access be supported by additional data? To date, the connection between status and between‐community relationships has been more theoretical than empirical. (3) Which poses stronger selection pressure in humans: benefits gained via intergroup tolerant encounters and association in the currency of between‐community resource access, or the cost of mortality risk from aggression and warfare,37 potentially reduced by intergroup tolerant encounters and association?

To answer the above questions and improve the accuracy of our characterizations of sociality in both humans and nonhuman great apes, researchers will need to collect targeted data assessing the predictors of intergroup behavior. For field researchers studying humans, we urge caution with respect to reliance on observational data and “complete” social networks. Asking participants about their social strategies for mitigating shortfalls,49 their preferences for same‐community vs between‐community relationships,41, 42 and their extra‐community ties85 may provide a more accurate picture of the flexibility of human sociality. Furthermore, the dedication of increased research effort to intergroup encounters and association in gorillas and bonobos, as well as habituation of neighboring groups, will improve our understanding of sociality in the group‐living nonhuman great apes.

In the present review, we opted not to unpack the nature of human “groups” nor human group psychology. Humans are adept at cognizing groups of various kinds—from groups formed in experimental contexts to interest‐based groups to ethnic or religious groups—and at recognizing their boundaries. A number of the papers and book chapters we reviewed here discuss potential derived functions of group living in humans (see Refs. 53, 54, 56, 69). Our larger point is that human reliance on resources that vary in their spatial and temporal availability often necessitates relationships spanning distance; in general, the group‐living great apes evidence flexible interest in intergroup encounters and association (Box 1), and it is likely that this flexible interest became even more important in the human lineage (Section 3.2). While relationships spanning distance sometimes span ethnolinguistic boundaries, for example, or religious boundaries, they do not necessarily. As such, questions of the proliferation of different types of human groups, and how ethnic groups may have been built on the scaffolding of social relationships through which nonlocal resources could be accessed (e.g.,83), we leave to other papers.

Given the lack of attention the benefits of intergroup tolerant encounter and association have received in evolutionary anthropology, the present review reflects initial theorizing about these incentives; as such, we have not explored the roles of constraints, including phylogeny and life history constraints, nor the affordances of a comparative approach with non‐primate species. Phylogeny and life history constraints likely affect the prevalence and flexibility of intergroup tolerance in different species of primates. For example, the relationship between intergroup tolerance and the ecological and social factors discussed here may partially reflect a third variable, phylogenetic signal. Whether such constraints explain existing observational data is a question to be answered by future work. Furthermore, we chose not to pursue a comparative approach with non‐primate species. Though the high incentives for intergroup tolerant encounter and association observed in humans may have better analogies among non‐primate vertebrates or even insects,2 our goal here was to explore intergroup tolerance in humans in the context of nonhuman primates rather than to find the closest‐match analogy for human behavior.

5 CONCLUSION
Intergroup behavior in primates is flexible, and the prevalence of intergroup tolerant encounters and association varies across species. To be sure, incentives for aggression vary, as discussed extensively in existing work; however, when incentives for aggression are low or absent, why would natural selection favor tolerant behavior toward extra‐group members—or even increased rates of intergroup tolerant encounter and association? Drawing inferences from the existing primatological literature, we highlighted benefits favoring intergroup tolerant encounter and association in the Primate order, including in group‐living nonhuman apes and humans, such as transfer, mating, and food acquisition. Humans are unique among primates in our high prevalence of intergroup tolerance, however, and data from across the social sciences suggest the relevance of the human foraging ecology—especially the spatial and temporal availability of resources on which we depend—in explaining the human pattern. Future research should work to better document the variability in intergroup behavior in the group‐living apes, especially in gorillas, bonobos, and humans, using methods of data collection designed specifically for this endeavor.