The logic of animal intergroup conflict: A review

Highlights

• Structured review of models of animal intergroup conflict behavior.

• Biologically relevant modeling choices are highlighted.

• Recent instructive empirical findings are pointed out.

Abstract

We review the literature on various approaches to modeling animal intergroup conflict behavior in theoretical biology, highlight the intricacies emerging in the process of adding due biological realism to such models, and point out recent empirical findings that can inspire future theorizing.

Introduction

More than two millennia ago, in his History of Animals, Aristotle observed that “There is enmity between such animals as dwell in the same localities or subsist on the same food. If the means of subsistence run short, creatures of like kind will fight each other” (Aristotle, 1984, p. 949). In his On the Origin of Species, Darwin even limned life itself metaphorically as the ‘struggle for existence’, acknowledging that all forms of life are inevitably entangled in conflicts (Darwin, 1859). While the horrible events of the first half of the 20th century temporarily led to a focus of prominent European biologists on the more peaceful sides of animal behavior (Lorenz, 1963, Tinbergen, 1968), the modern synthesis in evolutionary biology (Huxley, 1942), with its emphasis on the ‘egoistic gene’ (Dawkins, 1976), has continuously upheld conflict as one of the most important themes in biology. Thus, it might not be a coincidence that the development of evolutionary game theory and its subsequent success in theoretical biology, economics and other disciplines began with a groundbreaking paper on ‘the logic of animal conflict’ (Maynard Smith and Price, 1973).

Conflicts between forms of (pre-)life, be they viruses, unicellular organisms, or more complex living beings, arise when one interferes in some way with another’s structural integrity, resource supply, growth, dispersal or reproductive interests. Obviously, such a broad definition of conflict comprises not only direct conflicts in which at least one party actively seeks to harm the other, but also indirect conflicts in which parties are negatively affected by each other’s mere existence or proliferation. Well-known examples of such indirect conflicts are intraspecific evolutionary arms races between trees competing for access to sunlight by growing taller (Dawkins and Krebs, 1979). Examples of direct conflicts, on the other hand, include interspecific predator-prey and host-parasite relations (Barbosa and Castellanos, 2005, Galvani, 2003, Schoener, 1983), chemical warfare between microbe species (Czaran et al., 2002), and intraspecific contests and potentially lethal fights (Enquist and Leimar, 1990), e.g. over reproductive access (Clutton-Brock and Huchard, 2013), parental investment (Trivers, 1974), or territory (Willems et al., 2013).

The central role of both direct and indirect conflicts in the evolution of virtually all forms of life has led to an abundance of respective empirical studies (for overviews see: Hardy and Briffa, 2013, Huntingford and Turner, 1987), and, when conceptualized as ‘the struggle for existence’, indirect conflict is implicitly present in practically all work in theoretical biology. Compared to this omnipresence of indirect conflict, intraspecific direct conflicts have remained a somewhat peripheral topic. Still, the respective literature has produced a number of seminal models, most of them closely tailored to distinct instances of aggressive behaviors observed in the field (for complementary reviews see: Kokko, 2013, Sherratt and Mesterton-Gibbons, 2013). One likely reason for the comparably high degree of segmentation in the theoretical literature on direct conflicts is that biological theorizing often quickly exchanges ideas and arguments with naturalist field work, as exemplified by the observational studies on non-lethal stag fights in deer that inspired Maynard Smith and Price’s models (Maynard Smith, 1974, Maynard Smith and Parker, 1976, Maynard Smith and Price, 1973), and vice versa (Clutton-Brock et al., 1979). The concentration of theoretical work in biology on select phenomena resulting from this close interplay of field and desk work, as well as the need for theories to remain testable by empiricists (Fawcett and Higginson, 2012), may thus have led to the current array of relatively specialized theories on animal conflict behavior.

The aim of this review is to present a structured overview of the existing theoretical literature on intergroup conflict behavior in biology. In doing so, we draw on instructive works by colleagues from theoretical biology and economics (Dechenaux et al., 2015, Kokko, 2013, Sheremeta, 2015; Sherratt and Mesterton-Gibbons, 2013). We follow their ‘top-down’ approaches of organizing their overviews according to general models of the incentive structures potentially faced by competing individuals and groups. Consistent with most biological models of conflict behavior, we use game theoretical terminology (Broom and Rychtar, 2013, Brown, 2016, Maynard Smith, 1982; for an alternative modeling approach see, e.g.: Santarlasci et al., 2014).

Two earlier, highly informative overviews of approaches to modeling animal conflict behavior already exist (Kokko, 2013; Sherratt and Mesterton-Gibbons, 2013). However, these mainly focus on dyadic and triadic conflict models. We complement them here by putting special emphasis on models of intergroup conflicts.

Inevitably, our review cannot do justice to the vast body of empirical literature on animal conflict behavior (for overviews of this literature see: Hardy and Briffa, 2013, Huntingford and Turner, 1987). Additionally, although we include several methodologically instructive models tailored to ancestral human intergroup conflicts, we confine this review mostly to the study of non-human animal behavior (for comprehensive reviews of human intergroup conflict behavior see, e.g., Glowacki et al. (forthcoming); Böhm et al. (forthcoming); as well as the other papers collected in this issue; Keeley, 1996; and Gat, 2008). Thus, we hope that our review will be found useful by an interdisciplinary readership interested in theoretical insights into the logic and dynamics of animal conflict and also by theorists looking for a guide to the respective methodological toolbox used by theoretical biologists.