The logic of animal intergroup conflict: A review
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.
Section snippets
Approaches to modeling intergroup conflict in biology
As the classic Hawk-Dove game introduced by Maynard Smith and Price (1973) set the stage for the majority of subsequent theoretical approaches to studying conflict in biology, we will start out by briefly recapitulating its main characteristics. Subsequently, we will extend our formalization of the game to be applicable to intergroup conflicts and use this extended version of the game to structure our review of different modeling approaches to animal intergroup conflict behavior and dynamics.
In
Adding further biological realism
The potential extensions of intergroup conflict models outlined in Section 2 of course do not exhaust the set of factors potentially relevant in modeling animal conflict behavior, as they are limited to changes in assumptions about individuals’ characteristics and the strategic features of the conflicts in which they are engaged. Consequently, let us now turn to reviewing selected approaches for studying the evolutionary dynamics and modeling the structure of populations in which these
Instructive empirical works to inspire future theorizing
As Sections 2 and 3 have shown, the existing theoretical literature on animal intergroup conflicts is diverse. The works discussed above have identified a remarkable variety of assumptions and modeling components that sensitively affect a model’s dynamics and long-term predictions, including: (i) impact, (ii) contest success, and (iii) cost functions; (iv) sharing rules; (v) strategy revision protocols; (vi) meta-population and subgroup sizes; (vii) demographics; (viii) within-group
Discussion and outlook
In this paper, we outlined the existing theoretical literature on approaches to modeling animal intergroup conflict behavior in theoretical biology (Section 2), highlighted the intricacies emerging in the process of adding due biological realism to such models (Section 3), and pointed out recent empirical findings that can inspire future theorizing (Section 4); see Table 3 for a synopsis.
In summary, we have seen that a plethora of models of animal intergroup conflict behavior exists using
Funding
SG was partially supported by the U.S. Army Research Office under grant number W911NF-14-1-0637 and by the National Science Foundation under NSF Award #DBI-1300426.
Acknowledgements
We thank Luke Glowacki, Kelly Rooker, Erik Kimbrough and Roman Sheremeta, and our anonymous reviewers for helpful comments on earlier versions of this paper as well as Kevin Laughren for generating Fig. 1. This work was assisted through participation in the “Evolution & Warfare” Investigative Workshop at NIMBIOS, sponsored by the National Science Foundation through NSF Award #DBI-1300426, with additional support from The University of Tennessee, Knoxville.
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