Aggression heuristics underlie animal dominance hierarchies and provide evidence of group-level social information

Abstract

Members of a social species need to make appropriate decisions about who, how, and when to interact with others in their group. However, it has been difficult for researchers to detect the inputs to these decisions and, in particular, how much information individuals actually have about their social context. We present a method that can serve as a social assay to quantify how patterns of aggression depend upon information about the ranks of individuals within social dominance hierarchies. Applied to existing data on aggression in 172 social groups across 85 species in 23 orders, it reveals three main patterns of rank-dependent social dominance: the downward heuristic (aggress uniformly against lower-ranked opponents), close competitors (aggress against opponents ranked slightly below self), and bullying (aggress against opponents ranked much lower than self). The majority of the groups (133 groups, 77%) follow a downward heuristic, but a significant minority (38 groups, 22%) show more complex social dominance patterns (close competitors or bullying) consistent with higher levels of social information use. These patterns are not phylogenetically constrained and different groups within the same species can use different patterns, suggesting that heuristic use may depend on context and the structuring of aggression by social information should not be considered a fixed characteristic of a species. Our approach provides opportunities to study the use of social information within and across species and the evolution of social complexity and cognition.

Costs dictate strategic investment in dominance interactions

“…Such findings contrast with the very similar patterns observed across our two groups (different only in low-cost aggressive interactions among dyads with large rank differences and corresponding to low data densities) and in two social groups of monk parakeets [19]. Variation in strategies could appear from differences in the types of resources that individuals compete for [20], or if studies are conducted in different contexts.…”

“…Recent studies have suggested that rank differences may underlie patterns of aggressive interactions in group-living species [19,20]. Here, we extend current methods to incorporate dyadic opportunity to interact and-using an agent-based model-demonstrate that, if unaccounted for, such differences in opportunity can drive patterns that appear to be strategic (i.e.…”

“…The factors outlined above could reasonably generate apparent differences in strategies across systems. Further, when applying comparative methods across many datasets, Hobson et al [20] report differences in strategy between groups of the same species. Such findings contrast with the very similar patterns observed across our two groups (different only in low-cost aggressive interactions among dyads with large rank differences and corresponding to low data densities) and in two social groups of monk parakeets [19].…”

“…It has been suggested that certain interaction strategies demonstrate the use of 'higher-level' or 'global' social information [15,20] as individuals direct interactions in relation to relative rank difference. However, apparently complex strategies may instead be underpinned by simple mechanisms.…”

“…We develop a general framework for inferring whether individuals express rank difference-based strategies in dominance interactions. Our method builds on the approach proposed by Hobson & DeDeo [19], further developed by Hobson et al [20], in that it generates an estimate of the relationship between the propensity for individuals to interact and their differences in dominance rank. We refine this approach in two important ways.…”