Dangerous games and the emergence of social structure: evolving memory-based strategies for the generalized hawk-dove game

How can stable relationships emerge from repeated, pairwise interactions among competing individuals in a social group? In small groups, direct assessment of resource-holding potential, which is often linked to body size, can sort individuals into a dominance hierarchy. But in larger groups, memory of behavior in previous interactions may prove essential for social stability. In this study, I used a classifier-system model (similar to a genetic algorithm) to evolve strategies that individuals play in pairwise games that are potentially dangerous (i.e., fitness benefits of winning are outweighed by losing costs that result mainly from risk of injury). When the two possible responses by each player in a single interaction are designated C (= careful) and D (= daring), the average gain is highest if responses are complementary (i.e., one plays C and the other plays D). Stable dominance relationships, which depend on such complementarity across a sequence of interactions, are more common when both sizes are known to the contestants, when strategies are based on memory, and when combat is especially dangerous. Two key memory-based strategies generated by the classifier system (DorC and CAD) are particularly adept at achieving and maintaining complementarity; these strategies, which represent building blocks from which social structure can arise, are linked here with pairwise contests for the first time. When most individuals in the group differ in size, stable dominance relationships generally yield transitive hierarchies consistent with size. Empirical tests of these predicted patterns are proposed.