Reproductive skew and the evolution of conflict resolution: a synthesis of transactional and tug-of-war models

The formation of animal societies is a major transition in evolution. It is challenging to understand why societies are stable, given the reproductive conflicts inherent within them. Reproductive skew theory provides a compelling explanation for how and why reproductive conflicts are resolved. Indeed, some have suggested that skew theory represents a general theory of social evolution. Lamentably, skew theory is composed of many independent models, with the generality of each model being restricted by its assumptions. Here, we tackle this problem, using Hamilton's rule to predict the conditions under which assumptions of major classes of skew models (transactional and tug-of-war) apply. First, building on transactional models, we define the amount of reproduction that individuals can negotiate based on the threat of group dissolution (the outside option) and determine conditions under which groups will be stable (free of group dissolution). Second, building on tug-of-war models, we define the amount of reproduction that individuals can negotiate based on the threat of costly competition (the inside option) and determine conditions under which groups will be tranquil (free of costly competition). Finally, synthesizing transactional and tug-of-war approaches, we determine the conditions under which individuals will negotiate based on outside rather than inside options. Simply, individuals will negotiate using their outside option when it is greater than their inside option and vice versa. We conduct a post hoc test of all predictions in one simple animal society - the clown anemonefish, Amphiprion percula. The product is a more general and demonstrably testable model of reproductive skew, which should help to refocus the debate surrounding the utility of reproductive skew theory as a general theory of social evolution.