Resolution of evolutionary conflicts: costly behaviours enforce the evolution of cost-free competition

A resolution model for evolutionary conflicts of interest is proposed. We assume that two conflicting parties originally have different evolutionarily stable strategies (ESS) with respect to a continuous variable, but that only one value of the variable is simultaneously possible. Individuals from the two parties meet in antagonistic encounters. The side that invests more in the antagonistic behaviours mediating the conflict is able to adjust the variable in its preferred direction. It is shown that the extent of the conflict – that is, the difference between the ESS values of the parties – decreases with increases in investment to antagonistic behaviours. The precondition for this is that the total conflict cost increases with the intensity and frequency of the antagonistic encounters, and that these factors in turn are related to how much the variable is adjusted. The conflict costs then result in a change of ESS level that forces the stronger party to alter its preferred level of the conflict variable. Behaviours that give control in the conflict escalate in an ‘arms race’, which eventually leads to a compromise solution with one ESS shared by the parties and with a small or no realized cost of conflict behaviour. This result contrasts with the traditional view of evolutionary conflicts, that expressed costs of antagonistic behaviours are necessary for evolutionarily stable resolutions. The model is applied to sexual conflicts, where a stable resolution may result in which apparent conflict behaviours are suppressed, although both parties are ready to engage in costly conflicts. In an explicit model of the compromise resolution of a parent–offspring conflict over parental investment, we demonstrate that the conflict may be resolved so that offspring do not beg. At the solution, parents provide an amount of care intermediate between the original ESS level of the parents and offspring, and the new parental and offspring ESS levels are identical.