Runaway evolution from male-male competition

Abstract

Strikingly elaborate weapons and displays are widespread features of male contests for mates across the animal kingdom. Male combat is familiar to students of behavior, and yet while we understand how female mate choice results in a runaway process, how such evolutionary extremes arise from male-male competition is unclear. In a quantitative genetic model of sexual selection for a male signaling trait that mediates aggression in male-male contests, we show that an honest indicator of aggression can generate selection on itself by altering the social environment. This can cause selection to accelerate as the trait is elaborated, which can ultimately lead to runaway evolution. Thus, the key unifying feature of runaway sexual selection driven by either male-male competition or female mate choice is an evolving source of selection provided by the social environment. Our model identifies simple conditions generating runaway evolution and provides clear, testable predictions for empirical studies.