The evolutionary loss of a badge of status alters male competition in three-spine stickleback

Abstract

The evolutionary loss of sexual signals is taxonomically widespread and quite common. These signals are often important not only in mate choice but also in male competition for territories and females; yet, male competition has rarely been investigated in the context of signal loss. We asked whether the loss of red throat color in three-spine stickleback (Gasterosteus aculeatus), an ancestral signal important in both male competition and mate choice, is accompanied by changes to intrasexual selection. Several freshwater stickleback populations have lost the red throat, and instead develop black breeding coloration. Previous work demonstrated that sensory drive is likely responsible. We conducted trials that mimic contact between ancestral (red) and derived (black) types during territory and nest establishment under conditions mimicking the 2 water color environments (clear and red-shifted, respectively) where they are found. We found that the allocation of competition behaviors depends on male breeding color, but not water color or the interaction of the 2. Although the total number of aggressive behaviors did not vary between color types, black males directed more aggression toward red males. Assessing each male competition behavior separately revealed that this pattern was largely driven by black fish biasing charges toward opposite type males. Agonistic behavior between types may strengthen divergence in resource or habitat use between populations of males with and without the ancestral sexual signal.