Assortative mate preferences (i.e. preferring mates of a similar phenotype to one's self) can limit gene flow and accelerate reproductive isolation among young lineages. However, the expression of such mate preferences can be largely constrained by intrasexual competition in the other sex, and co-divergence of sexual traits and behaviors often occur in both sexes. Considering male-male competition in addition to female preferences in speciation-by-sexual-selection is therefore essential. Male-male competition can act in concert with female preference when males with the preferred trait are also better competitors, but conversely, females may be less able to choose their preferred mate when non-preferred males win competitions. We tested the hypothesis that male-male competition among divergent phenotypes constrains the expression of female preferences in a contact zone in which two differently colored lineages (red and blue) of the poison frog Oophaga pumilio meet. We assessed the ability of different colored males to (1) establish a territory, (2) defend a territory from an invader, and (3) acquire mates with or without a territory, using a combination of capture-mark-recapture, simulated territory intrusion and field mesocosm experiments.
We found that O. pumilio males of different phenotypes show color-biased aggression and differ in competitive ability, and that the result of a male territorial contest is likely to limit a female’s ability to choose a mate of her preferred color. Preliminary results show that blue males may be more successful in establishing a territory compared to red and intermediate males. When defending a territory, males biased their aggression toward the local color morph in both monomorphic populations that flank the contact zone, but did not discriminate between different colored opponents in the polymorphic contact zone. Finally, in field mesocosms, males that lost the territorial contest never engaged in courtship, even when they are of the female’s preferred color. Our results highlight the importance and complexity of the role that male competition plays in trait evolution and phenotypic divergence.