Male mask size is correlated with mating success in the common yellowthroat

Abstract

Many socially monogamous species have sexually dimorphic traits. The evolution of these traits is puzzling as sexual selection is often thought to be relatively weak in monogamous species. However, sexual selection in monogamous species could be stronger than generally believed if the males possessing more dimorphic traits gain a reproductive advantage by increasing the probability of: (1) gaining a mate, (2) mating with a female of relatively higher quality (more fecund), or (3) gaining extrapair fertilizations. We used a combination of DNA fingerprinting and microsatellite markers to study male traits associated with social and extrapair mating success of males in the common yellowthroat, Geothlypis frichas. In this species the male has a black facial mask that is absent in females. The mask varies almost two times in size among males on our study area. We found that males with larger masks gained both a social and extrapair mating advantage. In terms of social mating success, males with larger masks were more likely to gain a mate. However, there was no clear evidence that these males also gained more fecund mates. In a populationwide analysis, males with larger masks were also more likely to gain extrapair fertilizations and they sired a larger number of extrapair young. Similarly, in a pairwise comparison, extrapair sires had larger masks than the males that they cuckolded. Over the season, males that obtained extrapair fertilizations sired more young (within-pair and extrapair) than males that failed to gain extrapair fertilizations. Most of this difference was due to extrapair rather than within-pair mating success. These results suggest that sexual selection has the potential to influence mask size as a consequence of variation in both the social and extrapair components of mating success.