experimental test of sexual selection on plumage color
When two taxa that have diverged in a sexual trait come back into secondary contact before reproductive isolation is complete, sexual selection may actually promote gene flow between them and counteract the speciation process. I conducted a plumage manipulation experiment in an allopatric population of the orange Red-backed Fairy-wren subspecies to test the hypothesis that sexual selection on red plumage in this area has driven the observed asymmetrical introgression of red plumage. The experiment consisted of three groups of males that were captured at the beginning of the breeding season: control males that were not manipulated, sham males that were painted with a non-toxic colorless marker, and reddened males that were painted with a red marker. These males were then released into the population, and their subsequent mating behavior was monitored.
Reflectance spectrophotometry confirmed that the colorless marker did not alter the natural orange color, whereas the red marker accurately mimicked the natural color of the western subspecies. Experimental groups did not differ in the number of within-pair young sired or in the likelihood that they would be cuckolded. However, reddened males sired significantly more extra-pair young, resulting in higher reproductive success than control or sham males. These results suggest that female preference for red males as extra-pair mates drives the asymmetrical introgression of red plumage. Furthermore, this preference for red is likely due to a sensory bias, as females did not exhibit a preference for naturally occurring red males, only those that were manipulated to exhibit the phenotype of the foreign subspecies.
Reflectance spectrophotometry confirmed that the colorless marker did not alter the natural orange color, whereas the red marker accurately mimicked the natural color of the western subspecies. Experimental groups did not differ in the number of within-pair young sired or in the likelihood that they would be cuckolded. However, reddened males sired significantly more extra-pair young, resulting in higher reproductive success than control or sham males. These results suggest that female preference for red males as extra-pair mates drives the asymmetrical introgression of red plumage. Furthermore, this preference for red is likely due to a sensory bias, as females did not exhibit a preference for naturally occurring red males, only those that were manipulated to exhibit the phenotype of the foreign subspecies.
