Increasing sexual ornamentation during a biological invasion

Sexual selection during range expansions has seldom been investigated, but it could be important for speciation because expansions create peripatric populations and sexual ornaments mediate reproductive isolation. We compared carotenoid-based ornamentation of common waxbills (Estrilda astrild) across a recent biological invasion that is representative of typical expansions, in that it progressed from ecologically favorable environments, where the source populations inhabit, toward less favorable ones. We found that sexual ornamentation and sexual dichromatism augmented during the expansion, involving an increase in male color saturation and decrease in female ornamental area. We disfavor explanations based solely on environmental effects or differential dispersal because of the opposite changes in each sex and because environmental factors alone (availability of dietary carotenoids) would predict decreased ornamentation. Results instead suggest increasing sexual selection along the expansion. Sexual signals generally allow female choice of high-quality males and, thus, intersexual selection may increase during expansions because the fitness differential between mating with high- or low-quality males is larger in less favorable environments. This is potentially a general mechanism that, if it proves common, could have implications for macroevolution (e.g., young peripatric species having more elaborate sexual traits).