Jesus Martinez-Padilla, Carlos Camacho, David Canal, Jaime Muriel, David López-Idiaquez, Jaime Potti

Sexually selected phenotypes are key to understanding population and eco-evolutionary dynamics since the expression of these traits is tightly linked to reproduction and population viability. Parasites are major agents of selection that alter both phenotypic expression and fitness. However, the carry-over effects of parasites in early life in selection on phenotypes expressed later in life are unknown. We capitalised on a long-term monitored population of pied flycatchers, Ficedula hypoleuca, where nestlings are naturally exposed to nest-dwelling, blood-sucking blowfly larvae. We explored whether parasitism by blowflies during early life and breeding mediates selection on forehead patch size (FPS), a sexual and social signal in the study species. We also explored whether FPS of males is a reliable proxy of ectoparasite burden of their offspring. We found that the strength of selection on FPS increased with the abundance of blowfly larvae only during growth, but also, that male FPS was a reliable predictor of the blowflies burden of their offspring. We highlight the major role of parasites as agents of selection in early life, but not during reproduction in adulthood. We discuss the implications of our results on the direct and indirect benefits females may obtain by mating to highly ornamented males.