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A Test of the “Sexy Son” Hypothesis: Sons of Polygynous Collared Flycatchers Do Not Inherit Their Fathers’ Mating Status

Department of Animal Ecology/Ecology and Evolution, Evolutionary Biology Centre, Uppsala University, Norbyvägen 18D, SE‐752 36 Uppsala, Sweden

According to the original “sexy son” hypothesis, a female may benefit from pairing with an already‐mated male despite a reduction in fecundity because her sons inherit their father’s attractiveness. We used data from a long‐term study of collared flycatchers (Ficedula albicollis) collected during 24 years to test this prediction. Our results show that the sons of polygynously mated females fledged in poor condition and therefore did not inherit their father’s large forehead patch (a condition‐dependent display trait) or mating status. From the female’s perspective, polygynous pairing resulted in fewer recruited grandchildren than did a monogamous pairing. The reproductive value of sons did not outweigh the fecundity costs of polygyny because the low paternal care reduced the attractiveness of sons. When there are long‐lasting parental effects on offspring attractiveness, costs of polygyny may include the production of nonsexy sons.