Evidence for sexy sons in European starlings (Sturnus vulgaris)

The sexy son hypothesis suggests that females obtain future fitness benefits from mating with polygynous males through the inheritance by their sons of traits contributing to mating success. We tested this hypothesis in the facultatively polygynous European starling (Sturnus vulgaris) in two ways. We first compared sons of monogynous and polygynous free-living males for their abilities in nest site acquisition and female-directed behaviors in controlled aviary tests. We then investigated eggs laid in monogynous and polygynous matings for concentrations of maternal steroid hormones that could affect behavioral differentiation of sons. Sons of polygynous fathers defended more nest boxes and produced more courtship song than sons of monogynous fathers. Mean concentrations of maternal androstenedione (A4), 5-dihydrotestosterone (DHT), testosterone (T), and 17-estradiol (E) in yolks were not affected by mating status. However, the within-clutch pattern of A₄ depended on mating status, with higher concentrations in last than in first eggs of clutches of monogynous but not polygynous matings. These results suggest that (1) sons may inherit from their fathers behavioral qualities for the successful acquisition of nest sites and attraction of mates; (2) differential exposure to maternal steroid hormones during development in the egg is unlikely the cause of the behavioral differences between adult sons of monogynous and polygynous fathers; and (3) females adjust the within-clutch pattern of androgen to mating status, possibly in response to the expected contribution of their mate to nestling care.