Environmental determination of a sexually selected trait

Models of sexual selection usually assume that variation in the expression of sexual ornaments is determined largely by genetic, rather than environme ntal, factors(1). However, empirical support for this assumption conies fro m studies of species with little parental care(1,2), in which the influence of environmental factors may be limited(3,4), and from studies of just two species(5,6) with parental care, in both of which heritability estimates v ary hugely between years or populations(7,8). In the remaining studies of s pecies with parental care, it is not known whether resemblance in sexual or namentation between relatives was due to shared genes or shared patterns of care(3,4,9-15). Here we use cross-fostering experiments in house sparrows, Passer domesticus, to examine the relative roles of these effects. We demo nstrate that, although sons resemble their fathers with respect to sexual o rnamentation, this resemblance is mainly due to post-hatching environmental effects rather than shared genes. We also show that sons hatching early in the year have the largest ornaments. These results support models that emp hasize the importance of environmental sources of variation(16-18), such as direct paternal effects(4,19,20), on the expression of sexual ornaments, a nd agree with the general observation that sexually selected traits tend to be condition dependent(1). We urge the incorporation of gene-environment i nteractions into future models of sexual selection.