Artificial selection for increased comb size and its effects on other sexual characters and viability in gallus-domesticus (the domestic chicken)

The expression of secondary ornaments is often positively correlated with the bearer's condition. So far no study has reported on consequences of directional selection on male sexual ornaments in birds, i.e. it is unknown to what extent male sexual traits are genetically correlated and whether selection for secondary ornaments affects viability. In this study we estimate phenotypic and genotypic parameters of three male sexual traits, namely comb size, spur length and testes mass, by using two different lines of domestic chickens (Gallus domesticus). One of the two lines had been artificially selected for large comb size during nine generations whereas the other line was a randomly mated control line. As a result of the selection, male comb size was almost doubled. Data from the selection line indicate that comb size was negatively genetically correlated with testes mass and spur length. The cockerels from the selection line had accordingly lighter testes and shorter spurs than cockerels from the control line. A simple mate choice experiment with females and males from both lines indicated that preferred males had a larger joint expression of comb size and spur length than nonpreferred males. Data from three different selection lines, all with enlarged comb size, clearly reveal a negative effect of comb size on male viability