Sexual selection by female choice in a monogamous bird: Darwin's theory corroborated

Abstract

The Arctic Skua is a seabird that is polymorphic for dark, intermediate and pale phenotypes of plumage. The melanic, dark and intermediate males can find mates sooner in the breeding season than the non-melanic, pale males. When males are looking for new mates, melanic males find mates on average four days sooner than non-melanic males. There is no difference in the breeding dates of the females, nor in the breeding dates of males who have not changed their mates. The difference in the males' breeding dates is found only on change of mate. Finding a new mate takes about six days for a dark male, about eight days for an intermediate male, and about eleven days for a pale male. These differences give the melanic males an advantage by sexual selection: exactly as Darwin had postulated in his theory of sexual selection of monogamous birds, the earlier pairs are more successful in terms of the numbers of chicks they rear to fledging. The overall disadvantage of intermediates and pales relative to darks is given by the sexual selective coefficients The melanic males gain their advantage because the females prefer to mate with them. Models of female preference entail distributions of breeding dates that fit the data better than models of male competition. The earlier breeding of melanic males can be satisfactorily explained if 38 per cent of females prefer to mate with melanics rather than with non-melanics. In the model which fits the data best, 4 per cent of females prefer dark males and 34 per cent prefer either dark or intermediate males. Further evidence, which strongly supports the theory of female choice, is provided by the stability of the polymorphism, the existence of assortative mating, and the absence of any effect of male experience on the chances of finding a mate. The data of the Arctic Skua corroborate Darwin's theory of sexual selection by female choice in monogamous birds.