SEXUAL SELECTION AND CUCKOLDRY IN A MONOGAMOUS SONGBIRD - IMPLICATIONS FOR SEXUAL SELECTION THEORY

Sexual selection is generally assumed to be weaker in monogamous than in polygynous animals. Recently, though, extra-pair fertilizations hav e been hailed as an important force in generating variance in reproduc tive success among males in socially monogamous species, thereby incre asing the intensity of sexual selection. To see if extra-pair copulati ons contribute to variance in male reproductive success in the house f inch (Carpodacus mexicanus), we used DNA fingerprinting to determine t he paternity of chicks from 35 nests. This species is a socially monog amous passerine in which plumage brightness serves as a sexually selec ted indicator of male quality. Out of 119, nestlings 10 (8.3%) were fa thered by a male other than the attending male, but cuckoldry occurred randomly with respect to the plumage colouration, size, or age of the attending male. Thus extra-pair fertilizations do not generate varian ce in male reproductive success with respect to plumage colour. On the other hand, a strongly male-biased sex ratio and asynchronous breedin g by females may generate substantial variance in male reproductive su ccess and could explain the evolution of ornamental colouration.