MALE PHENOTYPE, FERTILITY, AND THE PURSUIT OF EXTRA-PAIR COPULATIONS BY FEMALE BIRDS

In field and laboratory studies of birds, positive associations betwee n male phenotype and success at obtaining extra-pair copulations or ex tra-pair fertilizations are often interpreted as providing evidence th at females are using extra-pair copulations to obtain indirect benefit s for their offspring, either through genes for increased viability, o r for a fisherian mating advantage. I describe a simple model, in whic h functional fertility (the success of ejaculates in fertilizing eggs) covaries with male phenotype, which can explain the observed associat ions equally well. Under such a model, females pursue extra-pair copul ations as insurance against the functional infertility of their mate, and obtain only direct benefits for themselves in their current reprod uctive event. Several studies of birds suggest that a relation between male phenotype and functional fertility is often likely to exist and that there are many potential causes of functional infertility. Non-ma nipulative field studies are unlikely to produce results which disting uish between the two hypotheses, and I discuss several alternative app roaches which may allow their resolution.