Models of preferential mating

In this chapter, I bring together and analyse the different lines of evidence on the mechanism of sexual selection in the Arctic Skua. The models to be fitted to the data have already been described in sections 9.2 and 9.4. They are all models of preferential mating, not male competition. As we proved in section 9.3, male competition cannot maintain a polymorphism without heterozygous advantage. Heterozygotes suffer a slight disadvantage compared to dark homozygotes (section 8.5). Since the polymorphism of the Arctic Skua is stable, models of male competition cannot plausibly be fitted to the data.

Males who are the objects of female preference will tend to be chosen as mates earlier in the breeding season. We may postulate that a certain proportion of females will prefer to mate with particular male phenotypes. If the males have already arrived on their territories or joined a ‘club’ of unmated males competing for the females' attention, then as groups of females come into breeding condition they will be able to choose males from among those who are still unmated. If we are to simulate this process of mate selection as it goes on through the breeding season, we must allow for the intervals of time which are required for courtship and mate selection. On average, as we saw in section 8.4.2, new pairs take an extra 7–8 days to breed compared to old, established pairs.