Do female grey seals select genetically diverse mates?

Like most mammals, grey seals, Halichoerus grypus, are thought to be polygy nous. However, genetic analysis reveals that as few as 1% of males gain rep roductive success that is significantly above average. At the same time the re appear to be more full siblings than can be accounted for by chance, imp lying some level of partner fidelity. We used an enlarged data set to show that, on average, maternal half-siblings who have different fathers are sig nificantly more diverse than expected from random mating. This unexpected p attern is too strong to be accounted for by influxes of males from other br eeding colonies and hence implies female choice for partner diversity. We a rgue that behavioural mate choice is an unlikely explanation and speculate that naturally occurring antisperm antibodies could modulate sperm competit ion through weak immunointolerance of sperm from previous partners. Whateve r the mechanism, choice for diverse partners will tend to increase the loca l effective population size. We used a simple model to show that such a str ategy could be favoured at the level of the individual in small closed or s emiclosed populations of long-lived species where polygyny would lead to a rapid increase in the level of inbreeding. (C) 2001 The Association for the Study of Animal Behaviour.