Do female grey seals select genetically diverse mates?

Like most mammals, grey seals, Halichoerus grypus, are thought to be polygynous. However, genetic analysis reveals that as few as 1% of males gain reproductive success that is significantly above average. At the same time there appear to be more full siblings than can be accounted for by chance, implying some level of partner fidelity. We used an enlarged data set to show that, on average, maternal half-siblings who have different fathers are significantly more diverse than expected from random mating. This unexpected pattern is too strong to be accounted for by influxes of males from other breeding colonies and hence implies female choice for partner diversity. We argue that behavioural mate choice is an unlikely explanation and speculate that naturally occurring antisperm antibodies could modulate sperm competition through weak immunointolerance of sperm from previous partners. Whatever the mechanism, choice for diverse partners will tend to increase the local effective population size. We used a simple model to show that such a strategy could be favoured at the level of the individual in small closed or semiclosed 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.