Molecular analysis of a promiscuous, fluctuating mating system

The Soay sheep population of Sr. Kilda fluctuates widely in population size and ses ratio. so that the level of male-male competition for mates varies from one rut to the nest. In this paper we investigate variation in indivi dual male breeding success in relation to age and population size at the ru t. and its outcome in terms of lifetime breeding success. Since both sexes are promiscuous, and census-based behavioural data do not predict paternity , we conducted the whole analysis on breeding success derived by molecular techniques. We assumed that every male living in our study area during the rut (N= 68-294 in different years) was a candidate father for each subseque nt lamb, and used the parentage inference software CERVUS 1.0, applied to u p to 17 allozyme and microsatellite loci; to infer paternity at 95% and 80% confidence. Using 945 paternities assigned at 80% confidence, we show that juvenile rams (aged 7 months) and yearling rams (aged 19 months, regularly obtained paternities and that mean individual breeding success varied inve rsely with levels of competition in the rut for all age classes of ram. The proportion of young (juvenile and yearling) and adult rams gaining one or more paternities showed similar variation with population size, but the sib ship size sired by young and adult breeders showed different patterns: adul t rams sired larger sibships at low population size. while the size of sibs hips sired by young rams was small across all population sizes. Variable br eeding success by young rams approximately halved the estimated coefficient of variation in lifetime breeding success of Soay rams. (C) 1999 The Linne an Society of London.