Male reproductive success in a promiscuous mammal: behavioural estimates compared with genetic paternity

Molecular techniques have enabled behavioural ecologists to reassess mating systems from a genetic perspective. Studies of paternity frequently reveal that mating behaviour does not always reflect parentage, and may bring to light alternative mating tactics. Here we present a comparison of behavioural and genetic measures of male reproductive success in a mammalian mating system in which both sexes are highly promiscuous. Rather than having a stable harem social structure, Soay rams (Ovis aries) on the island of Hirta, St Kilda, UK usually consort with individual oestrous ewes sequentially. Not all matings occur between consort pairs, however, and ewes have been seen to mate with up to 10 different rams on the same day. Using locus-specific polymorphism at five protein and 10 microsatellite DNA loci, we determined paternity for 236 lambs born into three cohorts, and compared paternity with estimates of mating success derived from more than one census of rutting behaviour. The correlation between the number of ewes with which each ram was witnessed in consort and the number of paternities assigned was positive and statistically significant, and rams that were observed in consort with a ewe were 18 times more likely to have sired her offspring than other candidate rams. However, most lambs (73%) were not sired by a ram seen in consort with the oestrous mother. Many juveniles, yearlings and some adult rams were rarely seen in consort with ewes, yet were assigned a significant number of paternities. These results suggest that mating tactics differ between age groups, and that alternative mating strategies among adults that do not involve forming consorts with many females also confer mating success. For these reasons, census-based observations of consort associations between individuals cannot be used to accurately estimate individual male reproductive success in this population.