Density-dependent variation in lifetime breeding success and natural and sexual selection in Soay rams

Variation in male lifetime breeding success (LBS) is central to understandi ng selection, yet it has rarely been measured in natural populations of lar ge mammals. Here, we first describe variation in the opportunity for select ion in cohorts of Soay rams (Ovis aries) on the archipelago of Sr. Kilda, S cotland, that were born during years of varying population density. Variati on in LBS is closely coupled with demography, as rams born in years of low density following population crashes enjoy greater LBS than do those born i n high-density years. Paradoxically the opportunity for selection was great est in the largest cohorts, those born in years of high population density, owing to low juvenile breeding success and overwinter survival Variation i n longevity and the contribution of nonbreeders were the most important com ponents of the total variance in LBS in cohorts born in years of high densi ty, while variation in fecundity was more important in cohorts born in low- density years. The opportunity for sexual selection is thus stronger in coh orts born in low-density years, as many rams in these cohorts survive to co mpete for mates as adults in subsequent ruts. Variation in population densi ty in the year of birth also influenced the intensity of selection. individ uals born in years of high population density underwent strong natural sele ction in favor of longer hindlimbs over their first winter. In contrast, in cohorts born in low-density years, there was no natural selection on hindl imb in the first year of life. Longer hindlimbs were associated with increa sed fecundity over the entire lifetime of individuals born in low-density y ears. Natural and sexual selection thus act on the same trait in the same d irection at different life-history stages in Soay rams, depending on the po pulation density experienced in the year of birth.