Correlates of early reproduction and apparent fitness consequences in male Soay sheep

Abstract

Life history trade-offs are ubiquitous across species and place constraints on the timing of life history events, including the optimal age at first reproduction. However, studies on lifetime breeding success of male mammals are rare due to sex-biased dispersal and the requirement for genetic paternity inferences. We studied the correlates and apparent fitness consequences of early life reproduction among males in a free-living population of Soay sheep (Ovis aries) on St Kilda, Scotland. We investigated the factors associated with early breeding success and the apparent consequences of early success for survival and future reproduction. We used genetic paternity inferences, population data, and individual morphology measurements collected over 30 years. We found that individuals born in years with low-density population size had the highest early life breeding success and singletons were more likely to be successful than twins. Individuals that bred successfully at 7 months were more likely to survive their first winter. For individuals that survived their first winter, early breeding success was not associated with later breeding success. As individual heterogeneity affects breeding success, we believe that variation in individual quality masks the costs of early reproduction in this population. Our findings provide no evidence for selection for delayed age at reproduction in male Soay sheep.