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 seven 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. Since individual heterogeneity affects breeding success, we believe that variation in individual quality masks costs of early reproduction in this population. Our findings provide no evidence for selection for delayed age at reproduction in male Soay sheep. 

This data consists of long-term field data and samples collected by research teams between 1986 and 2020. Data is collected three times each year; in late winter/early spring, summer, and autumn. Ten study area censuses are conducted on each expedition, yielding population size and sex ratio data. In spring, mortality checks determine which individuals died over winter. Here, we used data only from males that died in the study area during the study period. Lambs are caught soon after birth, weighed, sampled for genetic analysis, and tagged for lifelong identification. In August, individuals are caught, and up-to-date measurements of body size are collected. Soay sheep rut in November; at this time, 4–5 censuses of the study area are carried out per day to note the identity of pairs in consorts. During this time, any immigrant males from elsewhere on the island are also captured by darting, measured, sampled, and tagged. 

Paternity was inferred genetically using 431 Single Nucleotide Polymorphisms (SNPs) genotyped in virtually all study area individuals and the R pedigree reconstruction package SEQUOIA. A small number of paternities involving individuals not SNP-genotyped were inferred using microsatellite markers and the R package MasterBayes.