Data from: Demographic drivers of age-dependent sexual selection

Sexual selection has a critical role in evolution, and it is fundamental to identify what ecological factors drive its variation. Disentangling the ecological correlates of sexual selection over the long term, however, is challenging and has rarely been done in nature. We sought to assess how demographic changes influenced the intensity, direction and form of sexual selection and whether selective pressures varied with age. We tested whether breeder sex ratio, number of competitors and age structure influenced selection differentials on horn length of wild bighorn rams (Ovis canadensis) of different age classes on Ram Mountain, Alberta. We used 21 years of data including a detailed pedigree, demographic parameters and repeated morphological measurements. Sexual selection on horn length of males of all ages was directional and positive. Selection intensity increased with the number of competitors, reflecting male–male encounter rate during the rut, but was independent of breeder sex ratio or age structure. This result can also be linked to changes in population size because the number of competitors was highly correlated to total number of sheep. This demographic effect likely arises from age-dependent mating tactics. Males aged 2–4 years are weakly competitive and experienced stronger sexual selection as they accounted for a greater proportion of all males. Selection experienced by mature males appeared independent of demography. Our study provides a rare description of the demographic determinants of sexual selection in nature.性选择在演化中具有关键作用，明确驱动其变异的生态因子是一项基础性研究内容。然而，厘清长期尺度下性选择的生态关联极具挑战，且在自然种群中鲜有相关研究开展。本研究旨在探究种群动态变化如何影响性选择的强度、方向与模式，同时检验选择压力是否随年龄发生变化。我们以加拿大阿尔伯塔省拉姆山的野生大角羊（Ovis canadensis）种群为研究对象，检验了繁殖种群性比、竞争者数量以及年龄结构，是否会对不同年龄组公羊的角长产生选择差（selection differentials）。本研究使用了长达21年的观测数据集，涵盖详尽的系谱信息、种群动态参数以及重复测得的形态学测量数据。所有年龄组雄性大角羊的角长均受到定向性正选择。性选择强度随竞争者数量增多而升高，这一结果反映了发情交配期（rut）雄羊间的相遇频率，但选择强度与繁殖种群性比及年龄结构均无关联。由于竞争者数量与种群总个体数高度相关，上述结果也可与种群规模的变化联系起来。这种种群动态效应可能源于基于年龄的交配策略差异。2至4岁的雄羊竞争力较弱，且由于其在种群雄体中占比提升，受到的性选择强度也更强。成年雄羊所经历的性选择则似乎不受种群动态的影响。本研究罕见地阐明了自然种群中性选择的种群动态决定因子。