Data from: Sneaker males affect fighter male body size and sexual size dimorphism in salmon

Large male body size is typically favored by directional sexual selection through competition for mates. However, alternative male life-history phenotypes, such as 'sneakers', should decrease the strength of sexual selection acting on body size of large 'fighter' males. We tested this prediction with salmon species; in southern populations where sneakers are common, fighter males should be smaller than in northern populations where sneakers are rare, leading to geographical clines in sexual size dimorphism (SSD). Consistent with our prediction, fighter male body size and SSD (fighter male:female size) increase with latitude in species with sneaker males (Atlantic salmon, Salmo salar, and masu salmon, Oncorhynchus masou), but not in species without sneakers (chum salmon, O. keta, and pink salmon, O. gorbuscha). This is the first evidence that sneaker males affect SSD across populations and species, and suggests that alternative male mating strategies may shape the evolution of body size.

通常而言，通过配偶竞争开展的定向性选择（directional sexual selection）会青睐体型更大的雄性个体。然而，诸如‘偷配雄性’（sneakers）这类雄性生活史替代表型，会削弱针对‘争斗型雄性’体型的性选择强度。我们以鲑科鱼类为对象验证了这一预测：在偷配雄性较为常见的南部种群中，争斗型雄性的体型应小于偷配雄性罕见的北部种群，由此会导致性别大小二态性（sexual size dimorphism, SSD）出现地理渐变特征。与预测结果一致，存在偷配雄性的物种（大西洋鲑Salmo salar、马苏大麻哈鱼Oncorhynchus masou）中，争斗型雄性体型与性别大小二态性（争斗型雄性/雌性体型比）随纬度升高而增大；而无偷配雄性的物种（大麻哈鱼O. keta、粉鲑O. gorbuscha）则未呈现该趋势。本研究首次证实了偷配雄性可在种群与物种层面影响性别大小二态性，同时表明雄性交配的替代策略或许会驱动体型的演化。