Operational sex ratio but not density affects sexual selection in a fish

The operational sex ratio (OSR) and density are considered important factors affecting the strength of sexual selection. While there is increasing evidence that OSR and density affect the potential for sexual selection, few studies have addressed whether this is realized in phenotypic selection and how the two factors interact. We manipulated OSR (three levels) and male density (two levels) in 36 experimental breeding populations of Gobiusculus flavescens – a fish with paternal care. We measured mating competition behavior, the opportunity for selection (I), and selection on four morphological traits in males. We found sexual selection on two male traits, with the strongest selection being 20% of I. As predicted from OSR theory, increasing female scarcity caused males to become more competitive, concomitant with an increase in I and selection on morphological traits. Model simulations of I based on random mating (Imin) and maximum mate monopolization (Imax) demonstrated that the potential for sexual selection was close to its theoretical maximum across the range of OSRs. However, male density and its interaction with the OSR did not affect sexual selection. We argue that a multifaceted approach, combining mating behavior and selection analyses, can help us to understand how ecological factors affect sexual selection.

操作性别比（operational sex ratio, OSR）与种群密度被视为影响性选择强度的重要因素。尽管越来越多的证据表明操作性别比与种群密度会影响性选择的潜力，但鲜有研究探讨这一效应是否能在表型选择中得以体现，以及二者的交互作用机制。我们在36个实验繁殖种群中对操作性别比（设置3个梯度）与雄性密度（设置2个梯度）进行了操控，实验对象为具有父本抚育行为的双带缟虾虎（Gobiusculus flavescens）。我们测定了雄性的交配竞争行为、选择机会（opportunity for selection, I）以及4个形态性状所受的选择压力。研究发现，2个雄性形态性状受到性选择作用，其中最强的选择强度可达选择机会I的20%。正如操作性别比理论所预测的那样，雌性稀缺程度的提升会使雄性竞争加剧，同时伴随选择机会I与雄性形态性状选择压力的升高。基于随机交配（Imin）与最大配偶垄断（Imax）两种情景构建的选择机会I模型模拟结果显示，在所有操作性别比梯度下，性选择潜力均接近其理论最大值。但雄性密度及其与操作性别比的交互作用并未对性选择产生显著影响。我们认为，结合交配行为与选择分析的多维度研究方法，有助于揭示生态因子对性选择的作用机制。