Data from: Within-season variation in sexual selection in a fish with dynamic sex roles

The strength of sexual selection may vary between species, among populations, and within populations over time. While there is growing evidence that sexual selection may vary between years, less is known about variation in sexual selection within a season. Here, we investigate within-season variation in sexual selection in male two-spotted gobies (Gobiusculus flavescens). This marine fish experiences a seasonal change in the operational sex ratio from male- to female-biased, resulting in a dramatic decrease in male mating competition over the breeding season. We therefore expected stronger sexual selection on males early in the season. We sampled nests and nest-holding males early and late in the breeding season and used microsatellite markers to determine male mating and reproductive success. We first analysed sexual selection associated with the acquisition of nests by comparing nest-holding males to population samples. Among nest-holders, we calculated the potential strength of sexual selection and selection on phenotypic traits. We found remarkable within-season variation in sexual selection. Selection on male body size related to nest acquisition changed from positive to negative over the season. The opportunity for sexual selection among nest-holders was significantly greater early in the season rather than late in the season, partly due to more unmated males. Overall, our study documents a within-season change in sexual selection that corresponds with a predictable change in the operational sex ratio. We suggest that many species may experience within-season changes in sexual selection and that such dynamics are important for understanding how sexual selection operates in the wild.性选择的强度可能在物种间、种群间以及种群内部随时间产生波动。现有研究逐渐积累的证据显示，性选择在不同年份间存在差异，但学界对单个繁殖季内性选择的变异情况仍了解有限。本研究以雄性双斑虾虎鱼（Gobiusculus flavescens）为研究对象，探究了繁殖季内性选择的变异规律。该海洋鱼类的操作性别比（operational sex ratio）会随繁殖季节从雄性偏向性转为雌性偏向性，进而导致整个繁殖季内雄性的交配竞争程度大幅降低。据此我们推测，繁殖季早期的雄性个体将面临更强的性选择压力。我们在繁殖季的早期与晚期分别采集了巢穴及占据巢穴的雄性样本，并利用微卫星标记（microsatellite markers）测定了雄性的交配与繁殖成功率。首先，我们通过对比占据巢穴的雄性与种群样本，分析了与巢穴获取相关的性选择；在占据巢穴的雄性群体中，我们进一步计算了性选择的潜在强度以及表型性状所受的选择作用。研究结果表明，性选择在繁殖季内存在显著变异：与巢穴获取相关的雄性体型选择强度随季节从正向转为负向；占据巢穴的雄性群体中的性选择机会（opportunity for sexual selection）显著高于繁殖季晚期，这一差异部分源于未交配雄性数量的变化。总体而言，本研究记录了性选择随繁殖季发生的动态变化，且该变化与操作性别比的可预测性转变相契合。我们认为，多数物种可能都存在繁殖季内性选择的动态波动，这类动态机制对于理解野外环境中性选择的运作逻辑具有重要价值。