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发生变化。尽管已有越来越多的证据表明性选择在年份间存在差异，但学界对单个繁殖季内性选择的变化仍知之甚少。本研究针对雄性双斑虾虎鱼（Gobiusculus flavescens）的季内性选择变化展开探究，该海洋鱼类的操作性别比（operational sex ratio）会随季节发生转变，从雄性偏倚逐渐转为雌性偏倚，进而导致繁殖季内雄性的交配竞争程度急剧下降，据此我们推测繁殖季早期的雄性会面临更强的性选择。我们于繁殖季的早期与晚期分别采集巢穴及占据巢穴的雄性样本，并借助微卫星标记（microsatellite markers）测定雄性的交配成功率与繁殖成功率，首先通过对比占据巢穴的雄性与种群样本，分析与巢穴获取相关的性选择；在巢穴持有者群体内部，我们计算了性选择的潜在强度以及各表型性状所受的选择压力。研究发现季内性选择存在显著变化：与巢穴获取相关的雄性体型选择效应在繁殖季内从正向转为负向；巢穴持有者间的性选择机会在繁殖季早期显著高于晚期，这一现象部分源于早期未交配雄性的数量更多。总体而言，本研究记录了与可预测的操作性别比变化相对应的季内性选择动态变化，我们认为诸多物种可能都存在季内性选择的变化，这类动态机制对于理解性选择在自然环境中的运作方式至关重要。