Data from: Mate choice and the operational sex ratio: an experimental test with robotic crabs

The operational sex ratio (OSR) (sexually active males: receptive females) predicts the intensity of competition for mates. It is less clear, however, under what circumstances the OSR predicts the strength of sexual selection – that is, the extent to which variation in mating success is attributable to traits that increase the bearer's attractiveness and/or fighting ability. To establish causality experiments are required that manipulate OSR. Furthermore, if it is possible to control for any OSR-dependent changes in the chosen sex (e.g. changes in male courtship), we can directly test whether the OSR affects the behaviour of the choosing sex (e.g. female choice decisions). We conducted female mate choice experiments in the field using robotic models of male fiddler crabs (Uca mjoebergi). We used a novel design with two females tested sequentially per trial. As in nature, the choice of the first female to mate therefore affected the mates available to the next female. In general we detected significant sexual selection due to female choice for ‘males’ with larger claws. Importantly, the strength of sexual selection did not vary across five different OSR/density treatments. However, as the OSR decreased (hence the number of available males declined), females chose the ‘males’ with the largest claws available significantly more often than expected by chance. Possible reasons for this mismatch between the expected and observed effects of the OSR on the strength of sexual selection are discussed.操作性别比（Operational Sex Ratio, OSR，即性活跃雄性与接受交配雌性的数量比）可预测配偶竞争的激烈程度。然而，目前尚不清楚在何种情境下，OSR能够预测性选择的强度——即交配成功率的差异在多大程度上可归因于提升个体吸引力与/或战斗能力的性状。 为确立因果关系，需开展操控OSR的实验。此外，若能够控制择偶性别（例如雄性求偶行为）随OSR产生的变化，则可直接检验OSR是否会影响择偶性别（例如雌性的择偶决策）的行为表现。 本研究以敏手招潮蟹（Uca mjoebergi）的雄性机器人模型为实验对象，开展野外雌性择偶实验。我们采用了一项新颖的实验设计：每轮实验依次测试两只雌性。与自然情境一致，第一只雌性的交配选择会影响第二只雌性可获得的配偶资源。 总体而言，我们检测到雌性偏好拥有更大螯肢的"雄性"，由此产生了显著的性选择。值得注意的是，在五种不同的OSR与密度处理组中，性选择强度并未发生显著变化。然而，随着OSR降低（即可获得的雄性数量减少），雌性选择当前可获得的螯肢最大的"雄性"的概率显著高于随机预期水平。 本文将讨论OSR对性选择强度的预期效应与观测效应之间出现偏差的潜在原因。