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）可预测配偶竞争的强度。然而，目前尚不清楚在何种情境下，操作性别比能够预测性选择的强度——即交配成功率的变异在多大程度上可归因于提升个体吸引力和/或打斗能力的性状。为确立因果关系，需开展操控操作性别比的实验。此外，若能控制被选择性别（例如雄性求偶行为）随操作性别比变化的改变，我们便可直接检验操作性别比是否会影响选择性别（例如雌性择偶决策）的行为。我们利用雄性招潮蟹（Uca mjoebergi）的机器人模型开展了野外雌性择偶实验，采用了一项新颖的实验设计：每个试次依次测试两只雌性。正如自然情境中一样，第一只雌性的择偶选择会影响第二只雌性可获得的配偶资源。总体而言，我们检测到了因雌性偏好拥有更大螯肢的‘雄性’而产生的显著性选择。值得注意的是，性选择强度在五种不同的操作性别比/密度处理组间并未发生显著变化。但随着操作性别比降低（即可用雄性数量减少），雌性选择拥有现存最大螯肢的‘雄性’的频率显著高于随机概率下的预期水平。本文讨论了操作性别比对性选择强度的预期效应与观测效应之间出现不匹配的潜在原因。