Data from: Disentangling the contribution of sexual selection and ecology to the evolution of size dimorphism in pinnipeds

The positive relationship between sexual size dimorphism (SSD) and harem size across pinnipeds is often cited as a textbook example of sexual selection. It assumes that female aggregation selected for large male size via male-male competition. Yet, it is also conceivable that SSD evolved prior to polygyny due to ecological forces. We analysed eleven life history traits in 35 pinniped species to determine their co-evolutionary dynamics and infer their most likely evolutionary trajectories contrasting these two hypotheses. We find support for SSD having evolved prior to changes in the mating system, either as a consequence of niche partitioning during aquatic foraging or in combination with sexual selection on males to enforce copulations on females. Only subsequently did polygyny evolve, leading to further coevolution as the strength of sexual selection intensified. Evolutionary sequence analyses suggest a polar origin of pinnipeds and indicate that SSD and polygyny are intrinsically linked to a suite of ecological and life history traits. Overall, this study calls for the inclusion of ecological variables when studying sexual selection and argues for caution when assuming causality between coevolving traits. It provides novel insights into the role of sexual selection for the co-evolutionary dynamics of SSD and mating system.鳍足类动物（pinnipeds）中性大小二态性（sexual size dimorphism, SSD）与配偶群规模之间的正相关关系，常被引为性选择的经典教科书范例。该范例默认雌性聚集通过雄性间竞争驱动雄性体型演化增大。然而，也存在另一种可能：SSD的演化早于一雄多雌制（polygyny）的出现，其驱动因素为生态压力。我们对35个鳍足类物种的11项生活史特征开展分析，以明确其协同进化动力学过程，并对比上述两种假说以推断最具可能性的进化轨迹。研究结果支持SSD先于交配系统改变发生演化的结论：这一过程既可能源于水生觅食阶段的生态位分化，也可能与雄性为强迫雌性交配而经历的性选择共同作用。直至后续演化阶段，一雄多雌制才得以演化，且随着性选择强度的增强，二者进一步产生协同进化。进化序列分析表明鳍足类动物起源于极地环境，同时显示SSD与一雄多雌制均与一系列生态及生活史特征存在内在关联。综上，本研究呼吁在性选择研究中纳入生态变量，并提醒学界在假定协同进化性状间存在因果关系时需保持谨慎。本研究为SSD与交配系统的协同进化动力学中，性选择所扮演的角色提供了全新见解。