The intensity of sexual selection, body size and reproductive success in a mating system with male-male combat: Is bigger better?

Body size is a key selected trait in many animal systems: larger size is sexually selected for in males because it confers a reproductive advantage during contest competition for access to females, and larger females are naturally selected for fecundity. Herein, we used radio-telemetry to gather a large dataset of male-female interactions and DNA paternity analyses to characterize the intensity of sexual selection and the link between two body size metrics (body length and condition, the latter manipulated experimentally for males) and reproductive success in a population of puff adders (Bitis arietans). Our multiple estimates of the intensity of sexual selection generally indicated that males experienced stronger sexual selection than females. However, the Bateman gradients did not differ by sex, despite the fact that males increased reproductive success by mating with multiple females while females did not. We also found no strong evidence that females experienced indirect fitness benefits through multiple matings. Body size was not a key predictor of male reproductive success, and for females, body condition—but not body length—was the critical fecundity trait. Altogether, a combination of factors suggests that post–copulatory mechanisms of sexual selection (e.g., sperm competition, cryptic female choice) may play critical roles in this mating system and perhaps that of other snakes. We interpret our findings in the context of sexual conflict—a ubiquitous and potent driver of mating strategy evolution—to propose a scenario for the evolution of female promiscuity that is applicable to many other animal systems where males roam widely to locate females at high costs.

体型是诸多动物类群中受选择的关键性状：雄性个体体型更大会受到性选择青睐，因为在争夺雌性配偶的竞技竞争中，更大体型能赋予其繁殖优势；而雌性体型更大则会因生育力优势受到自然选择。本研究通过无线电遥测技术（radio-telemetry）收集了大量雌雄个体互动的数据集，并结合DNA亲权分析，对鼓腹咝蝰（Bitis arietans）种群的性选择强度，以及两种体型指标（体长与身体状况，其中雄性的身体状况已通过实验操控）与繁殖成功率之间的关联进行了刻画。我们对性选择强度的多项评估结果普遍显示，雄性所经历的性选择强度高于雌性。不过，尽管雄性通过与多只雌性交配提升了繁殖成功率，而雌性并未如此，但雌雄个体间的贝特曼梯度（Bateman gradients）并无显著差异。此外，我们未发现足够证据表明雌性可通过多次交配获得间接适合度收益。体型并非雄性繁殖成功率的关键预测因子；而对于雌性而言，身体状况而非体长才是决定其生育力的核心性状。综合来看，多项证据表明，性选择的交配后机制（如精子竞争、隐秘雌性选择）可能在该蛇类的交配系统中发挥关键作用，这一结论或许也适用于其他蛇类类群。本研究结合性冲突——这一普遍存在且强力驱动交配策略演化的选择压力——对研究结果进行解读，并提出了一套关于雌性多偶演化的假说框架，该框架可推广至诸多雄性需耗费高昂代价大范围搜寻雌性的其他动物类群。