Meek Males and Fighting Females: Sexually-Dimorphic Antipredator Behavior and Locomotor Performance Is Explained by Morphology in Bark Scorpions (Centruroides vittatus)

Sexual dimorphism can result from sexual or ecological selective pressures, but the importance of alternative reproductive roles and trait compensation in generating phenotypic differences between the sexes is poorly understood. We evaluated morphological and behavioral sexual dimorphism in striped bark scorpions (Centruroides vittatus). We propose that reproductive roles have driven sexually dimorphic body mass in this species which produces sex differences in locomotor performance. Poor locomotor performance in the females (due to the burden of being gravid) favors compensatory aggression as part of an alternative defensive strategy, while male morphology is coadapted to support a sprinting-based defensive strategy. We tested the effects of sex and morphology on stinging and sprinting performance and characterized overall differences between the sexes in aggressiveness towards simulated threats. Greater body mass was associated with higher sting rates and slower sprinting within sexes, which explained the greater aggression of females (the heavier sex) and, along with longer legs in males, the improved sprint performance in males. These findings suggest females are aggressive to compensate for locomotor costs of reproduction while males possess longer legs to enhance sprinting for predator evasion and mate finding. Sexual dimorphism in the metasoma (“tail”) was unrelated to stinging and sprinting performance and may best be explained by sexual selection.性二态性（sexual dimorphism）可由性选择或生态选择压力塑造，但替代生殖角色与性状代偿在塑造两性表型差异中的重要性仍未得到充分阐明。本研究针对条纹石蝎（Centruroides vittatus）的形态与行为性二态性展开了系统性评估。我们提出，该物种的生殖角色分化推动了体质量的性二态性，进而导致了运动能力的两性差异。雌性个体因怀卵负担导致运动能力下降，因此更易表现出代偿性攻击行为，以此作为替代性防御策略；而雄性形态则通过协同适应，支撑起基于冲刺的防御策略。我们测试了性别与形态特征对蜇刺频率及冲刺性能的影响，并分析了两性在面对模拟威胁时的攻击性整体差异。在同性个体中，体质量更大的个体蜇刺频率更高、冲刺速度更慢，这解释了雌性（体型更重的性别）攻击性更强的原因；同时结合雄性更长的步足，也阐明了雄性冲刺性能更优的机制。上述研究结果表明，雌性通过提升攻击性来代偿繁殖带来的运动成本，而雄性则拥有更长的步足以强化冲刺能力，从而更好地躲避天敌与寻找配偶。后体部（metasoma，俗称"尾部"）的性二态性与蜇刺频率及冲刺性能并无关联，其演化或可通过性选择得到最佳解释。