Data from: Reproductive skew drives patterns of sexual dimorphism in sponge-dwelling snapping shrimps

Sexual dimorphism is typically a result of strong sexual selection on male traits used in male–male competition and subsequent female choice. However, in social species where reproduction is monopolized by one or a few individuals in a group, selection on secondary sexual characteristics may be strong in both sexes. Indeed, sexual dimorphism is reduced in many cooperatively breeding vertebrates and eusocial insects with totipotent workers, presumably because of increased selection on female traits. Here, we examined the relationship between sexual dimorphism and sociality in eight species of Synalpheus snapping shrimps that vary in social structure and degree of reproductive skew. In species where reproduction was shared more equitably, most members of both sexes were physiologically capable of breeding. However, in species where reproduction was monopolized by a single individual, a large proportion of females—but not males—were reproductively inactive, suggesting stronger reproductive suppression and conflict among females. Moreover, as skew increased across species, proportional size of the major chela—the primary antagonistic weapon in snapping shrimps—increased among females and sexual dimorphism in major chela size declined. Thus, as reproductive skew increases among Synalpheus, female–female competition over reproduction appears to increase, resulting in decreased sexual dimorphism in weapon size.

性二态性 (sexual dimorphism) 通常是对雄性用于雄性间竞争及后续雌性选择的性状施加强烈性选择的结果。然而，在群体内繁殖被单个或少数个体垄断的社会性物种中，对第二性征的选择压力可能在两性中均较强。事实上，在许多合作繁殖的脊椎动物以及拥有全能工体的真社会性昆虫 (eusocial insects) 中，性二态性有所减弱，这可能源于雌性性状所承受的选择压力增强。本研究以8种社会结构和繁殖偏倚 (reproductive skew) 程度各不相同的枪虾属 (Synalpheus) 鼓虾为研究对象，探究了性二态性与社会性之间的关联。在繁殖分配更均等的物种中，两性的多数个体均具备生理繁殖能力；而在繁殖被单个个体垄断的物种中，大部分雌性而非雄性处于生殖不活跃状态，这表明雌性间存在更强的生殖抑制与冲突。此外，随着各物种繁殖偏倚程度升高，雌性的大螯 (major chela) ——鼓虾的主要对抗性武器——的相对尺寸逐渐增大，而两性间大螯尺寸的性二态性则逐渐降低。综上，在枪虾属物种中，随着繁殖偏倚程度升高，雌性间为争夺繁殖机会的竞争愈发激烈，最终导致武器尺寸的性二态性减弱。