Data from: The design of a beautiful weapon: compensation for opposing sexual selection on a trait with two functions

Male fiddler crabs, genus Uca, have one greatly enlarged claw with which they court females and threaten and fight other males. Longer claws are more effective signals but are thought to be less effective weapons because the relative closing force at the tip of the claw decreases with claw length. We studied claw morphology and fighting in Uca terpsichores and Uca beebei and found a mechanism that may resolve opposing selection for signaling and fighting ability. When males fought they delivered gripping forces not at the tips but at the tubercles on the inner margins of their claws’ fingers. As claws grow, these tubercles remain relatively close to the apex of the gape. Consequently, the mechanical advantage that governs the forces that can be delivered at these tubercles decreases only slightly with increasing claw length allowing the claw to be an effective signal and a powerful weapon. Animal weapons are exceptionally diverse in form and detail of armature and the causes of this diversity are poorly understood. We suggest that the designs of weapons may often reflect compensatory patterns of growth and placement of armature that enhances the weapon’s overall utility for multiple uses in competition for mates.

招潮蟹属（Uca）雄性个体生有一只极度膨大的螯足，用于向雌性个体求偶，同时用于威慑其他雄性并与之争斗。螯足越长，其作为信号展示的效果越强，但作为武器的效能却被认为越弱，这是因为螯足尖端的相对闭合力量会随螯足长度增加而下降。本研究以舞招潮蟹（Uca terpsichores）和毕氏招潮蟹（Uca beebei）为对象，分析其螯足形态与争斗行为，发现了一种可能解决信号展示与争斗能力之间拮抗选择的机制。雄性在争斗时，其施加的夹持力量并非来自螯足指尖，而是来自螯足指节内侧边缘的结节（tubercles）。随着螯足生长，这些结节始终相对靠近螯足钳口的尖端。因此，控制结节处可施加力量的机械优势仅随螯足长度增加小幅下降，使得螯足既能成为有效的信号展示工具，又能成为极具威力的武器。动物的武器在形态与附属结构（armature）细节上呈现出极高的多样性，而这种多样性的成因目前仍鲜为人知。我们认为，武器的结构设计往往反映出一种补偿性的生长模式与附属结构布局，这种模式能够提升武器在配偶竞争中多用途的整体效能。