Department of Biology, Gonzaga University, Spokane, WA 99258, USA.
BMC Evol Biol. 2013 Jul 15;13:137. doi: 10.1186/1471-2148-13-137.
Fiddler crabs, genus Uca, are classic examples of how intense sexual selection can produce exaggerated male traits. Throughout the genus the enlarged "major" cheliped (claw) of the male fiddler crab is used both as a signal for attracting females and as a weapon for combat with other males. However, the morphology of the major claw is highly variable across the approximately 100 species within the genus. Here we address variation, scaling, and correlated evolution in the mechanics of the major claw by analyzing the morphology and mechanical properties of the claws of 21 species of fiddler crabs from the Pacific, Gulf and Atlantic coasts of the Americas.
We find that the mechanics that produce claw closing forces, the sizes of claws and the mechanical strength of the cuticle of claws are all highly variable across the genus. Most variables scale isometrically with body size across species but claw force production scales allometrically with body size. Using phylogenetically independent contrasts, we find that the force that a claw can potentially produce is positively correlated with the strength of the cuticle on the claw where forces are delivered in a fight. There is also a negative correlation between the force that a claw can potentially produce and the size of the claw corrected for the mass of the claw.
These relationships suggest that there has been correlated evolution between force production and armoring, and that there is a tradeoff between claw mechanics for signaling and claw mechanics for fighting.
招潮蟹属（Uca）的螃蟹是强烈的性选择如何导致雄性特征明显夸张的典型例子。在整个属中，雄性招潮蟹增大的“主要”螯（爪子）既被用作吸引雌性的信号，又被用作与其他雄性螃蟹战斗的武器。然而，主要爪子的形态在属内的大约 100 个物种中高度多变。在这里，我们通过分析来自美洲太平洋、海湾和大西洋沿岸的 21 种招潮蟹的爪子形态和力学特性，来研究主要爪子的变异性、比例和相关进化。
我们发现，产生爪子关闭力的力学、爪子的大小以及爪子表皮的机械强度在整个属中都高度可变。大多数变量在物种间与身体大小按比例缩放，但爪子力的产生则按异速缩放。使用系统发育独立对比，我们发现爪子在战斗中施加力量的潜力与爪子表皮的强度呈正相关，爪子的力量与爪子的大小呈负相关，爪子的大小是根据爪子的质量校正的。
这些关系表明，在力的产生和装甲之间存在相关进化，并且在信号传递和战斗的爪子力学之间存在权衡。
