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Using three data sources, we measured claw lengths of the extremely large major claws of 70 species of fiddler crabs, spread over four major biogeographic regions and all 11 genera, as a function of carapace length. Despite a diversity of biogeography, body size, type of sexual signaling and mating behavior, and details of claw morphology, all species groupings follow the same linear relationship. No subgroupings could be determined, by region, among diverse genera, by ancestral-derived status, or by data source. Major claw size is known to be implicated in female selection of mates, which might be expected to diversify claw lengths among species and genera, as might differences in combat might be expected to diversify claw length, as it does in other claw traits. The constancy suggests a possible functional constraint on major claw length across the fiddler crabs, which is to be determined. Efficient folding of the claw as the male rapidly retreats into a burrow, or a protection of the carapace from frontal attack by predators or combatants in an initial aggressive encounter might select for this pervasive relationship. Tests are suggested. The sexually selected major claw is more variable than the naturally selected minor claw, but this difference in variability could be attributed partially to regeneration of the major cheliped and even variation in shape. A surprising similar trend of closing force index for the major claw exists for a large number of species spread throughout the genera and major regions occupied by fiddler crabs, but a considerable variation exists in a variety of functional traits. Thus, the constancy of length relationship stands out distinctly from a considerable background variation in form. As a sexually selected trait, the claw length constancy begs for an explanation.

Length of a sexually selected ornament-armament in fiddler crabs (Decapoda: Brachyura: Ocypodidae): One way, over deep time and space