Allometric constraints and the modulation of weapon evolution by mating system in fiddler crabs

Sexual selection potentially drives the evolution of exaggerated traits used in intrasexual contests. However, the extent of mating systems’ influence on weapon morphology remains unclear. In fiddler crab males, an exaggerated claw functions both as a weapon and a signaling tool, varying according to the species’ mating system. We examined claw evolution in male fiddler crabs, differentiating between two main mating strategies: 1. males defend their mating burrows (= ‘burrow’); 2. males do not mate in their own burrows (= ‘surface’). We measured claw morphological traits and tested whether the mating system affects their evolutionary rates, expecting ‘burrow’ species to exhibit higher evolutionary rates. In general, claw size scales isometrically with body size across species. Both systems showed no correlation between claw elements and mechanical advantage, indicating the necessity of maintaining a conspicuous signaling tool alongside an efficient lever system for grip strength as body size increases. Contrary to predictions, however, ‘burrow’ males exhibited lower evolutionary rates in claw traits than ‘surface’ males, suggesting stronger stabilizing selection. These findings highlight the nuanced effects of sexual selection on male fiddler crab weapon evolution, suggesting that mating systems can modulate evolutionary trajectories, yet functional demands for dual weapon-signal roles constrain claw morphology.