Fight intensity correlates with stronger and more mechanically efficient weapons in three species of Aegla crabs

In many species, individuals contest resources using specialized morphologies to overpower rivals, hereafter referred to as weapons. Despite their importance in fights, little is known about the selective forces affecting weapon evolution. This may be particularly important to understand why weapons are highly variable among species. Due to their role during fighting, we expect that whenever fighting becomes more important for individual fitness so should the intensity of selection on weapon strength and morphology (which affect the efficiency of a weapon during combat). If true, we expect species that fight more intensely to have stronger and more mechanically efficient weapons. We tested this idea using males of three species of Aegla crabs (A. longirostri, A. abtao, and A. denticulata) that vary in their fight intensity. We compared the muscle size, the mechanical advantage (a proxy for the efficiency of the movable finger of the claw), and the correlation between weapon biomechanics and overall weapon shape (a proxy for the efficiency of the entire claw) among the species. We found that species with more intense fights presented stronger claws, higher mechanical advantage, and less variation in the regression between biomechanics and overall shape. Interestingly, the species with the largest claws were not the most mechanically efficient, suggesting that weapon size is not the sole factor behind weapon evolution. We conclude that fight intensity might be an important factor affecting weapon biomechanics, which ultimately might lead to a better understanding of weapon evolution. Animals fight using specialized morphologies to overpower rivals—termed weapons. Given the importance of fighting on leaving descendants to the next generation, weapon features related to winning fights are probably under selection. If true, then species in which fighting is more important should have stronger and more mechanically efficient weapons. Our results suggest that this might be true: Aegla crabs that fight more intensely have stronger and more efficient weapons (their claws). Interestingly, we also show that size is not the sole predictor of a better claw—muscle mass and mechanical efficiency might be higher in smaller claws when compared to larger claws. Thus, weapon evolution might not be solely tied to weapon size, but also to weapon morphology and mechanical efficiency.