Locomotor hindrance of carrying an enlarged sexually selected structure on inclines for male fiddler crabs

Abstract

Abstract Sexual selection often results in males exhibiting exaggerated traits (e.g. bright colors, elaborate appendages) to attract potential mates and in some cases to also use as a weapon. These traits, however, can impose costs, such as an increase in energy expenditure and a decrease in locomotor performance, which could decrease foraging efficiency and increase an individual's vulnerability to predators. We examined the effect of the enlarged claw in male fiddler crabs U ca pugilator on ecologically relevant performance measures. We measured locomotor performance and kinematics during horizontal, uphill and downhill movements. Speed and stride mechanics were measured for clawed males, males after the claw was removed, and females while running on level (0°), uphill (15 and 30°) and downhill (−15 and −30°) slopes. Endurance capacities were measured on all crabs on horizontal and uphill inclines. Though claw removal had no significant effect on horizontal speeds, removal of the major claw significantly increased uphill speeds of male fiddler crabs at 15 and 30° inclines. Generally, as incline increased, the difference in performance between males with the enlarged claw and those with the claw removed increased. We also found that clawed males exhibit slower downhill speeds compared to clawless males and that claw removal significantly enhanced endurance on all inclines. This study indicates that an assessment of movement on level surfaces alone may not be entirely ecologically relevant when determining the actual costs of sexually selected ornaments.