Fast and fine versus strong and stout: a trade-off between chela closing force and speed across nine scorpion species

摘要

The functional trade-off between force and speed in simple muscle-lever systems is a well-known example of a possible biomechanical impedance to phenotypic diversity. Few clear interspecific examples exist of this trade-off. We report the closing speed and force of the chela (pincer), a simple muscle-lever system, in nine species of scorpions with diverse chela morphologies. Chela closing speed was calculated from three-dimensional landmark trajectories reconstructed from high-speed video. Closing force was measured using a piezo force gauge. Chela closing speed ranged from 83.5 to 682.5 mm/s. We found a significant negative correlation between maximum force and speed across the species. This correlation persisted when taking phylogenetic relationships into account, suggesting a functional trade-off. We found chela finger length to be correlated with lengthening of other pedipalp segments, and an increased gape angle. These findings suggest selection for increased gape and reach in longer-fingered species. We show that the variance in closing speed across species is due to an elongated out-lever, and that in-lever length and muscle contraction speed also contribute to the increased closing speed. We experimentally confirm the trade-off between force and speed in a muscle-lever system across species, and quantify the contributions of the lever and the muscle.