Biomechanical trade-offs and the evolutionary dynamics of weapon morphology: phylogenetic comparative evidence from scorpion pincers.

Pincer-like appendages are multifunctional structures that have evolved repeatedly within Arthropoda. In scorpions, they appear in the form of pedipalp chelae, providing an excellent example of form-function integration in evolutionary biology. Performance traits such as pinch force, closing speed, and mechanical resistance can be reliably predicted from chela morphology and directly influence fitness through a wide range of interactions. Despite this, the evolutionary trajectories underlying the morphological diversity of the scorpion chelae remain poorly understood. Phylogenetic comparative analyses were conducted on chela measurements from 547 species (ca. 19% of all known scorpion species) to examine evolutionary allometry and to test whether chela length, width, and height were characterized by similar evolutionary rates and stabilizing regimes. Positive allometry was detected across all dimensions, with height scaling most steeply with body size. Ornstein-Uhlenbeck models revealed distinct evolutionary dynamics: chela length exhibited the highest instantaneous evolutionary rate while simultaneously experiencing the strongest stabilizing selection, whereas height and width displayed comparable long-term variance despite differing rates, consistent with coordinated functional constraints on robustness. These findings demonstrate that high rates of phenotypic evolution do not necessarily imply weak selection but rather indicate the magnitude of stochastic forces that selection must counteract to maintain functional integrity.