From Armament to Ornament: Performance Trade-Offs in the Sexual Weaponry of Neotropical Electric Fishes

The evolution of sexual weaponry is thought to have marked effects on the underlying static allometry that builds them. These weapons can negatively affect organismal survivability by creating trade-offs between trait size and performance. Here we use three-dimensional geometric morphometrics to study the static allometry of two species of sexually dimorphic electric fishes ( Apteronotus rostratus and Compsaraia samueli ) in which mature males grow elongate jaws used in agonistic male-male interactions. We quantify jaw mechanical advantage between the sexes of both species to track changes in velocity and force transmission associated with the development of sexual weaponry. We find evidence for trade-offs between skull shape and mechanical advantage in C. samueli, where males with longer faces exhibit lower mechanical advantages, suggesting weaker bite forces. In contrast, males, and females of A. rostratus exhibit no difference in mechanical advantage associated with facial elongation. We hypothesize that differences in the functionality of the sexual weaponry between the two species may drive divergences in the allometric scaling of mechanical advantage.