Ant head shape evolved to compromise bite-induced mechanical demands

Heads of ant workers harbour the mouthparts and respective musculature, varying drastically in morphology. The mandible adductor muscles occupy most of the head's internal volume, and their contraction generates forces that could risk cuticle failure. Here we quantified ant worker plane head shape disparity and explored how it influences stress dissipation under biting conditions. We combined a geometric morphometric approach under a phylogenetic comparative framework and biomechanical simulations to test the hypothesis that idealized head shapes poorly explored by current ant lineages exhibit biomechanical limitations that could be negatively selected, while an idealized shape representing the median ant worker head shape would present a superior mechanical performance. Our results revealed that narrow heads with deep vertex depressions distribute stresses more evenly, but usually at high levels. Broader heads with convex posterior margins, in contrast, concentrate stresses around the mandibular articulations, making them more prone to failure under high bite forces. Most ant lineages exhibit a head shape that allows mild stresses to spread along the head while concentrating higher stresses around the mandibular articulations. This evolutionary pattern results in a limited morphospace occupation, with most ants clustering around a typical shape and only a few lineages exploring extreme morphologies.