Ant head shape evolved to compromise bite-induced mechanical demands

Abstract Morphological evolution is commonly associated with functional performance, and understanding this relationship can aid in comprehending lineage diversification. Ant worker heads harbour the mouthparts and respective musculature related to food processing, and are morphologically diverse. Here, we quantified ant worker plane head shape disparity and explored how it influences stress dissipation under bite loading. 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 a median ant worker head shape would present superior mechanical performance. Our results revealed that narrow heads with deep vertexal depressions distribute stresses more evenly, but usually at higher 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. Our framework suggests an evolutionary pattern of limited morphospace exploration, with most ants clustering around a typical shape and only a few lineages exploring extreme morphologies.