Functional Allometry of Sexually Selected Traits in Tetraopes tetrophthalmus Red Milkweed Beetles (Coleoptera: Cerambycidae)

The allometry of sexually selected traits is highly variable. Some traits scale steeply in relation to body size while other traits scale more shallowly. Still others scale proportionately to body size. One explanation for this variability is trait function. Sexually selected traits should scale differently according to the function that they perform—the functional allometry hypothesis. Here, we investigate the functional allometry hypothesis in red milkweed beetles, Tetraopes tetrophthalmus (Coleoptera: Cerambycidae), a species that possesses multifunctional sexually selected traits. Males use their mandibles to eat and as weapons to fight rivals. Their antennae act as sensory organs and as tactile signals during copulatory courtship. We measured these sexually selected traits along with body size in both males and females. We also measured thorax and leg size, traits presumably not under sexual selection, as control traits. We found that tactile signal traits used in courtship were negatively allometric, while weapons, traits supporting the weapon, and control traits were isometric. Although male mandible weapons were isometric, we found that female mandibles were negatively allometric. Finally, weapon shape was also examined as this trait is important to fighting outcomes. We found that male mandible weapons have a rugose medial curve with a midpoint ledge that female mandibles lack. These results support the functional allometry hypothesis for variation in trait allometry. We discuss how interactions between the functions that these traits perform may influence the evolution of morphology.