Canalized weapon development but high levels of overall phenotypic variation in an evolutionarily novel ant male...

Sexual selection is responsible for the evolution of exaggerated morphologies in males of many animal species. In extreme cases, this has led to the emergence of novel morphs, which differ discretely from ancestral phenotypes. Morphological studies of sexually selected traits in males have mainly focused on species in which trait expression is continuous. How trait expression varies in species with discrete male morphs is comparably understudied. We find that wingless and winged male morphs in the ant Cardiocondyla obscurior differ in size and variation of eight morphological traits, including mandibles used as weapons in fights between wingless males. Differences between morphs were consistent across populations, but the size of some traits varied by population origin, even though all animals were lab-reared under identical conditions. In contrast to weapons in males of many other species, the mandibles of wingless males showed negative allometry with body size. Mandibles exhibited similar levels of fluctuating asymmetry in both morphs; in wingless males, mandible asymmetry was negatively correlated with mandible length, suggesting that stable mandible development may be an indicator of overall quality of fighter males. Mandible length showed less variation in wingless males than in winged males, whereas all other traits were more variable in wingless males, including highly conserved traits, such as antenna segment number. Wingless males also exhibited more variation in trait size than female queens and workers. Together, these data point towards stabilizing selection on weapon phenotype but overall low levels of developmental stability and canalization in an evolutionarily novel ant morph.