Sexual dimorphism as a facilitator of worker caste evolution in ants

Ant societies are primarily composed of females, whereby labor is divided into reproductive, or queen, and non-reproductive, or worker, castes. Workers and reproductive queens can differ greatly in behavior, longevity, physiology, and morphology, but queen-worker differences are usually modest relative to the differences in males. Males are short-lived, typically do not provide the colony with labor, often look like a different species, and only occur seasonally. It is these differences that have historically led to their neglect in social insect research, but also why they may facilitate novel phenotypic variation - by increasing the phenotypic variability that is available for selection. In this study, worker variation in multivariate size-shape space paralleled male-queen variation. As worker variation increased within species, so did sexual variation. Across species in two independent genera, using head width as a proxy for body size, sexual size dimorphism correlated with worker polymorphism regardless of whether the ancestral condition was large or small worker/sexual dimorphism. Mounting molecular data support the hypothesis that queen-worker caste determination has co-opted many genes/pathways from sex determination. The molecular evidence, coupled with the observations from this study, leads to the hypothesis that sexual selection and selection on colony-level traits are non-independent, and that sexual dimorphism may even have facilitated the evolution of the distinct worker caste.