Selection on an extreme weapon in the frog legged leaf beetle (Sagra femorata)

Biologists have been fascinated with the extreme products of sexual selection for decades. However, relatively few studies have characterized patterns of selection acting on ornaments and weapons in the wild. Here, we measure selection on a wild population of weapon-bearing beetles (frog legged leaf beetles: Sagra femorata) for two consecutive breeding seasons. We consider variation in both weapon size (hindleg length), and in relative weapon size (deviations from the population average scaling relationship between hindleg length and body size), and provide evidence for directional selection on weapon size per se and stabilizing selection on a particular scaling relationship in this population. We suggest that whenever growth in body size is sensitive to external circumstance such as nutrition, then considering deviations from population-level scaling relationships will better reflect patterns of selection relevant to evolution of the ornament or weapon than will variation in trait size...