Female weapon allometry and combat support social selection in a necrophagous dung beetle

Summary: Weapons used in intraspecific combat are typically male-biased, yet females of the Amazonian dung beetle Coprophanaeus lancifer bear prominent cephalic horns and pronotal protuberances that are used during contests over carrion resources and burial burrows. Using field observations of 95 dyadic contests involving 163 individually marked beetles at Los Amigos Biological Station, Peru, we tested whether body size and cephalic horn length predict contest outcomes across male–male, female–female, and male–female dyads inside burrows and at surface level. Larger-bodied individuals with longer cephalic horns won most contests. Horn length exhibited positive allometry in both sexes, with allometric slopes statistically indistinguishable between males and females. Most contests (81 of 95; 85.3%) occurred inside carrion burial burrows, consistent with Emlen's hypothesis that cephalic horns are adapted for fighting within confined spaces. Per-capita contest participation did not differ between sexes, and contests occurred predominantly between individuals of the same sex. Together, these findings support the prediction that female weapons can evolve through social selection when females compete intensely for limiting reproductive resources.