Variation in cross‐sectional horn shape within and among rhinoceros beetle species

Sexual selection has equipped male rhinoceros beetles with large horns on their head and prothorax to aid in battle over access to females. Horns are used to pry and dislodge opponents from resource sites that attract females, so an optimal horn should be able both to withstand the high stresses imposed during fights, and to resist deflection in response to these loads. We examined the cross‐sectional morphology of horns using micro‐computed tomography scanning to determine how horn structure changes with horn length to withstand the different fighting loads. Specifically, we measured the second moment of area of horns within and among rhinoceros beetle species to assess whether changes in cross‐sectional morphology accompany changes in body size in order to maintain high strength and stiffness during fights. We find that the second moment of area of horns increases with body size both intra‐specifically and inter‐specifically, and that these relationships closely fit those predicted if horns have been selected to be strong and stiff fighting structures. Our results therefore support the hypothesis that rhinoceros beetle horns are structurally adapted for combat.