Evaluating the costs of a sexually selected weapon: big horns at a small price

Abstract

A major assumption of sexual selection theory is that ornaments and weapons are costly. Such costs should maintain the reliability of ornaments and weapons as indicators of male quality, and therefore explain why choosy females and rival males pay attention to these traits. However, honest signalling may not depend on costs if the penalty for cheating is sufficiently high, a situation that is likely to be true for most weapons because they are frequently tested during combat. We examined and summarized the costs of producing and carrying giant horns in the rhinoceros beetle, Trypoxylus dichotomus. Remarkably, we found no evidence for fitness costs. Previously we found that horns do not impair flight performance, and here we found that horns did not stunt the growth of other body structures or weaken the beetles' immune response. Finally, and most importantly, horns did not reduce male survival in the field. Collectively, our results provide strong evidence that the exaggerated horns of T. dichotomus are surprisingly inexpensive. We discuss why weapons may be inherently less costly than ornaments, and suggest that the lack of fitness costs offers a simple, yet unexpected, explanation for why rhinoceros beetle horns are both elaborate and diverse.