Costs of elaborate weapons in a rhinoceros beetle: how difficult is it to fly with a big horn?

The giant horns of rhinoceros beetles exemplify the extreme morphologies that can result from sexual selection. Ornaments and weapons help males obtain mates but may also impose fitness costs. Intuitively, exaggerated sexually selected traits should impair locomotion, yet compensatory morphologies often make it difficult to detect locomotor costs. Here, we tested whether horns of the rhinoceros beetle Trypoxylus dichotomus impair flight and whether males compensate by developing larger wings or flight muscles. Contrary to our expectation that horns are costly for flight, males flew as fast as females, and among males, horn length was not correlated with flight speed or distance flown. We found some evidence for compensations in the male flight apparatus; males had relatively larger wings and flight muscles than females, and males with long horns for their body size had larger wings than males with relatively short horns. Flight muscle mass, however, was unaffected by horn length. We conclude that T. dichotomus horns may have been costly in the past and led to morphological compensations in wing and flight muscle size, but they do not currently impose significant flight costs. Fitness costs are a central tenet of sexual selection theory, and the large horns of rhinoceros beetles are expected to impose particularly strong costs on locomotion. Given our finding that T. dichotomus horns are surprisingly easy to carry, future work will be needed to identify the potential costs that have been important in shaping the evolution of elaborate horn morphologies.