![]() ![]() In many systems, the size or intensity of sexual traits and displays does indeed reflect an equilibrium between mating benefits and survival costs. ![]() A central tenet of sexual selection theory is that sexual traits are costly, and that these traits will evolve to be increasingly exaggerated until naturally selected survival costs balance the reproductive benefits of further trait elaboration. Sexual selection drives the evolution of many of Nature's most conspicuous morphologies and displays. Structural failure of oversized horns may therefore oppose the continued exaggeration of horn length driven by male–male competition and set a mechanical limit on the maximum size of rhinoceros beetle horns. I found that safety factors decrease with increasing horn length, indicating that the risk of breakage is indeed highest for the longest horns. ![]() Safety factors were calculated as the ratio between the force required to break a horn and the maximum force exerted on a horn during a typical fight. Here, I tested this mechanical limit hypothesis by measuring safety factors across the full range of horn sizes. However, males sometimes fight vigorously enough to break their horns, so mechanical limits may set an upper bound on horn size. It is therefore unlikely that horn size is limited by the theoretical cost–benefit equilibrium. In Trypoxylus dichotomus, long horns confer a competitive advantage to males, yet previous studies have found that they do not incur survival costs. Theory predicts that sexual traits will evolve to be increasingly exaggerated until survival costs balance the reproductive benefits of further trait elaboration. The horns of giant rhinoceros beetles are a classic example of the elaborate morphologies that can result from sexual selection. ![]()
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