14.4 Friday, Jan. 4 Long and strong? Mechanical limits to maximum weapon size in a giant rhinoceros beetle MCCULLOUGH, E.L.*; TOBALSKE, B.W.; EMLEN, D.J.; University of Montana; University of Montana; University of Montana firstname.lastname@example.org
In the Japanese horned beetle (Trypoxylus dichotomus), males have a long, branched head horn that they use to compete for access to females. These horns can reach exaggerated proportions of up to two-thirds the length of the beetle’s body. Sexual selection theory predicts male ornaments and weapons will evolve until the fitness costs outweigh the reproductive benefits of further trait exaggeration. Interestingly, the giant horns of T. dichotomus do not incur substantial fitness costs, so it is unlikely that weapon size is limited by a cost-benefit equilibrium. However, males often damage and sometimes break their horns during intense male-male combats, suggesting that maximum horn size is set by mechanical constraints on horn strength. We tested this hypothesis by measuring the safety factors of horns across the full range of horn sizes. Horn safety factors were calculated as the ratio between the force required to break a beetle’s horn and the force a beetle would have to generate to dislodge a typical size-matched rival. In support of our hypothesis, we found that horn safety factors decreased as horn length increased. Large horns are therefore more likely to break and perform poorly in combat. We suggest that mechanical constraints have played an important role in shaping the evolution of the beetles’ elaborate horn morphologies.