Horn size predicts physical performance in the beetle Euoniticellus intermedius (Coleoptera: Scarabaeidae)

1. In many animals, the size of secondary sexual ornaments is known to be related to the probability of victory in fights between males, and hence to fighting ability. However, few studies have attempted to link fighting ability to any physical performance measures. 2. Here we show that horn size in the dung beetle Euoniticellus intermedius accurately predicts two types of whole-organism performance, independent of body size, that are likely to play an important role in male contests: the force required to pull a beetle out of a tunnel, and the distance the beetle was able to run before exhaustion (maximum exertion). 3. Body length is also a statistically significant predictor of force, but not of exertion, suggesting that horn size is a more reliable predictor of performance than body size alone, a result that is consistent with a previous finding that horn size becomes more important in determining victory in male-male contests as body size increases. 4. This study is the first to establish direct links between whole-organism performance abilities, male armaments and fighting ability in the same species. Our findings suggest that physiological performance capacities are important factors underlying the evolution of signal expression in E. intermedius, and should be considered in future studies of the evolution of animal signalling.