Sexual selection and horn allometry in the dung beetle Euoniticelus intermedius

Secondary sexual characters are often positively allometric, such that larger individuals of a species have proportionally larger traits. The sexually selected weapons found on many insect species, however, show declining allometric slopes with body size. Previous work on these curved allometric relations has focused on the role of competition for limited resources between growing body parts within pupae, but there is an alternative explanation that has received little attention: changes in the allometric slope may reflect changes in the fitness benefits associated with increases in horn size and body size change as an animal gets larger. The allometric curve of the horn of the male dung beetle Euoniticellus intermedius shows one of the largest declines in slope known and by setting up contests between males of similar, or different, sizes over access to females we were able to investigate the importance of horn and body size for males of different sizes and horn lengths. Both body size and horn size were important in determining contest outcome in small males, but horn size became more important as body size increased, so that in contests between large males it was by far the most important predictor of victory. These findings are consistent with theoretical predictions of how fitness functions can affect allometry and offer an alterna- tive to some of the previous verbal arguments made to explain positive allometry.