Sperm competition games played by dimorphic male beetles

Reproductive strategies often consist of two alternative tactics whereby ma les either compete for and guard females, or sneak copulations. By their na ture, alternative tactics expose males to differing risks of sperm competit ion; sneaks will always be subject to sperm competition but guards will be subject to sperm competition with low probability, dependent on the number of sneaks. Recent game-theoretical models predict that males in the sneak r ole should have the greater gametic expenditure but that the disparity in e xpenditure should decrease with increasing numbers of sneaks. Male dung bee tles in the genus Onthophagus can be separated into two morphs: major males have horns and guard females whereas minor males are hornless and sneak co pulations. Here we compare testis size and ejaculate characteristics betwee n these alternative morphs. We find that in O. binodis 30% of males are sne aks, and sneaks have larger testes, ejaculate volumes, and longer sperm tha n guards. In O. taurus 60% of males are sneaks and there are no differences in gametic traits. Our data thus provide empirical support for game-theore tical models of sperm competition.