Sperm competition games played by dimorphic male beetles: fertilization gains with equal mating access

Alternative mating tactics can generate asymmetry in the sperm competition risk between males within species. Theory predicts that adaptations to sper m competition should arise in males facing the greater risk. This predictio n is met in the dung beetle Onthophagus binodis where minor males which sne ak copulations had a greater expenditure on the ejaculate. In its congener Onthophagus taurus there is a reduced asymmetry in sperm competition risk s uch that both tactics have equal ejaculate expenditure. We used the irradia ted male technique to test,whether adaptations to sperm competition in mino r males result ill higher paternity. We found that for both species, on ave rage, each of two males gained equal numbers of fertilizations, confirming thr assumption that sperm compete in a raffle. There were no differences in the sperm competition success of major and minor males in O. taurus as pre dicted from their equal expenditure on their ejaculate. Contrary to expecta tions, there were also no differences in fertilization success between the male tactics in O. binodis. Thus, in O. binodis minor males must expend mor e on their ejaculate in order to obtain the same fertilization gains as maj or males.