Bitterling as models for studies of sperm competition

Game theory models of sperm competition predict that sperm expenditure should depend on the intensity of sperm competition. Specifically, there should be an increase in sperm expenditure when a male has one competitor, but a decrease with additional competitors. European bitterling, Rhodeus sericeus , are good models for testing this theory because sperm release by males is readily observable and territorial males regularly compete with variable numbers of sneakers. Females spawn in living freshwater mussels (Unionidae) and males release their sperm over the inhalent siphon both before and after egg laying. Our experiments confirmed that both territorial and sneaker males adjusted their sperm expenditure to the number of participating males, following the pattern predicted by game theory models. The pattern, however, was not always upheld: males increased their aggression level after a female had spawned and no longer adjusted their sperm expenditure to the number of sneaker males. Thus, adjusted sperm expenditure and mate guarding were two alternative responses to increased risk of sperm competition. Males adopted the tactic of sperm adjustment, when mate guarding and aggression against sneakers were costly in decreasing female willingness to spawn.