Presence of rivals reduces mating probability but does not affect ejaculate size in the freshwater crayfish Austropotamobius italicus.

Sperm competition models predict that males should adjust their sperm expenditure according to the risk and intensity of sperm competition, increasing it when competing with at least one other ejaculate and decreasing it when the number of competing ejaculates rises above two. However, when the raffle to fertilize eggs is strongly loaded due to sperm removal by subsequent rivals, it could be hypothesized that first-mating males progressively decrease their sperm expenditure and/or defer copulation with an increasing number of nearby competitors. Here, by varying experimentally the number of competitors around a mating pair, we analysed mating behaviour of both sexes and sperm expenditure in relation to variation in the immediate risk of sperm competition in the freshwater crayfish Austropotamobius italicus, a species where males remove all or part and feed on sperm laid by previous mating males. We found that male mating behaviour varied significantly with increasing number of rivals, because interactions between focal males and competitors, as well as male refusals to copulate, increased with number of the latter. As a consequence, the probability to reach an effective copulation decreased with increasing number of competitors. However, males released similar amounts of sperm independently of the number of surrounding competitors. Thus, the observed lack of variation in sperm expenditure in relation to the number of surrounding rivals suggests that freshwater crayfish males are unable to adjust the size of their ejaculates to short-term variation in the immediate risk of sperm competition.