Male mating behavior and ejaculate expenditure under sperm competition risk in the eastern mosquitofish

Theory predicts that males should tailor the size of their ejaculates according to temporal changes in the risk of sperm competition. Specifically, males are predicted to allocate more sperm to each mating event with increasing risk (i.e., the probability that the sperm from two males will compete for fertilization). We tested this hypothesis by using the eastern mosquitofish, a freshwater species of fish exhibiting a coercive mating system and internal fertilization. We manipulated the perception of sperm competition risk by adjusting the sex ratio under which males were maintained over 8 days. Males were housed either with three females and one male (simulating high sperm competition risk) or with four females (low risk). After the treatment, we presented each test male individually to an unfamiliar male-deprived female for 30 minutes and observed his mating behavior. We then artificially stripped the test males of sperm and recovered the ejaculates from the females. Our results revealed that males in the high-risk group performed higher levels of mating activity and sperm expenditure (i.e., used up more of their sperm reserves) than did low-risk males. A control experiment, in which test males were treated but did not participate in the mating trials, revealed no significant difference in the number of sperm stripped from high- and low-risk males, indicating that sperm production was not affected by the treatment. We did not detect a difference in the number of sperm retrieved from females among the groups, raising the possibility that some sperm are lost during mating activity, either through ejaculation with incomplete or interrupted penetration, or via female ejection. Copyright 2003.