Sperm competition effects on sperm production and expenditure in sailfin mollies, Poecilia latipinna

Sperm competition risk (SCR) models predict that if there is a low probability a male's ejaculate will compete with another male, individuals should invest less sperm in a mating, whereas if the probability of competition is high, males should invest more sperm. Alternatively, models of sperm competition intensity (SCI) predict that increased intensity of sperm competition leads to maximal sperm investment when a male faces a single competitor. Few studies have examined predictions of these models for males of varying size, and none have examined effects of sperm competition on both sperm production and expenditure. To examine effects of sperm competition on these variables, male sailfin mollies (Poecilia latipinna) from 3 size classes were randomly assigned to one of 3 treatments that manipulated SCR and SCI: 1) no risk, low intensity; 2) risk present, medium intensity; and 3) risk present, high intensity. Male sailfin mollies produced more sperm in the high-intensity treatment than in the low-intensity treatment. Sailfin mollies in the risk present treatments expended more sperm than those males in the risk absent treatment. There was no significant difference in sperm expenditure between the medium intensity and the high-intensity treatment, indicating that maximal sperm investment does not occur when the number of competing males is one. Furthermore, small males expended more sperm than medium and large males. These results suggest that male sailfin mollies respond as predicted to SCR, but not to SCI. I suggest that male size effects and sperm production should be considered in theoretical treatments of optimal male sperm investment strategies.