Male meadow voles respond differently to risk and intensity of sperm competition

There are 2 models of male adjustment of sperm investment in the ejaculate in relation to sperm competition. The risk model predicts that as risk of sperm competition increases, sperm investment also increases. This prediction has been supported in many species, including mammals. The intensity model involves the number of competing males copulating with the same female and predicts that males will allocate the highest sperm investment at low sperm competition intensity (SCI) and then decreasing sperm investments as SCI increases. Two alternative outcomes are that sperm investment is unaffected by SCI and that sperm investment increases as SCI increases. There are studies supporting all 3 possible outcomes in relation to SCI but no data on mammals. The present paper presents the first study of SCI in a mammal species, the meadow vole, Microtus pennsylvanicus. We used odors of conspecific males to simulate low and high intensities of sperm competition. We found that males allocate the highest sperm investment at low SCI and decrease significantly their sperm investment at high SCI. We also found that males allocate the lowest sperm investment at low sperm competition risk (SCR) and the highest sperm investment at high SCR. All these results agree with current theoretical models of sperm competition.