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

There are 2 models of male adjustment of sperm investment inthe ejaculate in relation to sperm competition. The "risk model"predicts that as "risk" of sperm competition increases, sperminvestment also increases. This prediction has been supportedin many species, including mammals. The "intensity model" involvesthe number of competing males copulating with the same femaleand predicts that males will allocate the highest sperm investmentat low sperm competition intensity (SCI) and then decreasingsperm investments as SCI increases. Two alternative outcomesare that sperm investment is unaffected by SCI and that sperminvestment increases as SCI increases. There are studies supportingall 3 possible outcomes in relation to SCI but no data on mammals.The present paper presents the first study of SCI in a mammalspecies, the meadow vole, Microtus pennsylvanicus. We used odorsof conspecific males to simulate low and high intensities ofsperm competition. We found that males allocate the highestsperm investment at low SCI and decrease significantly theirsperm investment at high SCI. We also found that males allocatethe lowest sperm investment at low sperm competition risk (SCR)and the highest sperm investment at high SCR. All these resultsagree with current theoretical models of sperm competition.