Multi-male mating, probability of conception, and litter size in the prairie vole (Microtus ochrogaster)

We conducted a mating experiment in the laboratory using prairie voles, Microtus ochrogaster, to document that multi-male mating (MMM) can occur in this supposedly monogamous species and to test two hypotheses for the advantages of MMM in female mammals. The two hypotheses are that MMM (1) increases the probability of pregnancy and (2) increases litter size. We also tested the hypothesis that multiple copulations, rather than multiple partners, increases litter size and/or probability of pregnancy. Females were given a choice of mating with any of three males, each of which was tethered in a separate compartment. The mate choice bouts were recorded on videotape. We recorded the number of copulations and number of males with which females mated over a 24 h period. Litter size and probability of pregnancy were not significantly different for females that mated with one, two or three males. Increasing numbers of copulations, independent from the number of males, also did not increase litter size but did significantly increase the probability of pregnancy. MMM, at least in prairie voles, must serve some function other than increasing litter size and probability of conception.