Patterns of sperm allocation across successive ejaculates in four species of voles (Microtus)

This study was designed to determine testes masses, total number of spermatozoa ejaculated per copulatory episode, and the pattern of sperm numbers in successive ejaculates in prairie voles (Microtus ochrogaster), montane voles (M. montanus), pine voles (M. pinetorum), and meadow voles (M. pennsylvanicus). Prairie voles displayed mean totals of 2.7 ejaculations and 30.5 X 10(6) spermatozoa before reaching a satiety criterion; montane voles 3.4 ejaculations and 19.0 X 10(6) spermatozoa, pine voles 2.4 ejaculations and 3.3 X 10(6) spermatozoa, and meadow voles 2.5 ejaculations and 25.5 X 10(6) spermatozoa. In all species the number of spermatozoa decreased in successive ejaculates. Significant species differences were noted for the total number of spermatozoa ejaculated and number of spermatozoa ejaculated in each of the first 3 ejaculates. Species differences also were noted for testes mass, with meadow voles having the largest testes and pine voles having the smallest. These data can be compared to similar data on laboratory rats and deer mice and related to recent theory regarding sperm numbers, testes sizes, and mating systems. In general, the species with large testes appear to ejaculate more spermatozoa. The significance of species differences in testes mass and total sperm numbers remains unclear, but may relate to the occurrence of multiple mating by females during a single receptive period.