Studies of sperm competition in two species of muroid rodents

Summary

• 1. In three experiments on the effects of mating order, wild-type male deer mice, Peromyscus maniculatus bairdi were tested in competition with brown recessive, wide-band agouti, and blonde mutant males. Females of the homozygous recessive genotype received two ejaculatory series from a male of each genotype; mating order was varied. There were no significant effects of mating order on litter composition. In a parallel experiment wild-type and albino male prairie voles, Microtus ochrogaster, mated for one ejaculation each with albino females. There was a significant advantage with respect to litter composition for the male ejaculating last. Together with data from other species, these results suggest that effects of mating order are highly species-specific and must be evaluated for each species before speculations are made regarding the effects of multiple matings.

• 2. Males of various genotypes differed with respect to differential fertilizing capacity. Wild-type male deer mice outcompeted both brown recessive and wideband agouti males. The combinations of blonde and wild-type deer mice and albino and wild-type prairie voles yielded strain differences in litter composition in one mating order but not in the other. Counts of the numbers of sperm from males of different genotypes in the uteri of females revealed no sigificant strain differences. Thus, strain differences are not simply a function of sperm numbers. 3. Female deer mice mated or exposed to more than one male had a lower probability of pregnancy than those mated for comparable numbers of ejaculations with one male in previous studies. Additional data on pregnancy initiation in females of the brown-recessive genotype mated with one male revealed them to be only slightly less likely to become pregnant than wild-type females. Strain of male had little effect. 4. No significant effects on litter composition were attributable to the length of the delay between matings by successive males. This was established both via correlational analyses of the data from the mating order studies and in an additional experiment in deer mice in which delays of 2 h were imposed between the last ejaculation by the first male and introduction of a second male. 5. Litter composition in deer mice reflects the number of ejaculations attained by each of two males. In one experiment, female deer mice mated first with a blonde male and then with a wild-type male. The blonde male delivered three ejaculations and the wild-type male one ejaculation in one condition and the wild-type male three ejaculations and the blonde male one in the other. Litter composition was significantly affected.