Sperm competition in deer mice (Peromyscus maniculatus bairdi)

Summary Six experiments were conducted to determine the effects of mode of estrus (cycling versus postpartum) and intervals between matings on sperm competition in deer mice. When there was no lengthy interval between matings by different males, sperm competition matings by different males, sperm competition with females in postpartum estrus was the same as that with females in cyclin estrus (i.e., no male order effect). Imposition of 4 h delays between males did not engender a mating advantage when females were in postpartum estrus. When 4 h intervals were imposed with females in cycling estrus and tests were begun relatively early in the dark phase, there was a secondmale advantage with respect to the number of pups sired. When minimal delays were imposed with females in cycling estrus the time of testing within the dark phase did not affect the pattern of sperm competition, but larger litters were born following matings later in the dark phase. When 4 h delays were imposed with the first male mating late in the dark phase a first-male advantage resulted. It is hypothesized that for females in cycling estrus there is a period of peak fertilizability. There is no straight-forward effect of delays between males per se; rather, effects must be analyzed in relation to the period of peak fertilizability. The timing of insemination appears less critical for postpartum females.