In the fall, migrating monarch butterflies are in reproductive diapause when they arrive at winter sites in California. Approximately 1/3 of the overwintering females contain sperm of summer males within their spermatheca. When these females were subjected to conditions that terminated diapause, they were able to produce offspring with near equal fecundity throughout the overwintering period. The mated females also benefited by the added male nutrients to produce eggs when their body energy reserves were low. Spring matings of virgin females are necessary to obtain spermatozoa to fertilize their eggs and possibly to receive male nutrients to supplement their energy reserves. Several possible survival advantages of early mating are: (1) previously mated females are better able to withstand the rapid ovarian development towards the end of the overwintering season than virgin females; (2) females need not mate again to insure the fecundity of her eggs; and (3) mating with previous summer males broadens the genetic plasticity of the species.