Coevolution between male ejaculates and female reproductive biology in eutherian mammals.

Among eutherian mammals, sperm competition has favoured the evolution of high sperm numbers and long spermatozoa, but it cannot account for all the variability observed. In this paper we argue that differences in female reproductive biology may also have influenced the evolution of sperm numbers as well as several other sperm traits. Sperm fertile lifespan is positively related to the interval between the onset of oestrus and ovulation, suggesting that males try to ensure that their spermatozoa will remain capable of effecting fertilization until ovulation occurs. We tentatively suggest that variation in sperm fertile lifespan may be related to differences in sperm size. The duration of capacitation is positively related to sperm fertile lifespan, suggesting that the time constraints imposed by the female have also influenced the duration of this process. When females possess long oviducts in relation to their body size, males produce relatively high sperm numbers to counteract the greater losses incurred. Contrary to our prediction, sperm size does not seem to be related to the thickness of the zona pellucida.