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 oe strus and ovulation, suggesting that males try to ensure that their sp ermatozoa will remain capable of effecting fertilization until ovulati on occurs. We tentatively suggest that variation in sperm fertile life span may be related to differences in sperm size. The duration of capa citation is positively related to sperm fertile lifespan, suggesting t hat the time constraints imposed by the female have also influenced th e duration of this process. When females possess long oviducts in rela tion to their body size, males produce relatively high sperm numbers t o counteract the greater losses incurred. Contrary to our prediction, sperm size does not seem to be related to the thickness of the zona pe llucida.