SPERM COMPETITION IN MAMMALS - A COMPARATIVE-STUDY OF MALE ROLES AND RELATIVE INVESTMENT IN SPERM PRODUCTION

1. Sperm competition (i.e. competition between sperm from different males to fertilize the ova of a female) is widespread among mammals. 2. Recent ESS models of sperm competition suggest that it may sometimes pay for males within a population to invest differentially in ejaculates. These models lead to the prediction that, in mammals, small males within a population should invest proportionately more in sperm production than large males, where breeding is continuous. 3. We test this prediction by comparing intraspecific relationships between body size and testis size in seasonal and continuous breeders. The results of our comparative analyses support the predictions derived from the ESS models.