The effect of genital stimulation on competitive fertilization success in house mice

In species with internal fertilization, copulation is characterized by the mechanical interaction of male and female genitalia. Genital interaction during copulation stimulates the neuroendocrine system, promoting the physiological changes necessary for the initiation of pregnancy. Previous studies on mice have shown that the baculum (penis bone) influences male reproductive success under a competitive fertilization scenario and that males perform less mating behaviour and ejaculate sooner under an elevated risk of sperm competition. Here we explore how mating behaviour and baculum morphology interact to influence a male's competitive fertilization success. We selected male and female house mice, Mus musculus domesticus, from families with breeding values at the extreme of baculum shape (narrow/wide) and recorded polyandrous matings to quantify mating behaviour. Our analyses revealed that competitive fertilization success was dependent on the relative baculum shape of competing males and the amount of mating behaviour that each male delivered. Our data thereby provide support for the stimulatory hypothesis for the evolution of the mammalian baculum.