On the evolution of the mammalian baculum: vaginal friction, prolonged intromission or induced ovulation?

The function of the mammalian baculum is a puzzling enigma in mammalian morphology. Three hypotheses for the evolution and persistence of the mammalian baculum have been proposed: the vaginal friction hypothesis; the prolonged intromission hypothesis; and the induced ovulation hypothesis. We tested these three hypotheses using phylogenetically corrected methods and data on North American carnivores. Baculum length was independent of sexual dimorphism and duration of copulation, thus refuting the vaginal friction and prolonged intromission hypotheses, respectively. Also, baculum length did not differ between induced ovulators and simultaneous ovulators, thus also refuting the induced ovulation hypothesis. We suggest that other aspects of carnivore life history, such as mating systems, may help explain the evolution of the mammalian baculum.