What is the Role of the Baculum in Reproduction?

In many carnivores, rodents, bats, primates, and some insectivores a boney rod, or baculum, occurs in the penis. The baculum can be very large, as in the walrus where it reaches 21 inches (54 centimeters) in length in some adult males. Several hypotheses have been advanced to explain the evolution of elongated bacula. The baculum might be important to stimulate the female and to induce the luteinizing hormone surge required for ovulation. Alternatively, the baculum might assist the male to attain intromission or to prolong intromission in those species which mate for extended periods.

The San Diego Zoo's CRES assembled the largest data set yet achieved to examine some of these questions. Baculum lengths and adult body weights for adult males of 315 species of carnivores, bats, and primates were analyzed to control for effects of body weight and phylogeny. It was found that intromisison duration showed a highly significant correlation with baculum length for those species where mating durations are known. It is unlikely that baculum length is correlated with the occurrence of induced ovulation in mammals, however. Thus, elongated bacula occur in spontaneous ovulators such as bushbabies, lorises, and stump-tailed macaques. Conversely, the baculum is small in various members of the cat family, despite the occurrence of induced ovulation in this group. The functions of the baculum during intromission may include strengthening of the penis and protection of the urethra during extended intromissions or facilitation of sperm transport within the female's reproductive tract. Nor are bacula essential for maintenance of intromission in all mammals; thus the baculum is absent in marsupials, many of which mate for extended periods.