When Penis Size Does Matter
Researchers have found that penis size does matter when it comes to male reproductive success. Using the promiscuous bank vole in a study of social dominance, researchers discovered that voles with a wider penis bone - called a baculum - were socially more dominant and reproductively successful than their contemporary males with a smaller penis bone.
A penis bone - also known as a “baculum” - is present in many species of animals such as gorillas, dogs, cats, raccoons, skunks, rabbits and even our close genetic cousin the chimpanzee. Baculum is Latin for “stick” or “staff.” Reproductively, a penis bone is believed to offer a reproductive advantage for some species where ovulation is spontaneous with genital stimulation as opposed to being a cycling hormonal event as in mankind. Functionally, a penis bone also allows a male to engage in prolonged periods of intercourse.
In a recent study published in the scientific journal Behavioral Ecology and Sociobiology, scientists interested in how the roles of social dominance play toward reproductive success turned to the male bank vole as an animal model for their experiments. Their goal was to find out what exactly contributes to social dominance success. Previous studies had equated social dominance with penis length. The researchers wanted to determine if penis morphology is a reflection of social status in the male vole.
Using a collection of voles from the wild as opposed to inbred laboratory specimens that may no longer possess innate social skills found in nature, pairs of young males were placed into cages with female nesting material. The males were observed and noted for which ones left more scent markings on the nesting material. The male that did the most scent marking was considered to be the more dominant male.
After the dominant males were selected, their penis bones were scanned and the images compared. What the scientists found was that it was not the length of a male vole’s penis bone that correlates with social dominance, but the width of the bone that does. They determined that the dominant male vole has a statistically significant wider penis bone than a non-dominant male.
The researchers concluded that dominant male bank voles may benefit from an enlarged penis bone under sperm competition and/or a cryptic female choice over whose sperm she chooses. They believe that differences in penile morphology according to male social status might be an important source of variation in male reproductive success.
A theory held by one of the researchers is that female bank voles need a certain amount of genital stimulation in order to release eggs and that a wider penis bone may provide this stimulation resulting in a greater reproductive success rate for the male. In some species of animals—such as rabbits—spontaneous ovulation in female occurs just by the mere presence of a nearby male. It may be that ovulation in some species may require a certain level of physical contact before egg release is an “all systems are go” message to the ovaries. A male with a wider penis bone, therefore, would then have a reproductive advantage over a male with a thinner penis bone. The lead author of the study believes that their results may be an important step toward reaching an understanding about the connection between genital structure and male reproductive success.
Although the modern human male does not possess a penis bone, he likely did early in evolution and lost it shortly after man and chimp split from the evolutionary tree of life. It’s possible that the loss of a penis bone could be due to genetic and environmental selective pressures when the female turned to cyclic hormonal egg release and a penis bone became a reproductive disadvantage to possess. Then again, perhaps penile fractures were enough of a selective pressure for man to find another way toward reproductive success.
Source: “Genital morphology linked to social status in the bank vole (Myodes glareolus)”; Behavioral Ecology and Sociobiology http://www.springerlink.com/content/3u8g60t723819034/