Hung Like a Harvest Mouse? (more rodent sex )

Scientist says, "Everything is relative."

Promiscuity is common among female rodents, leading to competition between the sperm of rival males over who fertilizes the eggs. It now seems that possessing a longer penis may give males an advantage in this competition, according to new research to be published in the March issue of The American Naturalist.
Dr. Steve Ramm, an evolutionary biologist at the University of Liverpool, UK, compared the relative size of the penis bone in several mammal groups: "The data for rodents seem pretty clear cut. Species where sexual competition between males is most intense also tend to have the longest penises. But, interestingly, a similar pattern was not detected in either primates or bats."

Understanding the reasons for these differences will require a better understanding of the precise mechanisms through which male rodents benefit from longer penises, something which comparative data alone cannot address.

Overall, the rodent with the longest penis bone relative to its body size in the study was the Western harvest mouse, Reithrodontomys megalotis. "Everything's relative of course," explains Dr. Ramm, "so although big for its body size the penis bone in R. megalotis is still only 7 to 8 mm long. I don’t think the phrase 'hung like a harvest mouse' will be catching on any time soon."[read on...]