Sperm competition is particularly fierce in those species where the female promiscuity level is high, like monkeys, bats or rodents (rats and mice).

A male wants to be sure that he is the offspring's father, so males will develop methods of overcompeting the sperm of their rivals, so that their sperm fertilizes the ovules.

Scientists have already found bizarre adaptations of the rodent's sperm to ensure the success in sperm competition, like their hooked shape that allows them to form speedy sperm "trains".

Now, researchers discovered that having a longer penis gives males an advantage in the sperm competition.

Dr. Steve Ramm, an evolutionary biologist at the University of Liverpool, UK, made a comparison between the relative size of the penis bone in several mammal groups known to present various levels of promiscuity. "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", said Ramm.

Further research will explain how the differences in penis length benefit the rodent males. The relatively longest penis bone to the body size was encountered in the Western harvest mouse, Reithrodontomys megalotis. "Everything's relative of course, 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", explained Ramm.

Genitalia research offers many clues about the evolution of a species: they evolve much more rapidly than other organs, as they do not affect survival (so, selection upon them is slighter) and many times they are the only method of establishing two closely related species. In fact, in some groups, like insects, the shape of the genitalia impedes crossing between closely related species.