"The data for rodents seem pretty clear cut," said study author Steve Ramm, an evolutionary biologist at the University of Liverpool, UK. "Species where sexual competition between males is most intense also tend to have the longest penises."
Female rodents are very promiscuous, therefore sexual competition is fierce between males trying to fertilize eggs, he said.
"Interestingly, a similar pattern was not detected in either primates or bats," Ramm said.
Ramm said he is unsure why the rodents benefit from having a longer penis.
Ramm found the Western harvest mouse (Reithrodontomys megalotis) had the lengthiest penis bone relative to its body mass of all rodents examined.
"Everything's relative, of course," Ramm said. "So although big for its body size, the penis bone in R. megalotis is still only 7 to 8 millimeters (0.27 to 0.31 inches) long. I don't think the phrase 'hung like a harvest mouse' will be catching on any time soon."
The study is detailed in the March issue of the journal American Naturalist.
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