It’s a striking fact of biology that genitalia are often among the most rapidly evolving and diverse aspects of animal morphology, so much so that taxonomists frequently use genitalia as the means of distinguishing closely related species. From the the spiny-headed penis of the seed beetle to the explosively-everting model of male waterfowl, a potentially general explanation to account for this great diversity is that rapid divergence in genital morphology is driven by sexual selection, the force identified by Darwin to explain characteristics that just don’t seem to make sense from the point of view of natural selection for survival. Broadly speaking, if there’s something about a male’s genitalia that makes him more successful in siring offspring, and that something is inherited, it will be sexually selected – genes for the “something” will tend to spread in the population. The hard part is measuring success in a meaningful way, and identifying what the “something” is.
In a new study led by Paula Stockley and conducted together with several other former colleagues at the University of Liverpool, UK, I investigated whether male genital morphology influences reproductive success in wild house mice. We found that one trait in particular – the width of the penis bone (yes, mice and many other mammals have a bone in their penis, called a baculum) – is a significant predictor of how many offspring a male will sire under natural conditions of competition with other males. This constitutes some of the first evidence for vertebrates that genital morphology really is linked to evolutionary fitness in the way the sexual selection hypothesis predicts, and is published today in the journal BMC Biology.
Update Sept. 2013: You wait years for experimental evidence that the baculum evolves under sexual selection, and then two studies come along at once. Leigh Simmons and Renee Firman at the University of Western Australia have just published a highly complementary study comparing baculum morphology across natural and experimentally evolved populations of house mice, drawing very similar conclusions. Their article was published online last month in Evolution. Together, the two studies provide compelling evidence for the influence of sexual selection on mammalian genitalia.
Stockley P, Ramm SA, Sherborne AL, Thom MDF, Paterson S & Hurst JL. Baculum morphology predicts reproductive success of male house mice under sexual selection. BMC Biology 2013, 11:66, doi: 10.1186/1741-7007-11-66
Image: a 56-cm long walrus baculum (via Wikipedia).