As body parts go, animal genitalia are particularly diverse – so diverse, in fact, that there’s a specific term for it: ‘genitalic extravagance’. One example of this is the baculum, or penis bone, found in the males of some mammalian species (humans don’t have one, though many other primates do), and which comes in a wild array of shapes. While the reasons for this diversity have been presumed to be related to the ‘evolutionary arms race’ of sexual selection – with males constantly competing with one another for an advantage in mating with females – the existing evidence in support of this is indirect and somewhat inconclusive, coming mostly from comparative evolutionary studies that compare features of the baculum between species with other proxy traits for sexual success.
In a recent article published in BMC Biology, Paula Stockley from the University of Liverpool, UK, and colleagues describe their experiments to measure the effects of sexual selection more directly by breeding mouse populations in an approximation of wild conditions and tracking the parentage of any new mice born through typing of microsatellites and other genetic markers. Counting the number of offspring from each father in this way allowed them to link the fathers’ traits to their success in reproducing – the ultimate measure of evolutionary fitness.
There are a large number of factors that determine mating success, many of them already known. Males that are large, or that are socially dominant, for example, will tend to father more offspring. The authors control for these other factors through their statistical analysis, and the results show that the width of the baculum – but not the length – correlates with breeding success. The study doesn’t tell us why the two are linked, though possibilities include increased stimulation of the female encouraging pregnancy, or dislodging of the sperm of other males from the females’ reproductive tract.
Written by Kester Jarvis, Senior Editor for BMC Biology.