The girth of a mouse penis bone depends on the stiffness of the competition. That’s what Leigh Simmons and Renée Firman at the University of Western Australia found after several generations of experimental evolution in mice. Over the course of the experiment, male mice developed thicker penis bones (or bacula, if you want to be scientific about it) when females were allowed to mate with multiple males. The study was published in the January issue of the journal Evolution.
Bacula are common in mammals, but missing in humans and a few other groups of mammals (also missing in humans, thankfully: penile spines). From Wikipedia (because it’s impossible to put it better than this): “The bone aids sexual intercourse by maintaining sufficient stiffness during sexual penetration.” Fun fact: species with bacula also have bones in the clitoris!
[Aside: There have been scattered reports of ossification (bone formation) in human penises, including the report of a 5 year old boy born with a baculum that had to be removed. Alas, that study is from 1965 and not available online, but this paper has a lovely summary of known cases of penile ossification.]
Male genitalia are a diverse, some might say beautiful, array of natural experiments by evolution (for some examples, see previous blog posts here and here and here). The prevailing wisdom is that this diversity is the result of sexual selection, the idea that forces such as female choosiness or the level of competition for females drives the evolution of sexual characteristics. But so far that hasn’t been much evidence that sexual selection causes differences in penises outside of insects (though see this article on how baculum size correlates with reproductive success in mice).
There has, however, been a lot of research on the baculum (perhaps out of penis bone envy?), and scientists have known for some time that there is a huge range of variation in baculum size and shape between species. To get an idea of just how much variation there is, check out this post at So Much Science. But there is also variation within species, and that’s the crucial ingredient for experimental evolution.
Experimental evolution is evolution in hyper drive. Scientists pick a trait that has some variation between individuals of the same species, forcefully apply a selective force, and see how the trait evolves over the course of several generations. For this experiment, Simmons and Firman first verified that there was sufficient natural variation in baculum morphology (size and/or shape) to make the experiment worthwhile. They collected 20 male and 20 female house mice from islands off the coast of Western Australia. These mice experience different levels of male-male competition, based on tests of multiple paternity in litters of mice. Mice from the island with the highest level of male-male competition had smaller, but thicker, bacula.
To test whether this could be a direct result of sexual selection, the researchers imposed strong sexual selection on mice that had previously been raised together under the control of their own libidos. Not anymore. Four groups were forced into monogamy—each female only had access to 1 male. This was the “low” sexual selection treatment. Another four groups were put under a high level of sexual selection: each female was mated to 3 males in a row during a single estrous cycle (4-6 days). This was continued for 27 mouse generations, or a little less than the length of 1 PhD.
After 27 generations, the variation in baculum length disappeared, but the variation in thickness increased. Mice under high sexual selection evolved thicker penis bones compared to those under low sexual selection. So the authors demonstrated that sexual selection can (and maybe did) drive differences in a male genital trait. Another trait, length, wasn’t affected by sexual selection, so the variation they saw in their 3 original natural populations must have been due to something else. Or it could just be random.
The scientists still don’t know why male mice with more competition need thicker penis bones. So I will leave that to you to speculate.
Simmons LW, & Firman RC (2014). Experimental evidence for the evolution of the Mammalian baculum by sexual selection. Evolution; international journal of organic evolution, 68 (1), 276-83 PMID: 24372607