Let me just say at the outset that this post is about the baculum. Some of you may not want to read about the bone found in the penises of many mammal species. I want to give you a chance to head off for tamer blogs. But you might want to stick around. There’s some real science below–and some evolution in action.

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Last week in the New York Times I wrote about the evolution of monogamy (here and here). The occasion for the articles were two new studies in which scientists analyzed hundreds or thousands of species of mammals, tracing the evolution of monogamy and other social arrangements. This big-picture approach to evolution can yield some important insights, but the finer details are hard to make out.

If you look at us humans (monogamy, polygamy, and assorted other stuff) versus chimpanzees (monogamy is for losers!), you’re only looking at the tips of two deep branches. Chimpanzees may be our closest living relatives, but our common ancestor with them lived about seven million years ago. After the two lineages split from that ancestor, they’ve been evolving in different directions ever since. We can make some inferences about what that evolution was like based on ourselves, chimpanzees, and other living mammal species. But this kind of research doesn’t give us a visceral sense of how the sexual habits of mammals evolve from generation to generation.

That’s why it was so interesting to come across a new study from Leigh W. Simmons and Renée Firman at the University of Western Australia. They’ve been able to observe sexual evolution of mammals unfold in their laboratory. Last week’s studies were like a satellite view of the continents. Here, we’re down on the ground.

How males and females live with one another depends on the conditions in which their species lives. If a single male can mate with lots of females, for example, he will end up with a lot of offspring. But if the females are spread out too far, he may not be able to guard them all from other males who want to mate with them too. In such cases, natural selection may favor males that prefer to stick with just one female.

View Images A seed beetle phallus. Photo by Johanna Ronn

In species where males compete with each other a lot for females, evolution may produce new pieces of anatomy. Some males evolve extravagant horns to fight off rivals. Even their genital anatomy can change. This is likely to happen when females mate with many males. The males fight against each other even during sex. Some male insects, for example, using spiny genitals to scrub out their competitors’ sperm.

Evolutionary biologists hypothesized that these extravagant sexual organs were the result of an evolutionary race between males. They found support for this idea when they compared individual males to each other. It turned out that the males that had the most offspring tended to have the spiniest penises.

More recently, researchers have started to watch these organs evolve. In 2011, for example, Swiss scientists reported a study they carried out on seed beetles, which have spines on their genitals for scrubbing away rival sperm.

The Swiss scientists isolated each male with just one female. In that arrangement, the males had no competition for mates. The researchers then let the beetles mate for 21 generations. The spines on the male’s genitals got measurably smaller. That’s just what scientists had predicted based on evolutionary theory. Without any competition from other males, there was no advantage to spiny penises.

View Images The bacula of rice rats and voles. From Stockley 2012 http://dx.doi.org/10.1016/j.cub.2012.11.001

Simmons has documented a lot of the evidence for this evolution of male genitals in insects, and now he and Firman have turned their attention to mammals. To be more precise, they’ve turned their attention to the baculum–a bone that they call “one of the most puzzling enigmas of mammal morphology.”

The baculum is long in some species and stubby in others; it can be straight or hooked, barbed or shaped like Neptune’s trident. In a few species, like our own, it’s just missing altogether.

Scientists have developed several possible explanations for its existence. Some have suggested that by making the penis rigid, the baculum lets a male deliver more sperm into a female. Those extra sperm may outnumber those of rival males. Others have suggested that the baculum helps the sperm travel further towards an egg. Still others have proposed that it stimulates the female, triggering ovulation.

All three hypotheses have something in common: the baculum evolves thank to its ability to translate mating into fathering. In June, some British researchers published a study that supported that idea. They studied house mice, a species in which females mate with many males each time they’re ovulating. The scientists found that male mice with a wider baculum had more mouse pups than other males.

Simmons and Firman took this research to the next logical step. They reasoned that this difference between male mice should drive the evolution of the baculum. To find out, they ran an experiment similar to the one run by the Swiss scientists. They created two groups of mice: promiscuous maters and monogamous maters.

The promiscuous females got to mate with three males in each cycle. The monogamous ones only got to mate with one. They bred the mice for 27 generations and then took a look at their bacula. As with the seed beetles, the baculum evolved. It became thicker in the promiscuous group and thinner in the monogamous one. For the first time, scientists had observed the baculum evolving.

The experiment still doesn’t solve the mystery of what the baculum for, but Simmons and Firman do have an idea about that–at least for mice. They think that the baculum helps male mice stimulate the female reproductive tract. That stimulation may make it more likely that the male fertilizes the female’s eggs, or raises the odds that a fertilized egg successfully implants itself in her uterus.

If the baculum is indeed driven by sexual selection in mammals, the question naturally arises: where’s ours? “Why human males lack a baculum remains enigmatic,” Paula Stackley, a biologist at the University of Liverpool, wrote last December in Current Biology.

Among primates, monogamous species tend to have much smaller bacula than species where males compete for mating. So it wouldn’t be crazy to assume that the shift towards monogamy in our ancestors made the human baculum disappear altogether (except for a very, very few scary cases).