Evolution loves penises. How else do you explain all the crazy penis shapes out there? Por ejemplo (slightly nsfw): the corkscrew duck penis, the spiny cat and chimp members, the sci-fi-esque spiked penis of a seed beetle, the 4-headed penis of the echidna, and, well, all of these.
Why so many ways to make a sperm delivery system?
The shape, size, and WTF-ness of penises in part depends on the mating system it’s a part of. For example, Muscovy ducks have crazy long, corkscrew-shaped penises, possibly because they’re part of a very competitive mating system. Males and females form tight pair-bonds, but this doesn’t stop rival males from trying to get in on the gene-spreading action. Males will often forcibly copulate with females, and the longer and twistier their penis, the more likely they’ll be able to get their sperm closer to the eggs.
But female ducks evolved countermeasures. Their genital tracts became twisted, too (in the opposite direction) to make it more difficult for a male to force his way in. And on it goes.
Other animals can get away with having penises that physically harm the female. Examples include bed bugs and seed beetles. It actually works out in the male’s favor to induce harm, because the female will have less lifespan to mate with other males. There’s no pressure from the female side to select for less knife-like dongs, because they end up having a crapload of babies anyway.
Basically, there are a zillion different ways of having sex and making babies. And there’s a special penis shape for each and every one.
Lock and key
Okay, so evolution can push penis shapes to crazy extremes–and it does so ridiculously quickly–but of course there must be limits. The key still has to fit the lock, so to speak. So, as penis shapes evolve in one direction, lady parts follow, as long as these new shapes are beneficial. But not every member of a species will mate with every other member of the species. Especially if there are pockets of this species that are isolated from one another.
What would happen if two populations, that were isolated for a long time, came together and, um, didn’t fit anymore? Could this be the driving force for making two species out of one? Researchers Janine Wojcieszek and Leigh Simmons at Murdoch University in Australia used millipedes to answer this question.
Millipedes basically all look the same. To me, anyway, and as it turns out, even to the people who study them. Related species of millipedes can generally only be told apart by the shape of their genitals. What better system to study the link between speciation and genital shape?
At this point, I have to point out that the genitalia in question here is not exactly a penis. Millipede males have specialized “legs” that act as their genitals, called gonopods. At the beginning of mating, the male will charge his gonopod with sperm. The gonopod then becomes the thing that is inserted into the female during mating.
The researchers chose one particular species of millipede to study, Antichiropus variabilis, which lives in Western Australia. By looking at the DNA of different populations of A. variabilis, it’s obvious that the populations have been isolated from each other for quite a while.
Their gonopods are also different, but the differences aren’t as big as you might expect given the DNA changes. The researchers interpreted this to mean that the lock-and-key constraint (it has to fit) was keeping evolution from running wild with their pseudo-penises. On the other hand, the differences between populations are greater than the differences between males of the same population. So, each population is settling on one gonopod shape and getting rid of variation. If you want a technical term for it, genital shape is under stabilizing selection in this species.
Okay, so they’ve been isolated for a while and their genitals look a little bit different. But can they still get it on, if given the chance? Or are their genitals pushing them to become separate species?
The experiment: try to mate males to females from a different population and see if they can still mate.
To make things simple, the researchers focused on three populations for this study: Serpentine National Park (S), Boonanarring Nature Reserve in Gingin (G), and Mersea Forest, North of Manjimup (M). They collected males from all three populations, but took females only from the S population. Females got to mate first to a male from their own population (S). After at least a day, the females then mated to a male from either the G or M population.
Did isolation turn these male millipedes’ gonopods into something unrecognizable as a tool for love?
Not for the G males, but the M males were out of luck. Not only did they spend less time having sex, but when they succeeded in doing so, they didn’t always seem quite able to keep things in place. About 40% of the males had to withdraw and insert their gonopods more than once. The authors note one particular male who tried 6 times to get it in there and then, after only 8 seconds total, walked away from the female. It just wasn’t meant to be. On the other hand, S and G males never inserted their gonopods more than once with a S female.
Even more interesting, but less measurable, the authors note that males from the divergent (M) population “exhibited mechanical difficulty in genital insertion.” That must have been fun times in the lab.
The reason for all this “mechanical difficulty”? It appears that the M males have evolved a more extreme shape to their gonopod. The image below is from Figure 3 of the paper (linked back to the full text of the article):
The saddest part of this for our heroes, the M males, is that they also fathered fewer offspring with the S ladies. Even though the second male to mate usually has a very large advantage, M males had a large disadvantage when it came to paternity.
All of this points to the M population being well on its way to forming a new species distinct from the S and G populations. So the study did confirm that changes in genital shape can be an important factor in speciation. But the interesting thing is that the changes don’t have to be because of any strong evolutionary force selecting for one shape over another. In this case, the species started out with a lot of variation in gonopod shape. Then, populations became isolated and one or another of the shapes that existed in the mix was randomly chosen to become the norm. Not for any reason, other than things work out best for everyone if there’s one shape for boy parts and one shape for girl parts. The M population just happened to settle on a different shape than S. With, I’m assuming, hilarious results.
Wojcieszek JM, & Simmons LW (2013). Divergence in genital morphology may contribute to mechanical reproductive isolation in a millipede. Ecology and evolution, 3 (2), 334-43 PMID: 23467632