Evolution, sex, and spiky penises

C. maculatus penis. Image via Wikipedia.

Male seed beetles (Callosobruchus maculatus) have long spikes covering their penises (or adeagus, if you want to be scientific about it). These spikes are thought to have evolved in response to female promiscuity, as a way of increasing the male’s chances of fertilizing a female’s eggs. Females, in response to the spikes, have evolved every man’s worst nightmare: spikes inside her vagina. Talk about a rocky relationship.

This is what evolutionary biologists call sexual conflict. Males and females of a species have a common goal: to pass on their genes to the next generation. Sometimes, however, males and females have conflicting best strategies for getting that done. Females may benefit by spreading out their relatively small supply of eggs across multiple males. Males, on the other hand, tend to benefit most when they mate with many females, and each female uses only his sperm to fertilize her eggs.

The spikes on seed beetle penises may have evolved from this kind of sexual conflict. To test this hypothesis, researchers Luis Catyetano, Alexei Maklalov, Robert Brooks and Russell Bonduriansky forced seed beetles to be monogamous for many generations. They then looked to see whether the male’s penises became less spiky, or if the the female’s vaginal spikes changed at all. They found that forced monogamy did reduce male spikes, but not female ones. The research was published in the current issue of Evolution.

Seed beetles mating. Ouch! Photo by F. de Crespigny.

The penis of the male seed beetle punctures the female’s reproductive tract and, eventually, all those injuries will kill her. Females even have to kick the male constantly during mating to lessen the severity of the injuries–but she still won’t be deterred from hooking up again.

Why would male beetles want to harm females like this?

The current best guess is that the harm is just a side effect. The spikes appear to act as an “anchor”, keeping the female from getting away before he’s finished. The fact that they also puncture her and cause her permanent harm is just collateral damage.

So what about this idea that the male evolved the spikes in response to females sleeping around? The authors of the study found that larger males had bigger spikes on their penises–bigger than would be expected if spike size and body size were linearly correlated. These large males had more potential to harm their mates. Under forced monogamy, the authors expected large males in particular to evolve smaller spikes.

Why? Well, there’s no longer any reason for those spines. They aren’t in competition with any other males, so it’s not like the female is going to stop mating and hook up with Mr. Hot Beetle on the next seed over. Perhaps more importantly, any harm the male inflicts on the female will now harm him to the same degree. His reproduction is now directly tied to hers, and only hers. So it’s in the male’s best interest to make sure she is healthy enough to pump out lots and lots of eggs.

The experimental evolution in this paper was a success. After only 18-21 generations, the authors saw changes in the spike length, bringing the ratio of body size to spike length closer to 1. This wasn’t enough time to see changes in female genital spikes. Since these probably evolved in response to male spikes in the first place, you would expect a lag period for her to respond to shorter male spike length.

But while we’re on the topic of sharp penises that harm females, I thought I would throw in a little bed-bug action as well. Bed-bugs mate using what is known as “traumatic insemination”. Males bypass the female genitals entirely and pierce them in a specialized location on their bellies. They then ejaculate into the female sperm storage organ directly.

A: Bed-bug penis. B: The female “sweet spot” where the male penetrates her. Figure from Stutt and Siva-Jothy.

As you might imagine, this has some costs for the female. Females don’t fight back–they basically let any male mate with them that comes along. And males? Well, in one experiment, “males moved through the culture copulating with any engorged adult females they encountered.” Do the females benefit at all from letting all those males pierce their exoskeleton? Apparently not. Females that only mated twice in an experimental setting had just as many offspring as females that were continuously mated, as they would be in nature (i.e.: your bed). But those females who were constantly being mated died much sooner than the females that only did it twice.

Seems like males have the advantage in this species. Maybe there just isn’t enough selective pressure for females to evolve defenses–after all, they’re still having lots and lots of babies whether they die sooner rather than later. And, honestly, I don’t feel so bad for them. Bed-bugs suck.

If you want to be freaked out, check out this video by Isabella Rossellini where she explains how bed-bugs do it, in her own unique way.


Cayetano, L., Maklakov, A., Brooks, R., & Bonduriansky, R. (2011). EVOLUTION OF MALE AND FEMALE GENITALIA FOLLOWING RELEASE FROM SEXUAL SELECTION Evolution, 65 (8), 2171-2183 DOI: 10.1111/j.1558-5646.2011.01309.x

Stutt, A. (2001). Traumatic insemination and sexual conflict in the bed bug Cimexlectularius Proceedings of the National Academy of Sciences, 98 (10), 5683-5687 DOI: 10.1073/pnas.101440698

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6 thoughts on “Evolution, sex, and spiky penises

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