Why did the seed beetle's penis evolve to look like this?

One of the evolutionary consequences of sexual reproduction is that males have to compete with each other for females. But as a recent study published in Current Biology reveals, this competition doesn't necessarily stop after the mating is done. In the case of seed beetles, the presence of promiscuous females has caused a completely different kind of competition: a penis best suited for fertilization. For the seed beetle, it would seem, size does truly matter.

The rapid and ongoing evolution of penis morphology isn't exclusive to beetles, of course. Males of many species continue to evolve genitals that have to do a better job than the competition's. As a result, there is an ongoing genital arms race among males as different configurations emerge. Interestingly, female genital traits tend to remain relatively unchanged over the course of a species' evolutionary history.

Why did the seed beetle's penis evolve to look like this?

To illustrate this effect, researchers from Uppsala University in Sweden and the University of Cincinnati conducted a series of experiments on seed beetles to see if they could demonstrate the force behind the rapid evolution of male genitals. They chose the seed beetle for two primary reasons, namely because females tend to mate with several males (thus heightening the need for competition among males), and because males have evolved a rather bizarre and freakish looking penis that's worthy of further study.

Beetle penis breeding

What makes the seed beetle penis unique is that it is lined with a series of spines. Michal Polak and his colleagues have suspected that the length of these spines confer a reproductive advantage — but they wanted to prove this in the lab. To do so, they decided to run a rather interesting experiment: They bred two different kinds of seed beetles: those with extraordinarily long genital spines, and those with extremely small spines.

Sure enough, the longer spined beetles were able to produce substantially more offspring.

But the scientists admitted that their beetle penis breeding program may have introduced a secondary characteristic that may have affected fertility. So, to rule out this possibility they brought out the lasers.

To prove that it was the spines and not something else, they sliced down the size of the long spines by using a micro-scale laser. Subsequent experiments once again demonstrated a definite correlation between the length of the spines and reproductive success.

So why the long spines?

Once the scientists were sure that the spines were linked to sexual success, they needed to come up with an answer as to why. One thing they observed was that the spines produced physical injures among the females during mating, and that some of the seminal fluid subsequently trickled into their bloodstream (they confirmed this by tracking the seminal fluid through high-resolution radioisotope labels). And in fact, the longer the spine, the more quickly the seminal fluid entered into the female's circulatory system.

But why would this be so important to reproduction given that the semen couldn't possibly fertilize the female through the bloodstream?

The answer, it would seem, may have to do with what's in the semen. Specifically, the scientists speculate that the seminal fluid contains bio-active molecules that play a vital role once they're in the female's circulatory system. It's possible, for example, that the molecules are making the females more responsive to mating, or that they might stimulate her reproductive tract during copulation (which could facilitate female uptake and the use of more of her current mate's sperm).

The scientists aren't entirely sure, but they're planning on looking into the proteins and peptides that make up these bio-markers to find out. But it's safe to say that the seed beetle semen is doing much more than just delivering sperm.

You can read the entire study at Current Biology.

Images: N.Kaval and Michal Polak.