Why Seed Beetles Are Caught in a “Sexual Arms Race”

The strange spiky penis of the cowpea beetle seems to drive the evolution of both male and female beetles

Beetle Penis 2
This is cowpea seed beetle penis, not a weapon from Game of Thrones Uppsala University

Sex in the animal kingdom is not all chocolate and roses.

Many species engage in what’s known as traumatic mating, in which the act of procreation is painful or even damaging. One species in particular is known for its less than happy foreplay. As Marlowe Hood at Agence France Presse reports, males cowpea seed beetles have evolved elaborate and imposing spiked penises that pierce their partners' reproductive tracts while they mate.

A team of researchers set out to figure out why. “The extraordinary genitalia and reproductive traits of males of this insect have spurred our interests for some time,” Göran Arnqvist, professor in animal ecology at the University of Uppsala in Sweden tells Ryan F. Mandelbaum at Gizmodo. “[We wanted to know] why males have such nasty genitalia and how females cope with that.”

Sex between the cowpea beetles is truly brutal. Once the beetles emerge from the beans where they spend their larval stage, reports Chris Simms at New Scientist, there's no dawdling—they're on the hunt for mates. But once the male beetles locates a female, he doesn’t dance or flash pretty colors; he just jumps aboard and struggles with the kicking female as he inserts his penis, Liam Doughety of the University of Western Australia tells Simms.

To find out how the females deal, the researchers monitored the scarring caused by copulation in female beetles in 13 independent populations collected from places including Benin, Brazil, California, Nigeria, South India and Yemen. The changes in the male and female genitals were then tracked over a decade.

What they found is that there was a correlation between scarring in the female's reproductive tract lining and length of the penis spikes as well as the thickness of the female beetles' tract lining. Because of this, the males and females seemed to be locked in what Doughety, lead author of the study in The Proceedings of the Royal Society Bcalls a "sexual arms race." 

The researchers believe that the harm the males cause force the females to evolve, growing thicker vaginal linings. “Because it’s so harmful, you get rapid changes,” Dougherty tells Simms. “And females that are less harmed have more offspring.”

As Hood reports, the finding seems almost counterintuitive to most evolutionary strategies. Why would a species develop a mating technique that injures the future reproductive success of the female, and might even kill her?

The spikes, however, may serve a purpose. Patty Brennan, a biologist at Mount Holyoke College, tells Simms that like snakes, the barbs may help the males hang on to the females while mating, increasing the chances of reproductive success. 

But the spikes also seem to inject chemicals into the females. "This may influence her behavior," Dougherty tells Simms. "Like in fruit flies, it might influence females to lay more eggs."

The method of mating, though brutal, does up the odds for successful beetle fertilization. “[T]he female well-being is sacrificed at the expense of male fitness," Dougherty tells Hood. “Traumatic mating has evolved because it increases male fertilization success.”

The team found that evolutionary changes happened not in just one population of beetles, but across several. “Seeing this in one population would have been cool,” Brennan tells Simms. “But showing it across lots of populations is very cool—that’s the scale at which evolution works.”

Seed beetles are not alone in traumatic mating. Hood points out that bed bug males use a hypodermic needle penises to simply pierce the female abdomen. And there are plenty of examples of sexual cannibalism including spiders and preying mantises. Even ducks have pretty violent sex. While the traumatic mating in seed beetles at least makes some sense, Dougherty tells Hood that overall, it's not yet clear how sexual conflict drives evolution in the rest of the animal kingdom.

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