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Ever wondered what constitutes extreme sex? Cowpea seed beetles certainly know – their sexual act is brutal, and it also seems to encourage a rapid evolutionary arms race between spiked penises and shielding tissue in females.

Extreme genital co-evolution happens in many types of animal, including ducks, fish and fruit flies. For example, female ducks of some species have evolved long and complex internal genitalia, complete with blind alleys, to thwart the unwanted advances of males intent on fowl play. And to counteract this, male ducks have evolved longer and more elaborate penises.

Now of the University of Western Australia and his colleagues have discovered how female cowpea seed beetles (Callosobruchus maculatus) are fighting back against the males’ vicious spiked penises (see picture, below).

Once beetles become sexually mature after emerging from the beans in which they live as larvae, they have only one thing on their to-do list – procreation. They don’t eat, or drink, they just look for partners.

“There’s not much courtship,” says Dougherty. “The smaller male jumps on the female and there’s a bit of a struggle. He sort of leans back when he’s fully in. Then there’s a period when the female starts kicking the male. Then they break apart after a few minutes.”

Sexual healing

To peek into what’s happening beneath the shells, Dougherty and his colleagues looked at female sex-related damage in 13 independently bred populations of the beetles. At first, they saw no link between scarring and the thickness of female reproductive tracts. But when male spike length was taken into account, it revealed that scarring was greatest when linings were thin and penis spikes were long.

“There isn’t a relationship between harm and the female tract until you invoke the male spike length,” Dougherty says. This is evidence that the spikes and tract-thickening are evolving in response to each other, he says.

“Seeing this in one population would have been cool,” says , a biologist at Mount Holyoke College in Massachusetts. “But showing it across lots of populations is very cool – that’s the scale at which evolution works.”

“It’s probably the harm caused by males that is causing the females to evolve. Because it’s so harmful, you get rapid changes,” says Dougherty. “And females that are less harmed have more offspring.”

Controlled by sperm

So why do the males even have the spikes? “Spikes are force concentrators,” says at the University of Massachusetts, who studies similar behaviour in vertebrates.

“They perform multiple functions,” says Brennan. “As barbs to help the males hold on to the female, just as in snakes.”

That could help the pair stick together, increasing the chance sperm will be transmitted. “Males that have longer spines have greater fertilisation success,” says Dougherty.

It could also damage the female enough that she will die shortly after laying eggs he has fertilised, meaning she will produce his young only, says Kelly.

There could be another reason, too. “When males pierce the female reproductive tract, they release chemicals directly into the female’s body,” says Dougherty. “This may influence her behaviour. Like in fruit flies, it might influence females to lay more eggs.”

“Thickening the tissue isn’t the only way females could be defending themselves,” says Kelly. “They could change the hydration level of the chitin in the tissue to increase toughness. They could also make the tissue more folded.”

Despite the race, there are no winners, because defeating the opposite sex would be somewhat counterproductive. “They survive in equilibrium,” says Brennan.

Read more: Longer ‘penis’ drives evolution of bigger brains in female fish