Ever wondered what constitutes extreme sex? Cowpea seed beetles certainly know ā 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 malesā vicious spiked penisesĀ (see picture, below).
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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 ā procreation. They donāt eat, or drink, they just look for partners.
āThereās not much courtship,ā says Dougherty. āThe smaller male jumps on the female and thereās a bit of a struggle. He sort of leans back when heās fully in. Then thereās a period when the female starts kicking the male. Then they break apart after a few minutes.ā
Sexual healing
To peek into whatās 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.
āThere isnāt a relationship between harm and the female tract until you invoke the male spike length,ā Dougherty says. This is evidence that the spikes and tract-thickening are evolving in response to each other, he says.
āSeeing this in one population would have been cool,ā says , a biologist at Mount Holyoke College in Massachusetts. āBut showing it across lots of populations is very cool ā thatās the scale at which evolution works.ā
āItās probably the harm caused by males that is causing the females to evolve. Because itās so harmful, you get rapid changes,ā says Dougherty. āAnd females that are less harmed have more offspring.ā
Controlled by sperm
So why do the males even have the spikes? āSpikes are force concentrators,ā says at the University of Massachusetts, who studies similar behaviour in vertebrates.
āThey perform multiple functions,ā says Brennan. āAs barbs to help the males hold on to the female, just as in snakes.ā
That could help the pair stick together, increasing the chance sperm will be transmitted. āMales that have longer spines have greater fertilisation success,ā 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. āWhen males pierce the female reproductive tract, they release chemicals directly into the femaleās body,ā says Dougherty. āThis may influence her behaviour. Like in fruit flies, it might influence females to lay more eggs.ā
āThickening the tissue isnāt the only way females could be defending themselves,ā says Kelly. āThey could change the hydration level of the chitin in the tissue to increase toughness. They could also make the tissue more folded.ā
Despite the race, there are no winners, because defeating the opposite sex would be somewhat counterproductive. āThey survive in equilibrium,ā says Brennan.
Proceedings of the Royal Society B
Read more: Longer āpenisā drives evolution of bigger brains in female fish
