The story of seed-beetle sex has often been told in a very particular way, with the male in the evolutionary driver’s seat, his hapless mate taken along for a grudging ride. A quick glance at the insect’s penis makes it easy to see why: The appendage is tipped with hundreds of sharp, hard spines that give it the appearance of an elaborate mace. This terrifying surfeit of spikes riddles the female’s reproductive tract with punctures and scrapes that can leave, as the biologist Göran Arnqvist puts it, some “quite massive scars.”
Prickly pricks are great for the male, who tends to father more offspring when his spines are particularly long. But the female, who usually endures penetrations from multiple mates, can sustain damage so extensive that it curbs her egg-laying abilities, or ushers in a tragic early death.
To guard against these dangers, females have evolved their own defensive arsenal, including an amped-up immune system, super-speedy wound-healing, and an ultra-thick reproductive tract, a quite literal girding of the loins. Across populations of Callosobruchus maculatus seed beetles, female tract thickness seems to have ratcheted up in lockstep with the length of penile spines—a sort of genital arms race, what researchers refer to as sexually antagonistic coevolution. It seems like a classic tale of harmful action and palliative reaction: “Males evolve something that is good for males, but is bad for females, so females adapt to that,” Arnqvist told me. The male thrusts; the female parries; the species, as a whole, trudges on.