The Pros and Cons of Spiky Genitals

Seed-beetle sex seems like a classic conflict between males and females, but new research gives it a twist.

a male and female seed beetle having sex
Julian Baur

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.

But Arnqvist, who has been studying the beetles for many years at Uppsala University, in Sweden, thinks it’s time for a reframing. He and his colleagues have been working to quantify how the female, too, can gain from these encounters, reaping rewards from even the most hellishly ornamented males. Her evolution, they argue in a new paper, is motivated not just by an avoidance of the injuries the male inflicts, but an attraction to the traits that a well-endowed suitor might offer. “I think we’ve seen this much too simply in the past,” said Arnqvist, who published the findings today in Proceedings of the Royal Society B.

Read: This common butterfly has an extraordinary sex life.

To study the myriad effects of penile spines on females, Arnqvist’s team had to manipulate evolution in real time. Genetics, they knew, played a big role in length. So they carefully bred two groups of beetles, segregating out males with extra-long or extra-short spikes until the populations’ privates were visibly distinct, then coupled them up with mates.

Females who bumped uglies with males from the long-spined lineage certainly sustained some internal damage—that’s inevitable, Arnqvist said. But their sons were gifted with grander, gougier spikes, which, by proxy, helped the females’ genes carry on. Their daughters, too, inherited a wealth of valuable traits: They had bodies that ballooned to bigger sizes and laid more eggs before they died.

a seed beetle penis
Johanna Rönn

The notion of seeking out high-quality fathers to sire high-quality kids isn’t new, of course. But the study from Arnqvist’s team adds a layer of complexity to our understanding of the evolutionary carrots and sticks faced by female beetles, who must juggle the immediate cost of genital wounding with the eventual benefit of producing an excellent brood. “You’re getting this push and pull in different directions,” Ainsley Seago, an entomologist at the Carnegie Museum of Natural History, told me. The two competing pressures operate on different timescales, but the perks are built to last, says Yoko Matsumura, a zoologist at the University of Kiel, in Germany, who wasn’t involved in the study: By partnering her genes with those of a high-quality male, the female helps her own legacy live on.

There may even be some more instantaneous positives for the females who couple up with superlatively spiked males. Arnqvist’s team found that females who mated with males from the long-spined lineage laid more eggs over their lifetime than those that had hooked up only with the stubby-spined. Males with naturally longer phallic accoutrements are thought to gain better access to the female’s hemolymph—the insect equivalent of blood, flowing outside her reproductive tract—where their seminal fluids can prime her body for reproduction. They might also, thanks to their genes, produce higher-quality ejaculate—top-shelf beetle juice. This rich cocktail can be up to 8 percent of the insect’s full weight (the equivalent of all the blood in the human body, by proportion), and teems with hundreds of ingredients, many of which are thought to enhance egg production, or supply the female’s body with much-needed nutrients. (After seed beetles mature into adults, they stop eating and focus their efforts solely on sex.)

The female seed beetle’s story still isn’t really a happy one. But maybe these findings cast her fate in a less dreadful light, says Blake Wyber, an evolutionary biologist at the University of Western Australia, who wasn’t involved in the study. The findings from Arnqvist and his team, Wyber notes, also help put the female’s perspective center stage—something widely lacking in research on animal mating. Wyber and his colleagues are attempting to do the same with their own population of seed beetles, using new techniques to measure how females’ reproductive tracts are changing over time, perhaps even within single individuals. “It’s not just, Woe is them,” Wyber told me. “They’re able to get benefits from this situation; they’re not just being used.” And that’s “totally what you would expect,” says Patricia Brennan, an evolutionary biologist at Mount Holyoke College, in Massachusetts, who wasn’t involved in the study. Without perks that play out in both sexes, she told me, “how would the system be maintained?”

Arnqvist and others think that these evolutionary themes will echo in other instances of sexual antagonism as well—among bed bugs, perhaps, whose males use their saber-like phalluses to stab female abdomens and ejaculate into the wound, or ducks, for whom sex involves a ballistic corkscrew penis that explosively everts from the male. Brennan’s research has centered on the latter example, and she’s shown how females can force males to jump through their own genital hoops. Lady ducks have cooked up extraordinarily labyrinthine vaginas, complete with twists, turns, and blind alleys to thwart unwanted phallic advances. (Matsumura is studying another type of beetle whose female genitalia are also coquettishly coiled.) Males, in turn, have scrambled to lengthen and elaborate upon their penises, in hopes of keeping pace.

Even the female seed beetle has impressive strategies to ensure her own success. She’ll sprint away from males when she’s not in the mood (all the more resolutely, Arnqvist told me, after she’s had her first sexual experience), and will kick impatiently at little lotharios who try to get fresh. She is also the master of her own violent sexual destiny: Her reproductive tract is armed with tiny rows of teeth that appear to puncture the packages of sperm and seminal fluid deposited by the penis, abetting the gushing forth of goodies. On the twisty, turny road that has led to jabby seed-beetle sex, females, too, have decidedly taken the wheel.