Fossil ostracodsGene Hunt / Smithsonian

The oldest penis ever found is 425 million years old, and belongs to an animal whose scientific name—Colymbosathon ecplecticos—means “astounding swimmer with a large penis.” Large is relative, though. The entire creature is just a fifth of an inch long, but for its size, its penis is still “large and stout,” according to its discoverers.

That’s not unusual for the ostracods—the ancient group of crustaceans to which Colymbosathon belongs. From their origins almost half a billion years ago, these animals have diversified into some 70,000 species. At first glance, they look like little seeds. Look closer, and you’ll see what appear to be distorted shrimps, encased in hard, clam-like shells. Male shells tend to be longer than female ones, because they have to accommodate a pair of large penises, and outrageously big sperm that, when uncoiled, can be six times as long as the ostracod itself. In some species, all of this reproductive gear can take up a third of the male’s shell.

Female (top) and male (below) of the ostracod Cypideis salebrosa. The male shell is longer to accommodate his genitals (shaded). (Maria João Fernandes Martins)

This anatomical extravagance is the result of intense sexual selection, where organisms evolve traits that give them an edge in the competition for mates. That competition leads to excessive body parts, like peacock tails or deer antlers. It leads to colorful plumage and flashy courtship displays. In many groups—ostracods, flies, ducks, dolphins—it sculpts genitals and sperm into an extraordinary diversity of shapes and sizes. And in the ostracods, it can lead to oblivion.

By studying dozens of fossilized ostracods, Maria João Fernandes Martins from the Smithsonian Institution and her colleagues have found that species where males are disproportionately bigger than females—and so invest more heavily in sex, and have larger penises—disappear far more quickly. They say that it’s not size that matters, but what you do with it; what ostracods do with it is go extinct.

Many scientists have tried to predict how sexual selection might influence a species’ fate. “Does it make organisms more susceptible to collapse because they are too big or too flashy?” says Patricia Brennan from the University of Massachusetts at Amherst. “Or does it make them better able to overcome challenges because they have better genes, or are somehow better adapted to their environments?”

There are several studies that support both of these conflicting possibilities, showing variously that sexual selection can protect against extinction, lead to it, or neither. But almost all of this research focused on animals that are still alive, and merely estimated their odds of going extinct by looking at population trends, local disappearances, or conservation status. “A lot of these results are ambiguous,” says Fernandes Martins. “When you work on living species, you’re not actually analyzing extinction.”

That’s why she and her team, led by Gene Hunt, decided to study ostracods. Their hard shells can withstand the ravages of time, so there are plenty of ostracod fossils to examine. Different species have distinctive shapes, so it’s easy to chart the rise and fall of particular lineages over time. Males are more elongated than females, so it’s also easy to tell the sexes apart—a rarity among prehistoric creatures. And among males, shell size reflects penis size, so that even when these soft organs have long since decayed, you can estimate their dimensions by looking at the entire animal.

Fernandes Martins did this for 93 ostracod species, which lived between 66 and 84 million years ago in what is now Mississippi. She found that those where males were biggest relative to females went extinct 10 times faster than those whose males were much smaller. On average, species whose males invested most heavily in their sexual organs persisted for just 1.6 million years. By constrast, species whose males stepped away from the sexual rat race persisted for 15.5 million years.

Perhaps that’s because it takes a lot of energy to build large sex organs and long sperm—energy that could go towards other adaptations. “In the short term, it’s beneficial for the individual, since he’ll have more babies,” says Fernandes Martins. “But in the long-term, it can be a problem. If you’re investing in reproduction, you’re not investing in coping with a changing environment.”

“It’s a great study,” says Brennan, “but it’s hard to predict how representative ostracods may be of larger [animals] that reproduce much more slowly and in smaller numbers.” Are elephant seals, whose males can outweigh females by more than four times, doomed to extinction? Is the peacock’s tail a death sentence? “We can’t answer that,” says Fernandes Martins. “It’s just one study.”

“But science, importantly, gets done one study at a time,” says Scott Pitnick from Syracuse University, who would love to see more work in this vein. “There may not be a singular rule. The reality is probably that sexual selection can protect species from extinction or increase extinction risk according to lots of variables that differ across species and environments.”

The ostracods also “highlights how important it is for us to understand sperm evolution if we hope to attain a full understanding of biodiversity,” Pitnick adds. Since Darwin, evolutionary biologists have tried to understand the evolution of extreme traits that seem costly and counterproductive, from heavy horns to conspicuous plumage. Pitnick thinks of giant sperm in the same way—as weapons and ornaments, much like an elk’s antlers or a stag beetle’s horns.

Fernandes Martins adds that female ostracods aren’t passive players. By mating with several males and selecting whose sperm gets to fertilize their eggs, they might be the final arbiters of any sexual competitions. This kind of cryptic choice is widespread in the animal kingdom, and is much harder to study, partly because female anatomy is often much more complicated.

“We know a lot less about the females, but we know that they have a story to tell,” says Fernandes Martins.