As a graduate student, everyone wants to be a dogma-buster. You’re always looking for the thing that makes no sense,” says Scott Pitnick. For him, that thing was giant sperm.

In 1995, Pitnick realized that males of the tiny fruit fly Drosophila bifurca produce gargantuan sperm cells that are 6 centimeters or 2.4 inches long. That’s 20 times longer than the fly itself, and a thousand times longer than the average human sperm. If a man produced sperm that big, it would stretch diagonally across a basketball court.

The megasperm seemed to contradict what we knew about how evolution governs the reproductive traits of animals. Sperm are meant to be small cells that male animals can produce in large numbers with little effort. They should be tiny, disposable, and numerous. So why did these particular flies evolve sperm that break all the rules.

Pitnick, who has a tattoo of a sperm cell coiling up his right arm, has spent the last 21 years trying to answer that question. For a start, he has come to see the megasperm not just as instruments of reproduction, but as sexual ornaments like a peacock’s tail or a deer’s antlers. “Biologists don’t think about these things as weapons or ornaments, but they absolutely are,” he says. “They meet all the right criteria.”

They’re obviously huge. What’s more, they get disproportionately huge relative to their owner’s size. An antelope’s horns will increase in size twice as fast as its body does, and a peacock’s tail will increase three times as fast. But the sperm of Pitnick’s flies grow 5.5 times faster than their bodies. They are, he claims, the most exaggerated ornaments in all of nature. They make peacock tails look understated. They make stag beetle horns seem modest. They take extremeness to an extreme.

Such exaggerated ornaments take a lot of energy to make, and only the fittest males can afford to carry them. By acting as visible, reliable badges of quality, these ornaments allow females to choose the best mates and they give males an edge over competitors.

But unlike tails and horns, giant sperm aren’t visible. Females can’t use them to select mates. That’s irrelevant, says Pitnick, because male competition doesn’t end with flashy courtship, butting heads, or even sex. It also continues inside female bodies. If females mate with many males, their sperm can compete inside her body for fertilization rights. This sperm competition fuels the evolution of numerous, fast-moving sperm, as well as with more absurd adaptations like toxic sperm, barbed sperm, traumatically delivered sperm, and cooperatively swimming sperm. And, perhaps, giant sperm.

But treating giant sperm as ornaments doesn’t solve the mystery of their origins. If anything, it just confuses matters further. Here’s why.

Typically, sperm are cheap to make and males do so in bulk, while eggs are expensive to make and females do so sparingly. So, as Pitnick says, “There’s just a lot more sperm than eggs out there.” This imbalance means that females are more successful if they’re selective about their partners, while males do best if they can mate with as many females as possible. So, females tend to be the choosier sex, and males the more competitive one. And the greater the ratio of sperm to eggs, the more intense male competition becomes.

It’s this intense competition that fuels the evolution of big, exaggerated male ornaments. It takes a lot of energy to produce and maintain these disproportionate structures, so their size advertises their owner’s health and quality. The stiffer the competition between males, the more elaborate these traits become.

In this context, the megasperm of fruit flies make no sense. It takes more energy to make bigger sperm, so species with giant sperm tend to make very few of them. In other words, their sperm are behaving like eggs—luxury cells produced at low volumes, rather than cheap ones that are mass-manufactured. In these situations, with few sperm competing for each egg, male competition should be very weak. But since the giant sperm are extreme ornaments, they must have evolved in the context of extreme competition. That’s what Pitnick calls the “big sperm paradox”: having few giant sperm implies both intense competition and weak competition.

Having set up the paradox, he now offers a resolution. It involves the crucial element that’s been largely absent from the discussion so far: the females.

Mathias Appel

When fruit flies mate, the male ejaculates his giant sperm as a tangled ball of yarn, which somehow unravel as they enter an equally long female organ called the seminal receptacle. The female stores them there until she is ready to fertilize her eggs. “In D. bifurca, the organ is nearly 8 centimeters long, and the sperm are essentially straightened out within it,” says Pitnick. “We don’t know how this happens, but it’s got to be cool.”

What’s clearer is that these storage organs favor males with longer sperm. The biggest cells are better at ousting out the shorter cells of rival males, while being themselves harder to displace. So females, simply by having labyrinthine reproductive tracts, inadvertently bias themselves towards males with longer sperm. You could say that they have a ‘preference’—it’s not an active one, but it’s just as real and potent as, say, a peahen eyeing up males with big tails.

Pitnick demonstrated this in 2002. Now, through intensive breeding experiments, he has shown that the length of a female’s storage organ is genetically linked to that of the male’s sperm. One cannot change without automatically changing the other. The same is true for other important traits, like mating frequency. As sperm become bigger in size and fewer in number, females get very few sperm per copulation, and so mate more often.

“Those genetic links between these traits are an important part of what’s driven this crazy evolution,” says Pitnick. It means that the male and female traits can runaway with each other, with increases in one driving increases in the other until both are absurdly exaggerated. Through simple, cumulative steps, nature dishes up the ludicrous.

But it’s not just the size of sperm that matters, but their number. By studying seven fruit fly species with sperm of varying lengths, Pitnick found that the bigger the sperm get, the more their numbers reflect the health of their owners. “In fly species with the most extreme sperm length, where females are re-mating like gangbusters, only the males of the best quality can produce enough sperm,” he explains. “Only the males with the best genes can capitalize on all this female mating.” And by mating with such males, females pass said good genes on to their offspring, increasing their own success.  

To recap, the big sperm paradox turns out not to be a paradox at all. Males have bigger sperm because their females have longer seminal receptacles and mate more frequently. That creates an intense sperm competition that can only be won by the fittest males, which have enough energy to produce more giant sperm. They may not mass-produce these cells, but even small improvements in their capacity can give them a big advantage. Or, to quote Monty Python, “Every sperm is sacred, every sperm is great.”

“This is a very complex story and an amazing amount of work” says Marlene Zuk from the University of Minnesota. “I think it helps us get out of the old, tired stereotype—sperm are cheap, eggs are expensive, that's why sexual jealousy occurs and men like going to strip clubs—that pervades some popular treatments of the evolution of sex differences.”

“These findings are important because they demonstrate that competitive reproductive success is not just about producing more or better sperm, but producing the best sperm for a particular environment,” adds Heidi Fisher from the University of Maryland.

That environment, of course, is the female’s body. Time and again, we see that questions about male genitals are really questions about female ones. In another study of diving beetles, Pitnick’s team found that all kinds of sperm traits, from head shape to cooperative swimming, were dictated by the structure of the female’s reproductive tract. And the now-infamous story about ballistic corkscrewing duck penises is really a story about twisting duck vaginas.

And yet, “it turns out that almost nobody still understands very much about female reproductive tracts,” says Pitnick. “They’re very rapidly evolving, they’re the selective environment of sperm, and they’re driving sperm form. But although sperm have been described for tens of thousands of species, we know almost nothing about what they’re doing inside females.”

There are various reasons for that, as I’ve written about before. Obviously, cavities are harder to study than tubes. But female genitals are more than passive locks; they also change shape, secrete hormones, and do things far more complicated than any penis.

There’s also the matter of longstanding gender stereotypes. Tellingly, in the most cited studies on sexual conflict, authors use active words like ‘intimidation’ and ‘coercion’ to describe males, but passive words like ‘resistance’ and ‘avoidance’ to describe females. Males have ‘adaptations’ and females have ‘counter-adaptations’. The implicit—and wrong—message is that males act, but females react.

“To me, the female reproductive tract is the greatest unexplored frontier for sexual selection research,” says Pitnick.