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.