All sperm perform the same basic job: They fertilize egg cells. But in a new study, researchers have figured out that size matters, and it's largely the female that pushes sperm to be big or small.
Sperm cells come in a huge variety of sizes. For instance, the parasitoid wasp Cotesia congregata produces little swimmers that are less than one-thousandth of a centimeter long, while fruit flies make sperm with 2.3-inch (6 cm) tails that coil tightly to fit inside their tiny bodies.
In the new study, the researchers set out to determine how sperm size varies among species and what might be driving the differences.
"We have all these studies that show evidence of natural selection pushing sperm size in various species to be either bigger or smaller, but we wanted to take more of a zoomed-out view and look for trends across species," said lead author Ariel Kahrl, a postdoctoral researcher in evolutionary biology at Stockholm University.
Related content: Sexy swimmers: 7 facts about sperm
Kahrl and her colleagues examined data from 3,200 species and discovered a governing principle that determines sperm size in a species: Females with small reproductive tracts drive the production of bigger sperm, and the need to spread sperm far and wide shrinks sperm across evolutionary timescales.
Here's why. For the most part, animals use two modes of sexual reproduction. One group — which includes mammals, insects and birds — are internal fertilizers that carry eggs inside their bodies. External fertilizers, by contrast, eject their eggs into the environment and hope for the best. Commonly, these species live in water, like fish and sea urchins. In both modes, tons of sperm are competing in a battle royal for the prize of fertilizing the egg, but the challenges of each mode exert incredible evolutionary pressure on sperm size.
"We found that external fertilizers tend to have really small sperm because they have to make a ton of it to reach the eggs," Kahrl said. External fertilization requires ejecting a cloud of sperm, typically into water. As the sperm spread, they become diluted, so the best strategy would be to produce as many sperm as possible to maximize the chance that at least one will reach an egg. Because an animal has a limited amount of energy to use for making sperm he can't afford to make them any bigger than they absolutely have to be.
It's a completely different situation for the internal fertilizers. "We think that for internal fertilizers, the female's reproductive tract influences the way sperm fight each other," said study co-author John Fitzpatrick, an assistant professor of biology who is also at Stockholm University. In internal fertilization, the sperm work in a tight space, so reproduction becomes less of a treasure hunt and more like a game of king of the hill. In this situation, bigger may be better for shoving other sperm out of the way, regardless of whether they came from the same father or different potential fathers.
"Some of these species make huge sperm, and if you're making enormous sperm, you don't make that many of them," Kahrl said. "These males coil up their sperm like a ball of yarn and pass it along."
In addition to internal and external fertilizers, the researchers examined a rarer third reproductive mode, called spermcasting. Spermcasting is like a combination of internal and external fertilization; for example, a river mussel might eject sperm into a stream, and that sperm would ride the currents until it is picked up by a stationary, filter-feeding female.
"With spermcasting, you have this dilution effect because the sperm are ejected into the water, but when the sperm enter the female, they evolve rapidly under the same types of pressures that we see in internal fertilizers," Fitzpatrick told Live Science. The spermcasters, though, have smaller swimmers, similar in size to the sperm of external fertilizers, likely because ejecting sperm into water incentivizes making more of them, forcing them to be small. But once those sperm are taken up by the female, the biggest sperm tend to win.
Despite being internal fertilizers, humans don't have monster sperm. Instead, human sperm measures a modest 0.002 inches (0.005 cm) long, well within the range seen in external fertilizers. That's because animals with bigger bodies have reproductive tracts that allow the sperm to spread out similarly to the way external fertilizers' sperm do.
The smaller the reproductive tract, the larger the sperm. And for a fruit fly, it's as cramped as it gets. "Fruit fly sperm is 20 times the length of the animal's body," Kahrl said.
The researchers published their findings 21 June in the journal Nature Ecology & Evolution.
Originally published on Live Science.