Why do some animals have sperm that are 20 times longer than their bodies?
All sperm do the same basic job: they fertilize the eggs. But in a new study, researchers have found that size matters, and it’s largely the female that causes sperm to be big or small.
Sperm come in a wide variety of sizes. For example, the parasitoid wasp Cotesia congregata produces small swimmers less than a thousandth of a centimeter long, while fruit flies make sperm with 2.3 inch (6 cm) tails that curl tightly to fit inside their little body.
In the new study, the researchers set out to find out how sperm size varies between species and what could be causing the differences.
“We have all of these studies that show evidence of natural selection causing sperm size in various species to be larger or smaller, but we wanted to take a more zoomed-in view and look for trends across species,” said lead author Ariel Kahrl. , postdoctoral researcher in evolutionary biology at Stockholm University.
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Kahrl and colleagues looked at data from 3,200 species and discovered a guiding principle that determines the size of sperm in a species: females with small reproductive tracts result in the production of larger sperm, and the need to shed sperm. on a large scale reduced sperm across evolutionary time scales.
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, on the other hand, throw their eggs back into the environment and hope for the best. Usually, these species live in water, such as fish and sea urchins. In both modes, tons of sperm compete in a battle royale for the price of egg fertilization, but the challenges of each mode put incredible evolutionary pressure on sperm size.
“We have found that external fertilizers tend to have very small sperm because they have to make a ton of them to reach the eggs,” Kahrl said. External fertilization requires the ejection of a cloud of sperm, usually into water. As the sperm spread, they dilute, so the best strategy would be to produce as many sperm as possible in order to maximize the chances that at least one will reach an egg. Because an animal has a limited amount of energy to use to produce sperm, it cannot afford to make it any bigger than it absolutely has to.
It’s a completely different situation for internal fertilizers. “We believe that for internal fertilizers, the female reproductive system influences the way sperm fight each other,” said study co-author John Fitzpatrick, assistant professor of biology who is also at the University. from Stockholm. In internal fertilization, sperm work in a small space, so reproduction becomes less of a treasure hunt than a game of king of the hill. In this situation, bigger may be better for repelling other sperm, whether they come from the same father or from different potential fathers.
“Some of these species produce huge sperm, and if you produce huge sperm, you don’t make that much,” Kahrl said. “These males roll up their sperm like a ball of wool and pass it on.”
In addition to internal and external fertilizers, the researchers looked at a third, rarer mode of reproduction called spermcasting. Semen is like a combination of internal and external fertilization; for example, a river mussel could eject sperm into a stream, and that sperm would ride the currents until it was picked up by a stationary female feeding by filtration.
“With spermcasting, you have this diluting effect because the sperm are ejected into the water, but when the sperm enter the female, they rapidly evolve under the same types of pressures that we see in internal fertilizers,” said Fitzpatrick at Live Science. Spermcasters, however, have smaller swimmers, similar in size to sperm from external fertilizers, possibly because ejecting sperm into the water prompts them to do more, forcing them to be small. But once these sperm are absorbed by the female, the larger sperm tend to win.
Although they are internal fertilizers, humans do not have monster sperm. Instead, human semen measures a modest 0.002 inch (0.005 cm) in length, well within the range seen in external fertilizers. This is because animals with larger bodies have reproductive tracts that allow sperm to spread in the same way that sperm from external fertilizers do.
The smaller the reproductive system, the larger the sperm. And for a fruit fly, it’s as cramped as it gets. “The fruit fly’s sperm is 20 times the length of the animal’s body,” Kahrl said.
The researchers published their results on June 21 in the journal Ecology and evolution of nature.
Originally posted on Live Science.