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“There are one or two reports of sperm surviving in the female tract for the next menstrual cycle, so up to 28 days, but they’re few and far between,” Christopher Barratta teacher of reproductive biology at the University of Dundee in the U.K., informed Live Science. “So most people say [sperm survive] up to seven days. We know this because a person still has a roughly 5% chance of getting pregnant even if they have sex as long as seven days before ovulation, he explained. “The anticipation is that the sperm cells can endure for that duration of time,” Barratt said.

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How do sperm last this long? Their longevity comes down to several factors, many of which researchers are still trying to understand. The seminal plasma that carries sperm into the body could play a role. This fluid contains ingredients that support and nourish sperm, including protein molecules and nutrients like zinc. And although sperm cells burn a lot of energy, they can switch to a more efficient method of using and generating energy when supplies are low, which might help them survive longer, Barratt said.

The female reproductive tract also plays a critical role in sperm’s survival by supplying them with energy in the form of glucose as they travel toward the egg, Barratt noted. “[Sperm] do have their own energy resources, however they will not last 5 days or 7 days,” Barratt said. “They do require to get it from the female system.”

What’s more, the cells of the tract appear to provide crucial rest stops for sperm along the way — for example, in the lower fallopian tube, or oviduct. Rather than racing to the egg, there’s evidence that sperm slow down and dock at these reservoirs, Barratt said. Here, they attach to surface cells and seem to take refuge, until they wriggle free for a final dash to the egg.

In fact, when researchers investigated this phenomenon in pigs, they found that sperm cells bind to special sugars, called glycans, within oviduct cells and that the sperm that did so lived longer than those that did not. Research on other mammal species also shows that sperm that bind to oviduct cells move, fertilize and live longer than those that don’t bind to these cells.

It’s possible that the oviduct cells deliver ingredients and trigger cellular changes that help to maintain the sperm and increase their staying power in the reproductive tract. “There’s a great deal of information on various particles that may pass in between the sperm and oviduct, and in reality, that you have the sperm signaling to the oviduct, themselves,” Barratt noted. However, studies on this process are difficult to do in humans, and are therefore limited. “I do not believe the system is yet clear,” Barratt said.

What is clear is that longer-living sperm provide a reproductive advantage in humans. Sperm that linger for several days in the body have a greater chance of overlapping with ovulation, the brief window of time when an ovary releases an egg to be fertilized. The sperm’s ability to wait it out in the oviductal reservoirs essentially ensures that hundreds of sperm are prepped and ready to go when an egg arrives.

“There’s more possibility of getting pregnant closer to ovulation, and the day of ovulation,” Barratt said. “If the sperm cells can be in there, in waiting, then that’s a benefit. Enduring for a longer time period considers that versatility.”

This benefit isn’t exclusive to humans. In fact, sperm show remarkable survival skills in other species, where they can last many times longer than human sperm. “We understand that the sperm of some types of bats have the ability to make it through for as much as 6 months in the female reproductive system,” Nixon said. “There are likewise some remarkable examples of sperm survival of a number of years in the female reproductive system of reptiles.”

How those particular sperm clung onto life is another fascinating mystery to unpick. “Researchers have still yet to exercise how this prolonged survival is achieved,” Nixon stated.

Emma Bryce is a London-based self-employed reporter who composes mostly about the environment, preservation and environment modification. She has actually composed for The Guardian, Wired Magazine, TED Ed, Anthropocene, China Dialogue, and Yale e360 to name a few, and has masters degree in science, health, and ecological reporting from New York University. Emma has actually been granted reporting grants from the European Journalism Centre, and in 2016 got an International Reporting Project fellowship to participate in the COP22 environment conference in Morocco.

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