Instead, their fitness seems to stem from their father’s exercise habits before they were even conceived. It’s a finding suggesting that running might benefit not just the exerciser, but also his unborn children.

“I was very surprised when I first saw the data,” says Yin, a biochemist at Nanjing University.

Yin’s team analyzed the molecules inside the exercising rodents’ sperm and found tiny bits of RNA—dubbed microRNAs—that were present in higher amounts than in the sperm of their idle littermates. When the scientists injected those molecules into unrelated embryos, they got animals just as fit as those that were born to exercising fathers.

That 2025 study adds to mounting evidence that sperm are more than wriggling vessels carrying DNA to an egg. Over the past two decades, studies in mice have detected microRNAs and other types of RNA fragments that surge and wane inside sperm cells in response to not just exercise or sloth but also fatty or sugary diets, daily stress, childhood trauma, heavy drinking and exposure to pesticides and other hazards. In step with these changes, researchers have documented developmental and metabolic changes and differing rates of depression in the males’ offspring.

And while it’s difficult to study the effect in people, researchers also have documented fluctuations in RNA fragments in the sperm of men who do or don’t exercise, smoke or eat excess sugar, as well as men with obesity or traumatic childhoods. Studies also report that children of parents who are overweight or who dealt with mental health stress are more likely to have those conditions, too. //

But though puzzles remain, recent studies show that not only are paternal RNA fragments transferred to a fertilized egg, but also that they are capable of inducing changes in the offspring at the doses found in sperm. //

Today, those studying the phenomenon are sure the effects exist but aren’t certain how they are transmitted. The end result, they believe, is adjustments to the activity of genes—a phenomenon known as epigenetics.

Such adjustments occur during normal development as tissues and organs adopt their different identities, which require certain genes to be active or to be turned off. //

Today, the idea that small RNAs carry environmental signals has the most direct evidence behind it. Although small RNAs are short-lived, they aren’t actively removed like other epigenetic marks. Somehow, the tiny bits of nucleic acid fluctuate in response to the environment, then find their way into sperm cells.b//

Whatever the mechanism, there’s enough evidence to rebalance parental responsibility, Teperino says. “Now it’s almost all on women,” he says. “When a couple is planning a family, the doctor gives the woman a list of rules to follow. This is not valid anymore—we need to at least give recommendations to both.”