On a bright afternoon in Jiangsu, China, Xin Yin is playing personal trainer to some mice. One by one, he sets the rodents on a miniature treadmill that starts slow and gradually speeds up. These littermates are born athletes, able to run farther with less lactic acid buildup than average laboratory mice.

The secret to their speediness isn’t carried in their genes—the animals come from the same genetic stock as a group of control mice. And they haven’t received any special training. 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.

Until recently, however, most evidence linking small sperm RNAs to environmental challenges and subsequent effects in offspring has been correlational. Attempts to pin down causality—by injecting RNAs directly into embryos—have often used far higher RNA concentrations than typically found in sperm. In fact, there was no proof that the RNA fragments even make it inside the egg.

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.

Epigenetic effects

Researchers first noticed the intergenerational effects of paternal lifestyle back in the 1960s, but it was decades before they started experimental investigations using animal models. 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. Epigenetic changes also occur throughout our lives, due to factors including exposures to certain chemicals, and activities such as smoking—and, maybe, exercise, stress, fatty diets, and more. Such changes can occur in myriad body cells, including those that give rise to sperm.

As evidence mounted that sperm somehow transmit environmental information to a male’s children, researchers started probing the epigenetic mechanisms that might be responsible. Several possibilities exist: methyl groups that turn down gene activity when they accumulate on genes, and acetyl groups that attach to the protein spools called histones, around which the DNA wraps. These tend to ramp up activity of nearby genes.

But methyl groups aren’t easily passed to the next generation: Fertilized eggs erase most of these marks from both sets of chromosomes before the embryo starts to divide. And mature sperm replaces most histones with its own proteins, limiting transmission of information this way.

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.

At first, researchers hypothesized that sperm manufacture these microscopic molecules in the testes, where stem cells morph into fledgling spermatozoa that are not yet fertile or able to swim. The problem, though, is that as they develop, sperm whittle down their insides to little more than the nucleus containing the male chromosomes and the mitochondria, cellular powerhouses that fuel the sperm’s odyssey to the egg.

New clues emerged in 2016, when Colin Conine and Upasna Sharma, postdocs in the lab of epigeneticist Oliver Rando at the University of Massachusetts’ Chan Medical School, and colleagues, cataloged the molecular makeup of sperm from male mice exposed to low protein diets. Sperm extracted from the testes and the epididymis—a convoluted tube that carries the sperm out of the testes—contain different RNA payloads. And small bubbles found in the walls of the epididymis—called epididymosomes—were found to carry a cargo of RNA fragments matching those found in mature sperm.

The team later confirmed their hunch: Sperm take up small RNAs from epididymosomes during their cruise through the winding tube, stockpiling environmental information.

Other groups later reported that movement through the epididymis was associated with a reconfiguration of small RNAs in the sperm of rodents exposed to environmental challenges. One group found that chemically activating an animal’s stress response just two weeks before conception—when sperm have already embarked on their epididymal journey—still produces metabolic changes in the offspring.

The epididymis connection has grown stronger with time. One 2020 study bred anxious mice by injecting sperm with epididymosomes from stressed rodents. Another 2020 study reproduced traits seen in pups reared from binge-drinking males by injecting epididymosomes from the alcohol-loving mice into sperm from teetotal animals. And in a study published earlier this year, Conine’s team found that epididymosomes also deliver some of the father’s messenger RNA—the molecule that cells use to build proteins—to sperm cells.

Doubts about RNA origins

But despite two decades of research, there are snags scientists can’t explain. Those unanswered questions are a major issue, says Kevin Mitchell, a geneticist and neuroscientist at Trinity College Dublin. “I’m really skeptical,” he says.

For one thing, there’s been little direct evidence that sperm pass this RNA to the egg, since it’s often difficult—sometimes impossible—to tell which parent an RNA fragment came from. This has been “one of the biggest doubts in the scientific community over epigenetic inheritance,” says Raffaele Teperino, a molecular epigeneticist and physiologist at Helmholtz Munich in Germany.

An important result came in 2024, when Teperino’s lab sourced two mouse strains with enough variation in their mitochondrial DNA that the team could identify which parent certain RNA fragments originated from. Using this tool, the team discovered RNA scraps in early embryos that must have come from the father. Still, Teperino says, a single study won’t sway skeptics.

And showing that male RNA gets into egg cells is only part of the problem. A sperm cell is thousands-fold smaller in volume than an ovum, making its supply of small RNAs a drop in the egg-cell ocean. How can it make any difference? “The dilution question is the most serious critique of paternal effects,” says Rando, who coauthored an article on the status of paternal epigenetic research in the 2025 Annual Review of Biochemistry.

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