Scientists have discovered a previously hidden biological “aging clock” in human sperm, revealing a new way that a father’s age might influence the health of his children.
The study, led by researchers at University of Utah Health and published in The EMBO Journal, shows that molecules called RNA inside sperm change in a predictable way as males age—both in mice and in humans.
For years, researchers have known that children born to older fathers face slightly higher risks of health problems, including obesity and stillbirth.
Most studies have focused on damage to sperm DNA as men age. But DNA is not the only thing sperm delivers to an egg.
Sperm also carry a complex collection of RNAs, molecules that help control how genes are turned on and off during early development.
Until now, these RNAs were difficult to study because many are short, chemically modified, and hard to detect using standard techniques.
To solve this problem, the research team developed a new sequencing method called PANDORA-seq, which allows scientists to see RNAs that were previously invisible.
Using this new tool, the researchers analyzed sperm from mice of different ages and discovered a striking pattern.
As males grew older, the RNA inside their sperm changed gradually—but around midlife, there was a sudden and dramatic shift. The team described this as an “aging cliff.”
Even more intriguing, they found evidence of a molecular clock. Over time, certain sperm RNAs became longer, while shorter RNA fragments became less common. When the researchers examined human sperm, they saw the same gradual lengthening of specific RNAs with age.
This result was unexpected. Scientists have long known that aging sperm DNA tends to break into smaller pieces. RNA, however, showed the opposite trend, growing longer rather than more fragmented. This suggests that sperm aging follows rules very different from what researchers had assumed.
The study also hints at why these RNA changes might matter. When the team introduced a mix of “old” sperm RNA into mouse embryonic stem cells—cells similar to those found in early embryos—the cells showed changes in genes linked to metabolism and neurodegenerative diseases. This suggests that aging-related RNA in sperm could influence how an embryo develops, potentially contributing to health risks later in life.
A key breakthrough came when researchers focused only on the sperm head, the part that actually enters the egg. The sperm tail contains many other RNAs that mask the signal. By isolating the head, the scientists were finally able to see the aging pattern clearly.
The team confirmed their findings in human samples using sperm bank resources at University of Utah Health, making this one of the strongest cross-species studies of sperm aging to date.
Researchers say the discovery could one day lead to new fertility tests or treatments that help men make informed reproductive decisions as they age. The next step is to identify the enzymes responsible for changing sperm RNA over time.
If scientists can learn how this molecular clock works—and how to slow it down—it may open new possibilities for improving sperm quality and protecting the health of future generations.
