When it comes to reproduction, female biology is often described in terms of a ticking clock. Women are born with most of their lifetime supply of eggs, meaning that a woman's age is usually the same as the age of her eggs. Older women therefore produce older eggs.
Authors
- Rebecca Dean
Research Fellow, Department of Biology, University of Oxford
- Irem Sepil
Lecturer in Evolutionary Biology, University of Oxford
- Krish Sanghvi
PhD Candidate, Department of Biology, University of Oxford
But male reproduction works differently. Sperm are produced continuously from puberty onwards and can be stored in the reproductive tract before ejaculation. That means a man's age is not necessarily the same as the age of his sperm . So what happens to sperm while they wait?
Men trying to conceive are often advised to remain sexually abstinent for several days to allow their sperm count to build up. It is true that abstinence increases sperm count . But the size of the ejaculate is not the only factor that determines fertility. Our new study shows that in men (and other male animals), sperm stored during sexual abstinence actually "ages" and deteriorates in quality.
We already know that male fertility declines with age . What has remained unclear is whether the time sperm spend in storage contributes to this decline.
Answering this question is particularly timely. Sexual activity appears to be declining , especially among young people. Combined with the global trend towards delayed parenthood, this may further exacerbate global fertility declines .
For our investigation, we collected semen data from 115 published studies involving nearly 55,000 men. We found that when men abstained from ejaculation, the health of their sperm dropped significantly. Sperm motility (their ability to swim) and viability decreased - and sperm DNA became more damaged.
We identified two likely causes. The first is oxidative stress - a form of biological "rust" that accumulates in sperm and can physically damage them. The second is energy depletion. Unlike most cells, sperm are highly active and have only a limited capacity to replenish their energy reserves. When stored for extended periods, they simply run out of fuel.
The World Health Organization advises against ejaculating two to seven days before providing a sperm sample for analysis, fertility treatments or procedures such as IVF. However, our findings suggest that even shorter periods may be better if sperm quality in the sample is to be improved.
This supports a recent discovery that ejaculating within 48 hours of providing a sample improves IVF treatment outcomes compared to longer durations of abstinence. It also aligns with a hypothesis in evolutionary biology.
We know that in primates, frequent ejaculation from masturbation improves the quality of ejaculates. Combined with our results, this suggests that male masturbation may have an adaptive benefit: it flushes out damaged, stored sperm.
Sperm does not just deteriorate inside males. It can also deteriorate after mating, when stored inside females. Human sperm only remains alive inside a woman for several days . However, in other animals such as queen ants, bees and female bats, sperm can be stored for several months or even years before eggs are fertilised.
The birds and the bees
To test whether sperm deterioration during storage is a widespread biological pattern, we examined data from 56 studies across 30 different animal species, including birds and bees, reptiles and other mammals. Here too, we found that sperm quality declined during storage.
Fathers who stored sperm before ejaculation, or mothers who stored it before fertilisation, produced embryos with lower chances of survival. We suspect this is not just due to damaged DNA. It may also be that stored sperm have a different gene expression profile - that is, a different pattern of which genes are actively switched on and being used - compared to freshly produced sperm.
Interestingly, sperm deteriorated at a slower rate inside females than inside males. This may be because females in several species have evolved specialised organs that secrete antioxidants , substances that nourish and protect the sperm they are storing, effectively extending their functional lifespan.
Whether in mice or men, sperm, much like eggs, have a "use-by date" after being produced. When sperm are stored for too long before fertilisation, they deteriorate in quality.
Crucially, however, our findings also point to a simple and potentially powerful intervention. Many fertility problems are driven by factors outside our control, such as environmental toxins, stress and genetics. But the duration of sperm storage is something that can be modified. Using freshly ejaculated sperm for fertilisation could therefore provide a meaningful boost to fertility outcomes by improving sperm quality.
Rebecca Dean receives funding from a Daphne Jackson Fellowship funded by NERC.
Irem Sepil receives funding from BBSRC and the Royal Society.
Irem Sepil receives funding from the Royal Society. Rebecca Dean receives funding from NERC (UKRI).
