The study suggests that regular ejaculation could give male fertility "a small but meaningful boost" - and that even a few days’ delay could matter for IVF success.
Sperm quality declines quickly when stored, whether in males or females, a team of researchers from the Department of Biology at the University of Oxford has found.
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Researchers carried out a meta-analysis of 115 human studies involving nearly 55,000 men, alongside 56 studies across 30 non-human species. They found that mature sperm deteriorates in storage independently of the male’s age, a process called post-meiotic sperm senescence.
In humans, longer periods of sexual abstinence were linked to increased sperm DNA damage and oxidative stress, as well as reduced sperm motility and viability.
"Because sperm are highly mobile and have minimal cytoplasm, they quickly exhaust their stored energy reserves and have limited capacity for repair. This makes storage particularly damaging compared to other types of cells," explained co-lead author Dr Rebecca Dean from the Department of Biology at the University of Oxford.
"Our study highlights how regular ejaculation can provide a small but meaningful boost to male fertility," she added.
Differences between males and females
Both male and female animals can store sperm as a reproductive strategy. In humans, sperm can survive in females for several days, but the effects of such storage have been less clear.
The study found that females are generally better than males at preserving sperm quality long-term.
"This likely reflects the evolution of female-specific adaptations, such as specialised storage organs that provide antioxidants to extend sperm viability,” said senior author Dr Irem Sepil, also from Oxford.
"These organs often secrete reproductive fluids to nourish sperm and could provide unexplored avenues for biomimicking technology to improve artificial sperm storage in the future."
Lead author Dr Krish Sanghvi added: "Ejaculates should be viewed as populations of individual sperm which undergo birth, death, ageing and selective mortality.
"The rates of these demographic processes can differ in males and females, mediating the ‘demographic’ structure of sperm populations and sex-specific differences in sperm storage effects."
Why is this study significant?
The study shows that sperm can “age” in both men and women, regardless of their actual age – something that's often overlooked in reproductive medicine.
Right now, World Health Organization guidelines say it’s fine to wait up to seven days before giving a sperm sample, but these new findings suggest that might be too long. Other research supports that ejaculating within 48 hours of providing a sample could improve IVF success.
"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," the researchers of the study wrote in The Conversation.
These results could help tweak fertility clinic protocols, guide breeding programmes for endangered animals, and give us a better understanding of how evolution has shaped ways to protect sperm during storage.