Sperm deteriorates in quality the longer it stays in the body

The health and performance of sperm decline while it is stored within male or female reproductive tracts, according to a major cross-species analysis.

Researchers from the University of Oxford compiled data from 115 human and 56 non‑human animal studies to examine how time spent in storage affects sperm health, fertilisation success and embryo development. The analysis drew on data from nearly 55,000 men and 30 animal species. This work provides a better understanding of the extent, causes and impact of post‑meiotic sperm senescence, a deterioration process that begins once sperm are fully mature.

'[Sperm] quickly exhaust their stored energy reserves and have limited capacity for repair,' said Dr Rebecca Dean, from the department of Biology at the University of Oxford and co-lead author of the study published in Proceedings of the Royal Society B: Biological Sciences. 'This makes storage particularly damaging compared to other types of cells.'

In humans, longer periods of abstinence were linked to sperm showing greater DNA fragmentation and oxidative stress, as well as reduced motility and viability. The effect was statistically significant, but modest. In addition, storage duration was not associated with a decline in fertilisation rate or embryo quality.

The authors identified two potential causes for the deterioration of sperm over time. First, mature sperm are highly mobile cells that contain very little cytoplasm, which makes them especially vulnerable to oxidative stress. Second, because they cannot produce antioxidants or replenish their energy reserves, they face high energy demands that further contribute to their decline.

Patterns in non‑human animals showed many similarities, with sperm performance consistently declining with storage duration across mammals, birds, reptiles, insects and fish. In several species, extended storage also reduced fertilisation success and embryo quality.

Females in many species can store sperm within their reproductive tracts – from a few days in humans, to months or even years in ants, bees or bats. Analyses on data from non-human animals showed that sperm deteriorated more slowly when stored inside females compared to males.

'This likely reflects the evolution of female-specific adaptations, such as specialised storage organs that provide antioxidants to extend sperm viability,' explained co-senior author Dr Irem Sepil, an evolutionary biologist from the University of 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.'

This study may also help refine fertility treatment guidance, as current World Health Organisation guidelines advise men to observe two to seven days of abstinence before providing a semen sample. Refraining from ejaculation increases sperm count, but, as the findings potentially indicate, it may also lead to a modest decline in performance. The authors therefore carefully caution against a one‑size‑fits‑all approach, stressing the importance of tailoring timing to achieve the best balance between sperm quantity and quality depending on the type of fertility treatment.