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 But male reproduction works differently. Sperm are produced continuously from puberty onwards and can be stored in the reproductive tract before ejaculation.
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.
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.
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 fertilized.
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 fertilization, produced embryos with lower chances of survival. We suspect this is not just due to damaged DNA.
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 fertilization, they deteriorate in quality.
Using freshly ejaculated sperm for fertilization could therefore provide a meaningful boost to fertility outcomes by improving sperm quality.
Sciencealert.com
