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Regular ejaculation — for example, by masturbation — produces higher quality sperm, a finding that has implications for fertility science and assisted reproductive technologies, according to a comprehensive new study published in Proceedings of the Royal Society B.
It’s well-established that sperm quality in many animals can deteriorate as males age, but less is known about how the age of sperm cells independently impacts reproductive outcomes. To fill in this gap, scientists co-led by Krish Sanghvi and Rebecca Dean of the University of Oxford conducted a meta-analysis of more than 115 studies about human sperm storage that cumulatively involved nearly 55,000 men, as well as 56 studies of 30 non-human species.
“Krish and I have done studies previously on the effects of male aging on fertility and offspring outcomes,” said Dean in a call with 404 Media. “So we have this joint interest in well: can sperm get old? What happens when sperm gets old? Do we see changes in fertility? Do we see changes in offspring outcomes?”
“We are sperm biologists, so these sorts of questions have been interesting to us,” added Sanghvi in the same call.
The results revealed that stored sperm deteriorates over time, resulting in DNA damage, reduced motility, and other defects that can affect fertilization and embryo outcomes. The discovery has implications for optimizing sperm quality for fertility clinics and captive breeding programs in animals, while also raising new questions about the complicated evolutionary mechanisms that govern reproduction across species.
Sperm cells in humans, and many other vertebrates, can be stored for several days in males before they are released through ejaculation or reabsorbed by the reproductive system. Sperm can also survive for several days in the human female reproductive system. Meanwhile, some female animals have evolved to store sperm for extremely long periods in order to optimize the timing and conditions of fertilization; for example, female bats can store sperm for months, while ant and bee queens can store it for years.
“There’s so much innovation within the animal world about how to maintain sperm and keep them alive and functioning,” said Dean. “In some species, the female is storing the sperm when the male who she’s mated with has long since died, but she’s keeping their sperm alive. It’s quite incredible.”
The team found that across these species, older sperm was more likely to show signs of deterioration that can influence fertility and embryo outcomes.
“In the animal dataset, we found an effect on fertilized embryos,” Sanghvi said. “Stored sperm led to lesser viability, or embryos actually becoming an offspring, basically, which is an intergenerational, almost evolutionary effect, which I thought was quite surprising. It means that the storage of sperm or abstinence is not only having an effect on the sperm itself, but it’s also doing something bad for the embryo at an early stage.”
“We found very weak effects in adult offspring, like offspring lifespan or offspring reproduction,” he added. “Those weren’t significant. But at the embryo stage, there was a stronger effect.”
Interestingly, insects that store sperm for weeks or months were able to maintain sperm quality at only a slightly higher level than insects that store it for years, underscoring that these animals have evolved sophisticated adaptations for long-term sperm storage. These amazing techniques across nature could inspire new approaches for future assisted reproduction in humans.
“Recently, there has been a huge boom in research on female reproductive fluids, or ovarian fluids, which are the chemicals in fluids released by females in their sperm storage organs,” Sanghvi said. “They have a lot of proteins that help with DNA repair, or the motility of sperm. They also have a lot of antioxidants, and small RNAs that change how sperm are transcribing their genes.”
“Maybe we can learn: what are the cocktails or chemicals in female storage? And can we actually replicate these molecules?” he added.
The study also sheds light on the possible evolutionary origins of masturbation, which has been observed in humans (a lot) as well as dozens of other species including dolphins, elephants, lions, and many primates. Masturbation may have emerged as a way to avoid leaving sperm in the tank for too long. Indeed, even species that don’t masturbate in the traditional sense of self-stimulation have still been observed offloading sperm in a practice called “sperm dumping.”
Masturbation has “been a sort of mystery,” Sanghvi said. “Sperm are costly to produce. For many animals it involves a lot of fluids, maturation, time, and energy investment. So why just get rid of it? A longstanding hypothesis has been that sperm are deteriorating within males when they are stored.”
“Crabs do something similar,” he continued. “Every time they moult, some crabs will just get rid of all their sperm. So again, why waste all of this? One reason could be that it’s getting rid of the stored old sperm.”
In addition to yielding new insights into the evolutionary tradeoffs of sperm storage, the study can inform the best approaches to assisted reproduction in humans, such as in vitro fertilization (IVF).
For example, the World Health Organization currently typically recommends that men undergo two to seven days of abstinence before donating semen in order to boost the number of sperm in the samples. However, the new study suggests the benefits of high sperm quantity should be weighed against sperm quality linked to the age of the sperm, which has implications for human fertility, as well as captive breeding programs for livestock and other animals.
To sum up, to ensure high sperm quality, avoid keeping too much spunk in your junk and regularly evacuate the ejaculate.
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