A new study has revealed that sperm quality in humans is subject to clear seasonal changes, being at its best during the summer months and at its lowest in the winter.
Researchers analyzed samples from more than 15,000 sperm donors in Denmark and the US state of Florida, and found a consistent pattern in both countries: the peak in sperm quality occurred in June and July, while the lowest levels were recorded in December and January.
This finding has led scientists to wonder whether men who volunteer to donate sperm in the summer have a better chance of acceptance, and whether couples trying to conceive might be more successful during that same season. However, researchers emphasize that this is merely a theory that needs to be proven.
Dr. Sherman Silber, a urologist not involved in the study, believes that these seasonal differences are biologically very small and practically do not affect real-life fertility.
When researchers tried to explain why sperm quality is higher in the summer and lower in the winter, they first turned to the most obvious factor: temperature. Since sperm production takes about 74 days, they examined the effect of temperature during the weeks leading up to ejaculation, but the results were surprising: they found no clear correlation between temperature and sperm quality.
This means that the secret isn't solely in temperature. Therefore, researchers have moved on to another explanation: lifestyle factors may be the real culprit, such as the type of food a man eats, how much he exercises, and his level of sun exposure.
But here the researchers stopped at an important point, as they did not measure these factors directly in their study, so they cannot be certain that they are the cause, and it remains just a strong belief.
In contrast to this lifestyle-related explanation, Dr. Silber offers a completely different explanation, one that is deeper and more rooted in the evolutionary history of humans.
Silber argues that this seasonal pattern is not a product of modern times, but rather an "evolutionary remnant" from our distant ancestors.
But he adds an important warning: this evolutionary influence is no longer as strong in modern humans as it is in animals, because humans have adapted to living in all seasons and have learned to provide food and warmth even in the harshest winter conditions, and therefore this "legacy" of our ancestors is now so weak that it does not really affect the chances of reproduction.
This isn't the first time seasonal variations in sperm quality have been observed, but previous findings have been conflicting. Some studies, like an Italian one, found that sperm motility peaks in the summer, just like the new study. Other studies, such as a large analysis of over 21,000 samples from southern China, found that the peak occurs in late winter and declines during the summer.
Researchers believe that regional differences in climate, humidity, and social behaviors may explain these discrepancies.
In addition to seasonal variation, the study found that a man's age plays an important role, with sperm motility being highest in men in their thirties and lowest in those under 25 and over 40.
Researchers also observed a significant decline in sperm quality in Denmark between 2019 and 2022, followed by a recovery in 2023, which may reflect the effects of the COVID-19 pandemic and associated lockdowns on work patterns, diet, and physical activity. In contrast, Florida saw a gradual increase in sperm quality from 2018 to 2024, a trend that remains unclear and requires further investigation.
