A groundbreaking study led by researchers at the University of Oxford reveals that as males age, their reproductive success typically declines—not due to a shortage of sperm, but because of a limitation in the quality of their seminal fluid. The findings, which challenge previous assumptions about aging and fertility, may have significant implications for improving fertility treatments and IVF technology.
While it has long been known that male fertility declines with age, this decline has typically been attributed to a decrease in sperm quantity and/or quality. However, the Oxford-led research, which focused on fruit flies (Drosophila melanogaster), challenges this conventional view. The study found that older male fruit flies actually produce more sperm than younger males due to its accumulation over time and transfer similar or even larger quantities of sperm during mating. Despite this, old males produce fewer offspring—a result not of sperm limitations, but of females storing fewer sperm from older males.
This finding led researchers to hypothesize that the decline in reproductive success in older males could be linked to poor seminal fluid quality. Seminal fluid, the non-sperm component of the ejaculate, plays a crucial role in fertilization, sperm storage, and egg-laying. In polygamous species, where males mate with multiple females, the quality of both sperm and seminal fluid is especially important, as ejaculate production is energetically costly.
To investigate whether seminal fluid indeed played a role, the researchers supplemented female fruit flies with seminal fluid from young males. This was achieved through a transgenic male that produced only seminal fluid, not sperm. The results were striking: when females received this supplementation, the reproductive success of aged males was restored, and they produced similar numbers of offspring as their younger counterparts. This suggests that the seminal fluid of younger males can significantly enhance the fertility of older males.
Dr. Krish Sanghvi, lead author of the study, explained, “Our findings show that different components of the ejaculate—sperm and seminal fluid—decline at different rates as males age. The reduced reproductive output in older males is likely due to deteriorating seminal fluid quality, declining sperm viability, and differential sperm storage by females, rather than a reduction in sperm numbers.”
The study also sheds light on the broader implications of seminal fluid in reproductive success. In polygamous mating systems, where the last male to mate often fathers the most offspring, the quality of seminal fluid from previous males can impact subsequent males’ reproductive success. This highlights the potential advantages of polygamous mating in boosting fertility.
These findings have critical implications for both animal fertility and human reproductive medicine. The research underscores the potential to reverse age-related declines in male fertility through the supplementation of high-quality seminal fluid. This approach could one day inform IVF strategies, where the seminal fluid of healthy, fertile donors could be used to supplement sperm from less fertile men.
Dr. Irem Sepil, senior author of the study, noted, “Future research will investigate whether seminal fluid supplementation could be applied to humans to enhance fertility, particularly for men with compromised reproductive health.” Dr. Sanghvi added, “We aim to further explore whether it’s just the presence of seminal fluid, or also its age, that affects male fertility, as well as the role of female sperm ejection in influencing sperm retention.
In conclusion, the research highlights the critical role that seminal fluid plays in male fertility and offers new insights into potential fertility-enhancing treatments for both animals and humans.
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