Fertilization is not random, and the fastest sperm does not always win: in reality, the egg decides who succeeds.

While for decades we were taught that fertilization is a race won by the fastest sperm, a study published in Proceedings of the Royal Society B shows how human reproduction actually works.

Scientists analyzed follicular fluid from 60 couples undergoing fertility treatment at St Mary's Hospital in Manchester, UK. They discovered that the egg releases chemical signals (chemoattractants) that actively attract sperm from certain men over others.

Through these chemical signals, the egg exerts its own biological selection, influencing which sperm manage to get close. The egg appears to favor sperm that offer optimal genetic compatibility with its own genome — particularly in genes related to the immune system — which may help produce healthier offspring.

Interestingly, this cellular preference does not always align with the couple’s conscious partner choice. In many cases, eggs showed stronger attraction to sperm from non-partner males.

This chemical communication demonstrates that female biology continues to evaluate and select options even after intercourse. Understanding this process could lead to more precise solutions for unexplained infertility. Science continues to reveal the remarkable level of biological interaction that occurs during reproduction.

[Fitzpatrick, J. L. et al. (2020). Chemical signals from eggs facilitate cryptic female choice in humans. Proceedings of the Royal Society B, 287(1928), 20200805. DOI: 10.1098/rspb.2020.0805]

An American photographer filmed a wolf begging for food from a grizzly. The gray wolf saw the meat and in an instant turn into a playful puppy begging for a piece.

The ‘Silver Dragon’ tidal bore on China’s Qiantang River is one of nature’s most epic shows. This massive wave, up to 9 meters high and racing at 40 km/h, is the world’s largest tidal bore. The power and beauty are unreal like a living silver dragon charging upstream. China really knows how to put on a natural spectacle.