Anish Moonka@anishmoonka
Researchers in Stockholm put a woman's egg in a dish next to sperm from two men: her partner and a stranger. Half the time, the egg pulled in more sperm from the stranger than from the man she'd chosen.
This was a 2020 study from Stockholm University and the University of Manchester, published in Proceedings of the Royal Society B. Until then, nobody had shown a human egg doing this.
The fluid that surrounds an egg in a woman's body releases chemicals that act like a scent trail. Sperm follow the scent. The team put samples in thin glass tubes and counted how many sperm from each man swam in. All the eggs and sperm came from real couples going through IVF treatment.
The pull was uneven. Eggs attracted 18 to 40 percent more sperm from one man over another, and inside the egg's fluid, sperm built up about 10 times more than in a plain comparison liquid. The team ran the experiment over and over, and the same pattern came out every time.
The bigger finding came in who got picked. In half the cases, more sperm got pulled toward a stranger than toward her partner. The same sperm performed differently depending on which woman's fluid it was in.
Sperm tails have a tiny gate on them. When the gate detects a hormone called progesterone (the same hormone tied to a woman's monthly cycle), it opens and the sperm kicks into a higher gear. Each woman's fluid has its own chemical mix, so different men's sperm respond differently. Some sperm get a strong push. Others barely react to the same fluid.
Roughly 1 in 4 UK couples in fertility treatment have no idea why they can't have a baby. Every standard test comes back normal. If the egg and sperm reject each other chemically, no test catches it. The cycle fails. They get no answer, and they have to go through the entire thing again.
In fertilization, the egg gets its own vote. The woman never feels it happen. Half the time in this study, that vote went to a stranger.