#Women have very judgmental eggs when it comes to sperm, study finds

A new study finds that eggs are especially choosy when it comes to picking sperm.
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A new study has found that a woman’s eggs are especially selective when choosing the most ideal sperm for fertilization.

Looking at leftover samples from 16 couples getting reproductive treatment, researchers from Sweden found that eggs release a chemical that attracts only certain types of sperm — regardless of whether they come from the woman’s chosen partner, according to the findings published Tuesday in the journal Proceedings of the Royal Society.

The selectiveness could be a way of sorting out the mate with the best genes, said study author John Fitzpatrick, an assistant professor in the department of zoology at Stockholm University in Sweden.

“We expected to see some sort of partner effect, but in half of the cases the eggs were attracting more sperm from a random male,” he told CNN. “The most likely explanation for this is that these chemical signals allow females to choose males who are more genetically compatible.”

The researchers knew that eggs release chemicals during ovulation known as chemoattractants, which Fitzpatrick says act like a “breadcrumb trail” for sperm to track down the egg. In this study, they found that as little as 18% of sperm can even find the trail. When they do pick up on the signal, the sperm swim more vigorously toward the egg, increasing the chances of fertilization.

Fitzpatrick and his colleagues believe this could potentially weed out less desirable sperm with fewer genetic variants, which would make the baby less immune to diseases. Attracting sperm with more diverse genes than the egg has could ultimately arm the baby with a better immune system.

This wouldn’t be the first time biology has taken the wheel, for better or for worse. Studies have found that women have evolved to prefer men with beards — even though beards potentially evolved to protect fellas from punches to the head. In that case, it’s probably for the best that eggs make the final call.