We all know the internal reproductive scene: The sperm swim towards the egg, wriggling their darnedest to be the first on the scene and fertilise the egg.
But science says there might be more at play than just the fastest swimmer – the egg actually has a fair bit to say in who breaks through.
The findings come out of Sweden this week.
They reveal that eggs have something like a tractor beam – in the form of a chemical – that essentially latches on to the wriggler it most desires.
“Human eggs release chemicals called chemo-attractants that attract sperm to unfertilised eggs. We wanted to know if eggs use these chemical signals to pick which sperm they attract,” said John Fitzpatrick, Associate Professor at Stockholm University and author of the study.
So we’ve got chemicals between mating partners before any hanky panky, and then chemicals inside the bodies post-hanky in the form of the egg sending out signals to essentially guide the way for the sperm.
Once the sperm make it through the fallopian tube to the site of the egg (of which only about 250 individual sperm arrive), the egg is surrounded in follicular fluid, which contain the guiding chemicals.
This study wanted to determine if this fluid’s chemicals had the final choice in which sperm got over the finish line.
Turns out, it does – with an interesting twist.
In the experiment – which used real-life couples – a woman’s egg was exposed to a cocktail containing the sperm of her partner, and that of a random male.
Although there may be chemicals interacting between partners – sexual chemistry being a physiological event – a woman’s egg won’t automatically preference her partner’s sperm over that of another man.
When the chemical tractor beam is choosing the winning sperm, it’s drawn to fitness – the quality of the sperm – over sentimental attachment.
As Professor Fitzgerald explained to CNN: “So when sperm go into the follicular fluid, they start to go straighter and they start to change the way they swim.”
“So depending on the strength of that signal, you can get different responses in how the sperm are responding to these female chemical signals within their follicular field.”
If the egg decides it likes the sperm, its chemicals tell it to swim faster. If it’s not a fan, the sperm is sent a go-slow message.
“Follicular fluid from one female was better at attracting sperm from one male, while follicular fluid from another female was better at attracting sperm from a different male,” Professor Fitzpatrick explained.
“This shows that interactions between human eggs and sperm depend on the specific identity of the women and men involved.”
Why does this happen? The sperm only has one job: Fertilise the egg.
The egg, however, has a serious job to do in choosing the highest quality or most genetically compatible sperm.
The study authors hope this finding can advance fertility treatments, and help benefit couples who struggle with fertility issues.
The findings were published in the scientific journal Proceedings of the Royal Society B.