High rates of human infertility may have evolutionary cause

10 percent of couples are infertile. Why? The reproductive organs of men and women are in an evolutionary arms race that's far from over.

About 10 percent of couples hoping for a baby have fertility problems.

Is pollution to blame? Stress? Eating habits?

Biologist Oren Hasson of Tel Aviv University thinks it's evolution, baby.

In a new study published in Biological Reviews, Hasson writes that the reproductive organs of men and women are in an evolutionary arms race that's far from over.

"The rate of human infertility is higher than we should expect it to be," Hasson said in a statement. "By now, evolution should have improved our reproductive success rate. Something else is going on."

Combining empirical evidence with a mathematical model developed with colleague Lewi Stone, the researchers suggest that the bodies of men and women have become reproductive antagonists, rather than partners.

They insist that women's bodies, over thousands of years of evolution, have forced sperm to become more competitive by rewarding the strongest, fastest sperm cells with penetration of the egg -- and thus fertilization.

In evolutionary response, men are overproducing "aggressive" sperm to increase chances of successful fertilization, the scientists say.

The problem? That aggression may be terminating the pregnancy before it starts.

It's all about timing: the first sperm to enter and bind with the egg triggers a series of biochemical responses to block other sperm from entering -- necessary because a second penetrating sperm would kill the egg.

But in the few minutes it takes for that blockade to complete, today's aggressive sperm may manage to penetrate the egg -- terminating fertilization just after it's begun.

"It's a delicate balance, and over time women's and men's bodies fine tune to each other. Sometimes, during the fine-tuning process, high rates of infertility can be seen. That's probably the reason for the very high rates of unexplained infertility in the last decades."

To avoid the fatal consequences of this "polyspermy," the female reproductive path has evolved to become less hospitable to male sperm, by "ejecting, diluting, diverting and killing" spermatozoa before they ever reach the egg.

Thus, a male-female fertility arms race, fueled by evolution and merely aggravated by factors such as stress and pollution.

(For the psychological side of infertility, read the touching "Life After Infertility Treatments Fail.")

This post was originally published on Smartplanet.com