Around 10 percent of all couples hoping for a baby have fertility problems. Problems conceiving are usually attributed to pollution or our stressful lifestyles, but evolutionary biologist Dr. Oren Hasson, of Tel Aviv University's Department of Zoology, offers a different theory. He believes the reproductive organs of men and women are involved in an ongoing evolutionary arms race.
"The rate of human infertility is higher than we should expect it to be. By now, evolution should have improved our reproductive success rate. Something else is going on," says Dr. Hasson. His investigations, published in the journal Biological Reviews, suggest that the bodies of men and women have become reproductive enemies, not reproductive partners.
In his analysis, Dr. Hasson describes how over thousands of years of evolution, women's bodies have forced sperm to become more competitive, rewarding the "super-sperm" - the strongest, fastest swimmers - with penetration of the egg. In response, men are over-producing these aggressive sperm, producing many dozens of millions of them to increase their chances for successful fertilization.
But these evolutionary strategies demonstrate the Law of Unintended Consequences, says Dr. Hasson. "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."
These unintended consequences are to do with timing. The first sperm to enter and bind with the egg triggers biochemical responses to block other sperm from entering. This blockade is necessary because a second penetrating sperm would kill the egg. However, in just the few minutes it takes for the "blockade" to work, today's over-competitive sperm may already be penetrating, terminating the fertilization just after it's begun.
According to Dr. Hasson, women's bodies have been developing defenses to this condition, known as polyspermy. "To avoid the fatal consequences of polyspermy, female reproductive tracts have evolved to become formidable barriers to sperm," he explains. "They eject, dilute, divert and kill spermatozoa so that only about a single spermatozoon gets into the vicinity of a viable egg at the right time."
Any small improvement in male sperm efficiency is matched by a response in the female reproductive system, Dr. Hasson argues. "This fuels the 'arms race' between the sexes and leads to the evolutionary cycle going on right now in the entire animal world."
Sperm, says Dr. Hasson, have also become more sensitive to environmental stressors like anxious lifestyles or polluted environments. "Armed only with short-sighted natural selection," he argues, "nature could not have foreseen those stressors. This is the pattern of any arms race. A greater investment in weapons and defenses entails greater risks and a more fragile equilibrium."
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Source: Tel Aviv University