An Evolutionary Reason For Male Infertility

In the past several years, numerous articles that have reported how men's sperm producing ability is decreasing, thus leading to reduced fertility. Now there is a report that suggests that men are producing sperm that are too good and this may also be contributing to infertility. According to Evolutionary Biologist Dr. Oren Hasson, some men are producing 'super-sperm' that are too aggressive to produce a pregnancy. These sperm move too fast to be stopped by the myriad of defenses set up in the women's reproductive system. The result is more than one of these sperm reach the egg and fertilize it, which causes an unviable polyspermy egg. Eggs fertilized by more than one sperm usually do not go on to produce a baby.

Although it only takes one sperm to fertilize an egg, in order for fertilization to reliably occur, many more sperm are required. Usually this means that hundreds or perhaps thousands of sperm need to be in the vicinity of the egg, ready and able to fertilize. However, there is significant attrition in sperm numbers as they overcome the barriers in the female reproductive tract during their quest for the egg. For those hundreds or thousands of sperm to reach the egg, millions of sperm need to be delivered by the male so that sufficient number of sperm survive the trip. Dr. Hasson theorizes the situation results from an 'evolutionary arms race' in which subtle changes over time occur to improve fertility. Women have evolved extra defenses which in turn compel sperm to evolve countermeasures. Men who produce 'normal' or 'non super' sperm must overcome the female defenses that were honed to ward off their kryptonian cousins.
Dr. Hassan based his theory on fertility statistics and mathematical models. Because he believes that stressful lifestyles, infection and pollution alone cannot account for all human infertility, his 'super sperm' hypothesis may provide an additional explanation for infertility.