The Science of Sex: Count Down

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One egg, one sperm. That’s all it takes to score a reproductive bulls-eye. Of course, as anyone who has watched the Discovery Channel knows, a maverick sperm takes a flood of its buddies along for the ride — between one hundred million and seven hundred million tail-snapping semen-surfing spermatozoa in each ejaculation.


Without all those “extras,” a man’s reproductive potential can drop radically. For several years, a debate has raged about this topic: according to some researchers, average sperm counts are declining — and doing so at such an alarming pace as to threaten the extinction of Homo sapiens in just a few decades. A new meta-study may add to the growing alarm.


The issue of declining sperm counts first came to public attention in 1992, when Danish researchers announced that, in Western countries as a whole, the average concentration of sperm in semen had dropped by nearly half between 1938 and 1990 — from 113 to 66 million sperm per milliliter. Related studies have reported a decline in the average volume of a man’s ejaculate, an increase in the proportion of abnormal sperm in the semen and an increase in the incidence of undescended testicles and testicular cancer.


Scary as these findings sound, there has also been criticism of the analytical techniques involved. In 1999, a Columbia University research group argued that the decline was an illusion caused by quirks of geography. Much of the early data came from New York, they said, whereas the more recent data came from all over. New Yorkers have higher sperm counts than men elsewhere — according to the New York researchers at least! — so that gave the false impression of a decline in sperm counts over time. The only solution to such a debate is of course more data, better analyzed.


Last week I spoke with Shanna Swan, a biostatistician at the University of Missouri who published a massive re-analysis of all the available data in the October issue of Environmental Health Perspectives. The news isn’t good. The decline in sperm counts is real, she says, at least in North America and Europe. In a finding which belies the New York critique, a French group studied sperm counts in just one city — Paris — and found a decline closely matching the Danish results, as well as a whopping increase in the number of abnormal sperm. And in some countries, sperm counts are already reaching levels that potentially affect fertility. Swan mentioned a just-published study of 708 military recruits in Denmark, in which the median (middle) sperm count was 41 million per milliliter — almost half the recruits were in the territory below 40 million, the critical level at which fertility begins to be impaired, according to a 1998 study.


If this trend continues in the future, male infertility will become a pandemic. That’s because, for a spermatozoon, surfing the Tunnel of Love is the most extreme of extreme sports — the X- (and Y-) Games in miniature. The chances are about 100 million to one against any particular sperm making it to the finish, so you’ve got to put enormous numbers on the starting line if you want a decent chance of a winner. Why the female reproductive tract was designed to be such a sperm-shredding minefield, we may never know: perhaps it was to select the one true, genetically superior survivor, the Schwarzenegger among sperm.


If the decline is real, what is it due to? Possibilities range from the popularity of clinging Calvin Klein briefs (which keep testes too hot) to an excessive amount of sex (which doesn’t allow production to keep up with demand). Swan dismisses this latter explanation because researchers carefully control how long a man has been abstinent before he donates a specimen.


But one theory is particularly ominous: the suggestion that man-made, estrogen-like toxins in the environment are interfering with sperm production. This idea got a jump-start years ago when researchers discovered that crop-duster pilots, who were exposed to the insecticide DDT, were losing their fertility due to low sperm counts. DDT has now been off the market for decades, but DDT and its estrogen-like metabolites still lurk in the environment. One hundred tons of DDT coat the floor of the Pacific Ocean just off Los Angeles, for example, and have played havoc with the reproduction of marine life and birds in the region. Estrogen-like compounds also leach out of many plastics, and even from dental sealants and composites. These compounds can either mimic or block the action of the body’s own estrogens.


Estrogens may be known as female hormones, but they’re crucial to men’s bodies too. They are required for normal development of the testes, and for concentrating newly-manufactured sperm. So, it’s quite plausible that pollutants with estrogen-like or estrogen-blocking activity could adversely effect sperm counts.


We’re still a long way from being certain that sperm counts are in fact declining, let alone from pinning down the cause. But it’s a problem that we can’t afford to ignore. We need more sperm count studies, to see which countries and which populations are being affected. We need to find out whether levels of phytoestrogen pollutants sufficient to disrupt sperm production do occur in the bodies of men and pregnant women, and if so, how they get there. And if these pollutants are indeed the culprits, we should redouble our efforts to control their production, use and disposal. Otherwise we may wake up one day and find that the younger generation is, well, missing.