If scientists from Mars were to study the human male's reproductive system they would probably conclude that he is destined for rapid extinction. Compared to other mammals, humans produce relatively low numbers of viable sperm.
As many as one in five healthy young men between the ages of 18 and 25 produce abnormal sperm counts. Even the sperm they do produce is often of poor quality. In fact only five to 15 percent of their sperm is, on average, good enough to be classed as "normal" - and these are young, healthy men. By contrast, more than 90 percent of the sperm of a domestic bull or ram, or even laboratory rat, are normal.
Men also suffer a disproportionate incidence of reproductive problems, from congenital defects and undescended testes to cancer and impotency. As these also affect fertility, it's a minor miracle men are able to sire any children at all.
In fact, an increasing number of men are finding themselves childless. Among the one in seven couples now classed as infertile, the "male factor" is the most commonly identified cause.
Next year marks the 20th anniversary of the 1991 WHO conference where a Danish scientist first alerted the world to the fact that Western men are suffering an infertility crisis.
Professor Niels Skakkebaek of the University of Copenhagen found sperm counts had fallen by about a half over the preceding 50 years.
Sperm counts in the 1940s were typically well above 100m sperm cells per millilitre, but they have dropped to an average of about 60m per ml. Other studies found 15 to 20 percent of young men now have sperm counts of less than 20m per ml, which is technically defined as abnormal.
Experts in human reproductive biology were astonished by the Danish study. The declining trend seemed to indicate that men were on a path to becoming completely infertile within a few generations (although recent studies suggest the fall in sperm counts may have bottomed out).
Skakkebaek could offer no explanation other than to suggest that the fall may have something to do with the equally alarming rise in other reproductive disorders, such as cancer of the testes and cryptorchidism, the incomplete descent of the testes into the scrotum.
Experts began to talk of a new phenomenon affecting the human male, a collection of disorders known as testicular dysgenesis syndrome. They wanted to know what was causing it, because the changes were occurring too quickly to be genetic. It must have something to do with changing lifestyles or the environment of men, and almost everything was suggested, from exposure to chemical pollutants to the modern fashion for tight underpants.
There is now an emerging consensus that whatever is exacerbating the problems of male infertility, it probably starts in the womb. It is not the lifestyle of men that is a problem, but that of their mothers.
The process of sperm production, called spermatogenesis, starts in adolescence, but the groundwork is laid down in the few months before and immediately after birth. Studies point to a crucial "window" of testicular development that begins in the growing foetus and ends in the first six months of life. Interfere with this critical developmental period, and a baby boy will suffer the lifetime consequences of being a suboptimally fertile man.
"It's most likely a reflection of the fact that many environmental and lifestyle changes over the past 50 years are inherently detrimental to sperm production," says Professor Richard Sharpe, fertility research expert at the Medical Research Council. "It may be that different factors come together to have a combined effect."
In one study men whose pregnant mothers were exposed to high levels of toxic dioxins as a result of the 1976 industrial accident in Seveso, Italy, have been found to have low sperm counts. But men exposed to dioxins in adulthood showed no such effect. Another study found women who ate large amounts of beef during pregnancy, a diet rich in potentially damaging chemicals called polycyclic aromatic hydrocarbons (PAHs), had sons with relatively low sperm counts. But men who eat beef are not affected.
Meanwhile, studies of migrants between Sweden and Finland showed a man's lifetime risk of testicular cancer tends to follow the country he was born in rather than the country where he was brought up. It was his mother's environment when she was pregnant with him, rather than his own later on, that seems to have largely determined a man's risk of testicular cancer.
One of the strongest pieces of evidence in support comes from studies of smokers. A man who smokes typically reduces his sperm count by a modest 15 percent. However, a man whose mother smoked during pregnancy has a fairly dramatic decrease in sperm counts of up to 40 percent - which also tends to be irreversible.
But the key question now is to identify the relevant lifestyle and environmental factors.
This is proving tricky. Obesity, for instance, is a growing problem and it has been linked with reproductive problems.
Chemicals in the environment, especially those that can mimic female sex hormones - or block male sex hormones, specifically testosterone - may also be to blame.
It is proving difficult to find a significant link between male reproductive problems and exposure to the many environmental chemicals that may have weak oestrogenic or androgen-blocking properties. They may be as wide-ranging as pesticides, traffic fumes, plastics and even soya beans. Sharpe says that much of the evidence to date is weak or non-existent. - The Independent