There are many funny stories about sperm. My favourite: it takes millions of male sperm to find a giant female egg because men won’t ask for directions.

Sperm are among the smallest cells in the human body; the female egg, or oocyte, is one of the largest. The egg is visible to the naked eye and—relative to the sperm—like Godzilla.

Women produce a single mature egg per month, while men produce millions of sperms daily. Those might appear to be excellent odds, but since no more than 200 sperms actually make it to the gates, and it requires only one sperm and one egg to spark the magic of human reproduction, the odds are drastically pared.

So, it’s important that the male sperm factory keeps production and stock levels high. It would appear this isn’t a problem, given humanity’s fecundity, quadrupling as it has to 7 billion from a century ago.

But like the larger global economy, the human male sperm factory seems to be tottering. In the wake of a series of studies this decade indicating falling sperm count, the latest—and largest—released last week in France said the sperm count of Frenchmen fell by a third between 1989 and 2005.

The tests, which used data from men aged between 18 and 70, revealed a decrease of more than 23 million sperms per millilitre of semen for an average 35-year-old man.

That’s a big decline over 16 years, but many specialists are asking if a drop from 73.6 million sperms per ml to 49.9 million per ml is as significant as it’s being made out to be. The new count is still in the normal range; infertile territory starts at a count below 20 million per ml.

Counting sperms over decades is tricky, and it is by no means conclusively established that the count is falling. It is worthwhile to consider three caveats:

One, science changes and so do laboratory techniques, which means it might be hard to compare a modern-day test with one conducted in the 1980s. Two, lifestyle and environment, including smoking, alcohol, pollution, medication and a sedentary life, can cause an individual’s sperm count to fluctuate. Three, there are studies that reported no declines.

The world was first alerted that something might be amiss with sperm when a team of Danish scientists led by endocrinologist Niels Skakkebaek reviewed 61 international studies between 1938 and 1992 and reported that, over half a century, sperm count in healthy men appeared to have declined by 50%.

Skakkebaek and team’s 1992 study said that the average count was down from 113 million per ml to 66 million. They also pointed out that the current normal of 20 million was a steep drop from 60 million, the accepted normal in 1938.

Skakkebaek faced ridicule. His methods and calculations were questioned, but too many studies across the Western world found similar evidence—notwithstanding the caveats. In addition, many studies also reported a decline in the quality of sperm.

Before going further, let’s ask what might be causing the sperm drain?

There are many hypotheses: from tight underwear to unhealthy diets to the effects of a variety of pollutants called “endocrine disruptors". These chemicals could be throwing the hormone system out of kilter.

In India, it is clear that overall fertility is falling, much more in the south than the north. While the desire to have fewer children is the primary cause, it isn’t clear if a sperm drain is playing a role. In urban areas, though there is relevant data, infertility specialists report a steep fall in sperm count.

While details are investigated and conclusions disputed, it is impossible to refute plunging fertility levels in many European countries and Japan. About one in five men between the ages of 18 and 25 have sperm count low enough to mar their fertility, several European studies show. Add to this the fact that women are having fewer babies later in life, and a perfect storm of future depopulation appears to be looming over much of the developed world (an exception is the US, where the fertility rate is higher than in Europe).

It isn’t as simple as that, of course. Falling sperm count and fertility levels certainly have implications for the next generation, but immigration tends to keep countries and economies robust. Immigrants typically exhibit high fertility levels, lower than their mother country but higher than the host. It is no surprise that the country most hostile to immigration, Japan, is ageing more rapidly than any other.

As immigrants assimilate, their fertility levels drop. Immigrant women led the US’s lowest-ever birth rate recorded in 2011, blamed substantially on the downturn. But could the stress of joblessness or working long hours for low pay also be causing sperm count to drop?

There are no correlations and causalities available, but it’s worth asking the question—and may be men should start asking for directions.

Samar Halarnkar is a Bangalore-based journalist. This is a fortnightly column that explores the cutting edge of science and technology. Comments are welcome at frontiermail@livemint.com. To read Samar Halarnkar’s previous columns, go to www.livemint.com/frontiermail

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