SAY “biological clock” and most people think women — most people know that a woman’s ovaries stop producing eggs at some point and that natural fertility ends with menopause. But conventional wisdom holds that men can father children as easily at 75 as at 25. Stories such as Saul Bellow, who became a father at 85, help to sustain a widespread myth: that women have a gun to their heads that men never have to face.

Unfortunately, conventional wisdom is wrong: men have biological clocks too, but this fundamental fact about life has been very slow to reach the public for many reasons, not least because an entire industry has arisen in the past decade for “fixing” female infertility with procedures such as IVF. Now men are waiting as long as women to have children — the percentage of births to US fathers between the ages of 30 and 34 rose from 19 to 26 per cent between 1970 and 1999; the percentage to fathers older than 35 rose from 14 to 21 per cent.

Yet most people, even many doctors, don’t know the startling facts about male sexuality and male infertility, such as that men older than 35 are twice as likely to be infertile as men 25 or younger. An estimated one man in 10 trying to conceive is either infertile or subfertile, and in around 40 per cent of couples experiencing problems conceiving, the problem lies with the man.

The male biological clock isn’t like a woman’s. It “ticks” at a different rate and doesn’t strike a “midnight” toll of an absolute end to fertility. The male equivalent of menopause is “andropause”, the steady drop in the levels of androgens (male sex hormones), accompanied by related declines in sperm count, sperm health, sexual desire and sexual performance. The male biological clock refers to the health status of four of a man ’s key physical factors: semen, sperm, testosterone and erections. The time shown on a man’s biological clock is a function of how much semen he ejaculates, how many sperm he produces and how healthy they are, his level of testosterone, and the quality and reliability of his erections. In turn this is a function of the man’s overall health: what he eats, how much he exercises, what illnesses and accidents he has sustained in life, whether he smokes and or abuses drugs, what chemicals he is exposed to, and a host of other external factors.

SEMEN and SPERM

Contrary to what many men think, semen is not produced in the testicles but by the small plum-size prostate gland which sits just below the bladder. The average volume of semen expelled by a healthy man in one ejaculation is about a teaspoon (3 to 5 cubic centimetres), which contains roughly 250 million sperm. (The average sperm count is between 60 million and 80 million per millilitre; between 20 and 40 million is considered marginal, below 20 million poor.) Men with counts below 20 million may still be fertile, but it may take longer for them to initiate a pregnancy and the chances are greater that a pregnancy will not occur.

The number, motility (swimming ability) and shape of sperm generally decline with age, though other factors can speed the biological clock. Heat, for example, is bad for sperm. Sperm production plummets in the days following a high fever. Anything that unnaturally warms the testicles, such as taking frequent hot baths, will similarly hurt sperm. Research also shows that a range of genetic problems in sperm get worse with age, such as haemophilia A, neurofibromatosis, Marfan syndrome, and polycystic kidney disease. Older men are much more likely to father a child with Down’s syndrome when their partner is older than 35. A study has found that half the cases of Down’s syndrome in children born to women older than 35 are likely to be sperm-related.

TESTOSTERONE

Testosterone drives characteristics such as facial hair, muscle development, and interest in sex. The normal range of testosterone in men is between 300 and 1,100 nanograms (ng) per decilitre (dl) of blood. Men with levels below 300 ng/dl tend to have little interest in sex and are usually nonconfrontational, socially inhibited, and physically weak. They are also often very intellectual, creative, expressive and likeable. Men with higher-than normal testosterone tend to be just the reverse: obsessed with sex, competitive, aggressive, extroverted, physical and tending toward more action orientated activities or careers. (One study of testosterone levels among actors, clergymen, football players, physicians, firefighters, professors, and salesmen found that there was only one statistically significant difference in average testosterone levels — that between clergymen, whose levels were low, and both actors and football players, whose levels were high.)

Testosterone levels usually begin a slow downhill slide of about 1 per cent per year starting around the age of 30. Since the average American man can expect to live to 74, an annual drop of 1 per cent means a 44 per cent drop in all. More importantly, men with low-normal testosterone levels — 400 ng/dl, say — might hit the threshold of clinically significant testosterone loss by age 55. The signs of below-normal testosterone levels include fatigue, depressed mood, low or absent sex drive, muscle weakness, sleep disorders, and a general feeling of malaise.

A 2004 article in The New England Journal of Medicine estimated that between two and four million men in the US have hypogonadism (defined as testosterone levels below 325 mg/dl), and the prevalence increases with age. Unfortunately, few men are getting the help they need. It’s estimated that only 5 per cent of the millions of men with the condition are being treated, despite a booming business in testosterone replacement therapies of many kinds. (Sales of prescription testosterone products have increased by 500 per cent since 1993.)

ERECTIONS

The last measure of a man’s sexual health is the quality of reliability of his erections. Erectile dysfunction is the persistent inability to achieve or maintain an erection adequate for intercourse and is caused by an interplay of normal ageing and a host of other factors which speed up the biological clock, including smoking, diabetes, alcohol, prescription medicines, obesity, high cholesterol levels, injury and a sedentary lifestyle. In short, what’s bad for the heart is bad for the penis.