When it comes to muscle power, most women can't hold a candle to their macho mates. But if you conclude this makes them the weaker sex, then think again. Of recent years the male of our species seems to have been undergoing a crisis of masculinity. And forget self-obsessed misgivings, it is the cold light of science that is putting maleness under the microscope.

Weedy seed
Given that humans have come to dominate the planet, it is surprising to find that, compared to other mammals, human males have very low fertility. Roughly one in seven couples in the West seek treatment for infertility, mostly because of problems in the quality of the male seed. Compared with that of other mammals, human sperm turns out to be weakest of the lot. It has a low fertilising rate and worse, it is prone to genetic damage. Gene damage can have devastating consequences. For example, fathers who smoke heavily may increase the risk of cancer in their children by a factor of four.

So why is human sperm so weedy? In part, it is because sperm have evolved into fast, lightweight swimmers to race up the fallopian tubes of women. On the road to agility they have jettisoned many of the constituents that protect other cells from harm, such as the proteins that repair genetic damage. They also lack antioxidants that play a vital role in soaking up toxins produced as our bodies use oxygen to make energy. The toxins that build up in sperm contribute to the high rate of chromosomal damage and are a major cause of male infertility.

Another major contributing factor to the malfunction of male seed is the impoverished Y chromosome itself.

Building a boy
Most of us have 23 pairs of chromosomes that sit in each one of our body cells. One pair are the sex chromosomes and they govern our sexual development — XX in girls and XY in boys.

Chromosomes are long lengths of up to 1500 genes, parcelled together by proteins. But it is the Y chromosome that carries the single gene that triggers the whole intricate process of turning a female foetus into a male. Building a boy is a complicated business that requires the cooperation of hundreds of genes — sorry boys, all foetuses are female for the first six weeks! Once triggered, a coalition of male building genes on the Y leap into action. Many Y genes are highly specific and are only active in building and maintaining the male sex organs.

Shrinking genes
Chromosomes come packaged in pairs. The other 22 sets of chromosomes in our cells consist of well-matched partners, that carry the same number of genes. But, the XY pairing that determines male sex are odd bedfellows. The X is three times as long as Y; poor old Y is a runt with only a few dozen genes, while X has 1500. And yet, the discovery that Y contains just a few genes that have almost exact counterparts on the X leads to the likely explanation that the two share a distant ancestor, and are evolved from a pair of matching chromosomes.

What all this tells us is that Y has shrunk over millions of years of human evolution and it has lost almost all of its original genes. The Y that you boys inherit now is a shadow of its former self. It contains the relics of several genes that no longer function, suggesting that decay is an ongoing process. A team of Australian scientists have estimated that 'at its current rate of decay Y will self-destruct in 10 million years'.

Y so different?
In our body cells, chromosomes exist in matching pairs, but when we produce eggs and sperm the matching pairs split up. Each egg or sperm cell goes on its journey with 23 single chromosomes. Then, when egg and sperm meet, the normal number of chromosomes is restored to 23 pairs (46 in total) in the baby. In the process of making sperm, the chromosomes in each matching pair temporarily bond and exchange stretches of their genes between one another. Then they split up and only one of the pair is packaged into each sperm.

Before X and Y became so different, they too would have swapped genes. But, along the evolutionary way, there would have been an advantage to keeping male-making genes together on the same chromosome. With the fertility of the male at the heart of his lasting survival; inheriting half a quota of male-making genes would be an evolutionary dead end.

Gene swapping had to stop. Isolating X from Y guaranteed that male-making genes were inherited together so that the Y chromosome and the men that carried it would endure the test of time. The rest is history. Now only the very tip of the X and Y chromosomes can engage in gene trading — a vestige of a more united past.

Defences down
Y is the most vulnerable of all the 23 chromosomes. It accumulates mutations faster than any of the others. A mutated Y is the cause of infertility in 5-15% of severely infertile men, many of whom have been found to have large chunks of the Y missing. Even tiny deletions on the Y can trigger lowered fertility. It is a double whammy — some of the genes that are vital to male fertility are clustered on the most defenceless of all chromosomes. Worse still, fertility is the essence of evolutionary endurance. So, why is Y such a weakling?

First, Y does not have a twin in the way that all other chromosomes do. If the genes on Y are damaged, there is no matching partner to act as a blueprint for the repair of the damage.

Second, Y chromosomes always pass into the next generation via sperm, never via the egg. But, sperm doesn't protect its chromosomes from damage like other cells do. Sperm present an especially risky environment for the little unmatched Y.

And third, men produce thousands of sperm every minute, requiring millions upon millions of cell divisions to take place. Every time a cell divides it has to make a copy of the original chromosomes — and the potential for errors in copying is hugely increased. And mistakes make mutations.

The result — Y is on its way out. Before you reach for the whiskey bottle, boys, take heart, the death of Y doesn't have to mean the death of the male.

New start
It is true that in the world of a diminishing Y, males will gradually become less and less fertile. But, in the long term, evolution will favour any male who carries mutations on any chromosome that makes him more fertile.

If it is of any comfort, men need only turn to the example of a small, burrowing rodent. The mole vole has already moved into a Y-free zone and still has males. No-one knows how this has happened, but on the way they must have evolved new genes on other chromosomes that govern the process of making males.

There are two species of mole vole with different types of sex chromosome, which means that they can't mate. One hot topic of research is to find out whether the original species of mole vole split into two when it gave up the Y. If so, this could have implications for our own evolution.

If we humans lose our Y, then it's possible that new sex chromosome systems could arise in the human population. And like the mole vole, people born under one system would not be able to mate with those born under another. Think of a world in which several species of humans co-exist, as Homo sapiens and Neanderthal man did?

The dear old male might be saved after all.