By Mike Leahy with Hilary MacQueen

Although I'm a biologist, I rarely think much about what the cloudy white stuff contains, but in fact semen is far more complex than my endearing childhood image of mini tadpoles swimming around in thin wallpaper paste.

There are many features of human semen that are very familiar to most people, but their importance isn't appreciated. For a start, the ejaculate isn't of a uniform consistency. As the man ejaculates, the first 'spurt' contains far more sperm than subsequent spurts. This is why the withdrawal method of contraception is so dangerous. If the man is only a split second too late then the most potent portion of semen ends up exactly where he or his partner doesn't want it. In fact, it has been suggested that should the couple mistime things, they are best off not bothering to withdraw at all, but please don't try that one at home.

Sperm: the active ingredient
First, let's talk numbers. The quantity of sperm cells that men produce varies widely. In general, it is said that men may produce between two millilitres (2ml) and 5ml of semen each time they ejaculate, and that each millilitre may contain from 20 million to 300 million sperm cells. That means a fertile man may produce between 40 million and 1800 million sperm cells in total, though the majority produce between 40 and 60 million sperm cells per millilitre, giving an average total of 80 to 300 million sperm per ejaculation.

Before going ahead with this episode of Lab Rats, Zeron and I were checked out at our local hospitals and our sperm count calculated. Of course, had we known the results before filming, the programme would have been a farce, so the results were kept secret from us. Without the pressure of filming, the long hours, lack of privacy and tiredness, I produced about 300 million spermatozoa in 4ml of semen and Zeron pumped out over 80 million in nearly 3ml, so both of us were found to be perfectly fertile. This was cool, because neither of us would have wanted to find out that we were infertile, and because Zeron has kids it would have been a big worry for him had it been shown that he was! (After all, up to one in seven British children are not the genetic offspring of the man considered to be their father.)

But sperm count alone is not the only factor that determines fertility. To understand the biology behind male fertility it must first be understood how sperm are produced, how they are transported from the man, and then how they fight their way to their final prize - the egg that may be waiting for them at the end of the female genital tract. Just to think about it makes me feel guilty, because a rough calculation shows that I've disappointed about 7,500,000,000,000 of the little fellas.

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