Slippery dicks and X-rated iguanas

There are many literary stars (such as Stephen Jay Gould, Richard Dawkins and Matt Ridley) in the firmament of writers on evolution, and to a man they write with dash and persuasive logic. David Bainbridge is one such and in his latest book he takes the reader through the glories of the X chromosome at a cracking pace. Olivia Judson is a welcome female addition to this boys' club, and she can more than hold her own - as an award-winning writer who is a Stanford alumna with an Oxford doctorate, currently a research fellow at Imperial College London. But for our purposes she's Dr Tatiana, zoological agony aunt to the whole animal kingdom, answering letters from bed bugs to salamanders about their sexual problems with enough wit to make you laugh out loud.

Bainbridge will make many female friends with his eulogy to the X chromosome, the female chromosome with which women are doubly blessed.

Men are allowed only one and it is just as well they are because the X chromosome is not simply the most interesting chromosome, it determines whether we live at all. The fact that women have a double dose of it means that every woman is two genetically different animals stirred together. In what more all-encompassing way could women be more complex than men? By contrast, all of a man's cells are just about genetically identical to each other.

Male superiority may be an ancient fortress, but all early embryos form with the intention of becoming a girl, implying that being female is the dominant state. The male Y chromosome has to wrest the embryo away from being a girl. And despite the fact that a man in chromosomal terms is 50 per cent Y (XY), this is the only service his poor Y chromosome performs for him.

While the X chromosome is loaded with essential life-giving genes, the Y chromosome, a shrunken waif in comparison, contains virtually none, self-evident as women spend their whole lives without it. The Y chromosome is a wasteland full of junk fragments of damaged genes. The only exception to this bleak picture is a single gene, named Sry , the switch, which when flicked on for a mere two days in a four-week-old embryo, sets off a cascade of events that starts moulding the embryo into a boy. The Y chromosome turns out to be just a vehicle to carry the Sry gene that is so obsessed with controlling gender it has become incapable of doing anything else.

How did the Y chromosome become so tacky? Within the past 30 years genetic science has come up with compelling evidence that the X and Y were originally a robust matching pair swapping genes over most of their length.

At some point they became estranged and parted for ever. When the Y went its separate way it consigned itself to a grim future: it could no longer enjoy the exchange of damaged genes for good genes from the X and it can never exchange old genes for new with other Ys because it hardly ever meets up with another one in the same cell. The X fared much better than its old partner. It was much more sociable - every X could spruce itself up by swapping genes with the other X in the cells of females. The X has remained forever young and not suffered the degradation that has blighted Y.

The presence of one X chromosome or two determines not only our gender but most of our sexual behaviour, and to track its evolution we could hardly do better than dip into Dr Tatiana's postbag. It is a little unnerving to find that she and I are asked the same kind of questions, though she tends to deal with animals of the lower orders, whereas my correspondents are at the top of the vertebrate tree.

Here's one of hers. "Dear Dr Tatiana, I'm a marine iguana, and I'm appalled by the behaviour of young iguanas of today. I keep encountering groups of youths masturbating at meI I'm sure they didn't act this way in Darwin's time. How can I make them stop?"

It is even more unnerving that we dole out the same kind of reassurance. My readers too are beset by worries about masturbation, but it turns out that many primates of both genders masturbate a lot. Some female West African monkeys use their hands to stimulate themselves during sex - advice I frequently give to anorgasmic women.

And in the spirit of reciprocity here is one of mine on the vexed question of genital size. "I am a 28-year-old male and I am still a virgin. The main reason is the size of my penis. I've read that the average size of a man's penis is about 3-4in and double when erect. My penis is about 1.5ins and when erect is only about 3.5-4in. I tried sex one night with my girlfriend and it was a complete disaster."

My litany is that size doesn't matter; Dr Tatiana adds a new dimension: "There's often no relationship between the dimensions of a man and the dimensions of his privates: bigger men do not necessarily have bigger bits. Indeed... it's often the opposite. The California singing fish takes this to extremes. Males have either big brains or big balls." Plus ça change .

Dr Tatiana calls into question that other general law of nature: that males can be promiscuous but females must remain chaste. From stick insects to chimpanzees (and according to my postbag, Homo sapiens too) females are hardly ever faithful. In species after species, rampant promiscuity is no malfunction. Rather, females and their offspring benefit from it. Female rabbits and Gunnison's prairie dogs both show higher rates of conception if they mate with several partners. The female sand lizard hatches out more eggs the more lovers she's had. The female slippery dick (a pale fish that lives on coral reefs) will have more of her eggs fertilised if she spawns with a gang than if she spawns with just one chap. Gang-bangs are common in the animal kingdom.

This is partly because lions, rats and many more require the vigorous stimulation of copulation with several partners before they can conceive, and the same urge may lie behind the common female sex fantasy of going solo with the Arsenal football team.

Females need this excitement so much that they will trade food for sex - the way to a man's heart and all that. Female field grasshoppers will exchange nourishment for sex with as many as 25 different males.

Promiscuous females lay more batches of eggs and more eggs per batch than females who are monogamous.

It comes as no surprise that males, from drones to men, object to this lack of specificity and some go to great lengths to counteract it. "Dear Dr Tatiana, I'm a queen bee, and I'm worried. All my lovers leave their genitals inside me then drop dead. Is this normal?" It's all down to male jealousy. In leaving their genitals behind, her lovers are trying to block up the queen bee in the hope that she'll not be able to mate with another.

What an effective chastity belt! But not advice I can pass on to my readers.

Miriam Stoppard, FRCP, is the health advice columnist in The Daily Mirror .