HILDREN, sit up straight and stop canoodling in the back there! This is Sex Ed revision and there's a lot to take in. The curriculum is entirely different from last year because of all we have been learning from the studies of evolutionary biologists.

You all remember last year's bottom line:

Thou shalt have one wife only; who
Would be at the expense of two?

Please delete that from your notebooks. We're through with hypocrisy and rational expectations. Write this instead:

The gods of science now decree:
Thy wives shall number one point three.

As the conviction of a Utah polygamist on Friday reminds us, monogamy is not the only way to procreate. It's just a theme 'round which nature has designed many variations. How naturally monogamous is man? Biologists who study primates figure that polygyny — a male's acquisition of many females — is mirrored in the ratio of male to female body size, with the larger the ratio, the higher degree of polygyny. Male gorillas, who run large harems, are twice as large as females. Male chimps, who in groups dominate smaller bands of females, are about a third larger than females. But men are just 15 percent larger than women, suggesting that "mild polygyny" is the natural condition of the human species, according to Dr. Alison Jolly of Princeton University.

Only since the advent of DNA paternity tests some 15 years ago have biologists started to pay attention to polyandry, a female's taking of more than one partner. Among reed bunting couples, for example, they found that 55 percent of chicks are not sired by the father of record, and the same turned out to be true in every group of creatures studied. "In the majority of species, right across the animal kingdom, the general pattern is for females to copulate with more than one male," writes Dr. Tim Birkhead in his book "Promiscuity" (Harvard, 2000).

Biologists began to see female sexual behavior with new eyes. Those male blackbirds fluttering around their mates weren't displaying loyal devotion at all — they were guarding their untrustworthy partners from the attentions of other males. So the question arose: what do females gain from promiscuity? Quite a lot, it turns out. If a mate's sperm is no good, an extra partner may help. And by sleeping around, a female can buy protection for her children since a male who might otherwise kill them may be led to think he is their father.

Under polyandry, males must compete not just mano a mano but even sperma a sperma. "Sperm competition" is a hot new field of study among biologists, as is "cryptic female choice," a female's unconscious selection of one individual's sperm over another's. No one really knows how much of this goes on in our own species. Since female chimpanzees mate often (one female observed by Jane Goodall copulated with 7 males 84 times in 8 days) male chimpanzees need enormous testes so that their sperm can flood out the competition. Gorillas don't tolerate polyandry so they need only small testes. People seem to fall somewhere in between. "Sperm competition is possible in Homo sapiens," says Dr. Alan F. Dixson in "Primate Sexuality" (Oxford University Press, 1998), "though whether it has played a significant role during human evolution remains highly debatable."

Those DNA paternity tests so revealing of animal mores cannot, for ethical reasons, be applied to human populations.

But genetic counselors, who often stumble across cases of discrepant paternity, believe some 10 percent of American children may be born this way. We have inherited our own version of the primate mating system, one in which both sexes possess an inclination to monogamy that falls somewhat short of total embrace.

That, children, is the lesson for today. And next time you look over a cradle, into those innocent eyes and half-formed smile, remember the old saying, "Mother's baby, father's maybe." and resolve to have a serious talk one day with your own mother, when you are a little older.