and use spare sons as nannies for subsequent broods.
Among zebra finches, as Nancy Burley of the University of California at Santa Cruz discovered, ' attractive' males mated with 'unattractive ' females usually have more sons than daughters, and vice versa. Attractiveness in this species can be altered by the simple expedient of putting red (attractive) or green (unattractive) bands on the male 's legs, and black (attractive) or light blue (unattractive) on the female 's legs. This makes them more or less desirable to other zebra finches as mates.'
But we are not birds: The only way to be certain of rearing a boy is to kill a girl child at birth and start again, or to use amnio-centesis to identify the gender of the fetus and then abort it if it 's a girl. These repugnant practices are undoubtedly on offer in various parts of the world. The Chinese, deprived of the chance to have more than one child, killed more than
It is possible that selective spontaneous abortion also explains much of the animal data. In the case of the coypu, studied by Morris Gosling of the University of East Anglia, females in good condition miscarry whole litters if they are too female-biased, and they start again. Magnus Nordborg of Stanford University, who has studied the implications of sex-selective infanticide in China, believes that such biased miscarriage could .explain the baboon data. But it seems a wasteful way to proceed: 68
There are many well-established natural factors that bias the sex ratio of human offspring, proving that it is at least possible. The most famous is the returning-soldier effect. During and GENETIC MUTINY AND GENDER
::: 123 :::
immediately after major wars, more sons are born than usual in the belligerent countries as if to replace the men that died. (This would make little sense; the men born after wars will mate with their contemporaries, not with those widowed by the war). Older fathers are more likely to have girls, but older mothers are more likely to have boys. Women with infectious hepatitis or schizophre-nia have slightly more daughters than sons: So do women who smoke or drink. So did women who gave birth after the thick London smog of 1952. So do the wives of test pilots, abalone divers, clergymen, and anesthetists. In parts of Australia that depend on rainfall for drinking water, there is a clear drop in the proportion of sons born 320 days after a heavy storm fills the dams and churns up the mud. Women with multiple sclerosis have more sons, as do women who consume small amounts of arsenic.'
Finding the logic in this plethora of statistics is beyond most scientists at this stage. William James of the Medical Research Council in London has for some years been elaborating a hypothesis that hormones can influence the relative success of X
and Y sperm: There is a good deal of circumstantial evidence that high levels of the hormone gonadotrophin in the mother can increase the proportion of daughters and that testosterone in the father can increase the proportion of sons.'°
Indeed, Valerie Grant 's theory suggests a hormonal explanation for the returning-soldier effect: that during wars women adopt more dominant roles, which affects their hormone levels and their tendency to have sons. Hormones and social status are closely related in many species; and so, as we have seen, are social status and sex ratio of offspring. How the hormones work, nobody knows, but it is possible that they change the consistency of the mucus in the cervix or even that they alter the acidity of the vagina. Putting baking soda in the vagina of a rabbit was proved to affect the sex ratio of its babies as early as 1932.'
Moreover, a hormone theory would tackle one of the most persistent objections to the Trivers-Willard theory: that there seems to be no genetic control of the sex ratio. The failure of animal breeders to produce a strain that can bias the gender of its off-
::: 124:::
spring is glaring: It is not for want of trying: As Richard Dawkins put it: 'Cattle breeders have had no trouble in breeding for high milk yield, high beef production, large size, small size, hornless-ness, resistance to various diseases, and fearlessness in fighting bulls: It would obviously be of immense interest to the dairy industry if cattle could be bred with a bias toward producing heifer calves rather than bull calves: All attempts to do this have singular-ly failed.''
The poultry industry is even more desperate to learn how to breed chickens that lay eggs that hatch into chicks of only one gender: At present it employs teams of highly trained Koreans, who guard a close secret that enables them to sex day-old chicks at great speed (though a computer program may soon match them'): They travel all over the world plying their peculiar trade. It is hard to believe that nature is simply unable to do what the Korean experts can do so easily.
Yet this objection is easily answered once the hormonal theory is taken into account. Munching enchiladas in sight of the Pacific Ocean one day, Robert Trivers explained to me why the failure to breed sex-biased animals is entirely understandable: Suppose you find a cow that produces only heifer calves: With whom do you mate those heifers to perpetuate the strain? With ordinary bulls—
diluting the genes in half at once.
Another way of putting it is that the very fact that one segment of the population is having sons makes it rewarding for the other segment to have daughters. Every animal is the child of one male and one female. So if dominant animals are having sons, then it will pay subordinate ones to have daughters. The sex ratio of the population as a whole will always revert to I:I, however biased it becomes in one part of the population, because if it strays from that, it will pay somebody to have more of the rare gender. This insight occurred first to Sir Ronald Fisher, a Cambridge mathematician and biologist, in the 1920s, and Trivers believes it lies at the heart of why the ability to manipulate the sex ratio is never in the genes:
Besides, if social rank is a principal determinant of sex ratio, it would be
::: 125 :::
almost by definition something that cannot be in the genes. Breeding for high social rank is a futile exercise in Red Queen running.
Rank is relative. 'You can't breed for subordinate cows, ' said Trivers as he munched. 'You just create a new hierarchy and reset the thermostat. If all your cows are more subordinate, then the least subordinate will be the most dominant and have appropriate levels of hormones: ' Instead, rank determines hormones, which determine sex ratio of offspring.'
REASON'S CONVERGENT CONCLUSION
