The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [62]
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 them72). They travel all over the world plying their peculiar trade.
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 one to one, 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 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 crazy to put it in the genes, for social rank is 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,’ Trivers said. ‘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 the sex ratio of offspring.73
Reason’s Convergent Conclusion
Trivers–Willard predicts that evolution would build in an unconscious mechanism for altering the sex ratio of an individual’s progeny. But we like to think we are rational, conscious decision makers. And a reasoning person can arrive at the same conclusions as evolution. Some of the strongest data to support Trivers–Willard come not from animals but from the human cultural rediscovery of the same logic.
Many cultures bias their legacies, parental care, sustenance and favouritism to sons at the expense of daughters. Until recently, this was seen as just another example of sexism or the cruel fact that sons often have more economic value than daughters. But, by explicitly using the logic of Trivers–Willard, anthropologists have now begun to notice that male favouritism is far from universal and that female favouritism occurs exactly where you would most expect it.
Contrary to popular belief a preference for boys over girls is not universal. Indeed, there is a close relationship between social status and the degree to which sons are preferred. Laura Betzig of the University of Michigan noticed that, in feudal times, lords favoured their sons, but peasants were more likely to leave possessions to daughters. While their feudal superiors killed or neglected daughters or banished them to convents, peasants left them more possessions. Sexism was more a feature of élites than of the unchronicled masses.74
As Sarah Blaffer Hrdy of the University of California at Davis has concluded, wherever you look in the historical record, the élites favoured sons more than other classes: farmers in eighteenth-century Germany, castes in nineteenth-century India, genealogies in medieval Portugal, wills in modern Canada and pastoralists in modern Africa. This favouritism took the form of inheritance