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Primogeniture and Primatology

In the course of the neo-Darwinian revolution of the 1960s and 1970s, Britain and America each produced a grand old revolutionary whose intellectual dominance remains secure to this day: John Maynard Smith and George Williams respectively. But each country also produced a brilliant young Turk whose precocious intellect exploded upon the world of biology like a flare. Britain’s prodigy was Bill Hamilton, whom we have already met. America’s was Robert Trivers, who as a Harvard student in the early 1970s conceived a whole raft of new ideas that proved far ahead of their time. Trivers is a legend in biology, as he is the first ingenuously to confirm. Unconventional to the point of eccentricity, he divides his time between watching lizards in Jamaica and thinking in a redwood grove near Santa Cruz, California. One of his most provocative ideas, conceived jointly with a fellow student, Dan Willard, in 1973, may hold the key to understanding one of the most potent and yet simple questions a human being ever asks: ‘Is it a boy or a girl?’47

It is a curious statistical fact that between them all forty-two presidents of the United States have had ninety sons and only sixty-one daughters. A sex ratio of sixty per cent male in such a large sample is markedly different from the population at large, though how it came about nobody can guess – probably by pure chance. Yet presidents are not alone. Royalty, aristocrats and even well-off American settlers have all consistently produced slightly more sons than daughters. So do well-fed opossums, hamsters and coypus and high-ranking spider monkeys. The Trivers–Willard theory links these diverse facts.48

Trivers and Willard realized that the same general principle of sex allocation, which determines the gender of nematodes and fish, applies even to those creatures that cannot change sex but that take care of their young. They predicted that animals would be found to have some systematic control over the sex ratio of their own young. Think of it as a competition to have the most grandchildren. If males are polygamous, a successful son can give you far more grandchildren than a successful daughter, and an unsuccessful son will do far worse than an unsuccessful daughter because he will fail to win any mates at all. A son is a high-risk-high-reward reproductive option compared to a daughter. A mother in good condition gives her offspring a good start in life, increasing the chances of her sons winning harems as they mature. A mother in poor condition is likely to produce a feeble son who will fail to mate at all, whereas her daughters can join harems and reproduce even when not in top condition. So you should have sons if you have reason to think they will do well and daughters if you have reason to think they will do poorly – relative to others in the population.49

Therefore, said Trivers and Willard, especially in polygamous animals, parents in good condition probably have male-biased litters of young; parents in poor condition probably have female-biased litters. Initially this was scoffed at as far-fetched conjecture, but gradually it has received grudging respect and empirical support.

Consider the case of the Venezuelan opossum, a marsupial that looks like a large rat and lives in burrows. Steve Austad and Mel Sunquist of Harvard were intent on disproving the Trivers–Willard theory. They trapped and marked forty virgin female opossums in their burrows in Venezuela. Then they fed twenty of them each with 125 gram of sardines every two days by leaving the sardines outside the burrows, no doubt to the delight and astonishment of the opossums. Every month thereafter they trapped the animals again, opened their pouches and sexed their babies. Among the two hundred and fifty-six young belonging to the mothers who had not been fed sardines, the ratio of males to females was exactly one to one. Among the two hundred and seventy from mothers who had