If you include Barbara and Jenna Bush, daughters of the forty-third president of the United States, it is a curious statistical fact that all the presidents have between them had ninety sons and only sixty-three daughters. A sex ratio of 6o percent 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, coypus, and high-ranking spider monkeys. The Trivers-Willard theory links these diverse facts.49
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
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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 with 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:'°
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 farfetched 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. Steven 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
The reason? The well-fed opossums had bigger babies; bigger males were much more likely to win a harem of females in later life than smaller males: Bigger females were not much more likely to have more babies than small females: Hence, the mother opossums were investing in the gender most likely to reward them with many grandchildren.
GENETIC MUTINY AND GENDER
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Opossums are not alone. Hamsters reared in the laboratory can be made to have female-biased litters by keeping them hungry during adolescence or pregnancy. Among coypus (large aquatic rodents), females in good condition give birth to male-biased litters; those in poor condition give birth to female-biased litters. In white-tailed deer, older mothers or yearlings in poor condition have female fawns more often than by chance alone: So do rats kept in conditions of stress. But in many ungulates (hoofed animals), stress or poor habitat has the opposite effect, inducing a male-biased sex ratio.52
Some of these effects can be easily explained by rival theories. Because males are often bigger than females, male embryos generally grow faster and are more of a strain on the mother.
Therefore, it pays a hungry hamster or a weak deer to miscarry a male-biased litter and retain a female-biased one: Moreover, proving biased sex ratios at birth is not easy, and there have been so many negative results that some scientists maintain the positive ones are merely statistical flukes. (If you toss: a coin long enough, sooner or later you will get twenty heads in a row.) But neither explanation can address the opossum study and others like it. By the late 1980s many biologists were convinced that Trivers and Willard were right at least some of the time:'
The most intriguing results, however, were those that concerned social status. Tim Clutton-Brock of Cambridge University studied red deer on the island of Rhum off the Scottish coast. He found that the mother 's condition had little effect on the gender of her calves, but her rank within the social group did have an effect: Dominant females were slightly more likely to have sons than daughters:'
Clutton-Brock's results alerted primatologists,who had long suspected biased sex ratios in various species of monkey. In the Peruvian spider monkeys studied by Meg Symington, there was a clear association between rank and gender of offspring: Of twenty-one offspring born to lowest-ranked females, twenty-one were female; of eight born to highest-ranked females, six were male; those in the middle ranks had an equal sex ratio.'
But an even greater surprise was in store when other monkeys revealed their gender preferences. Among baboons, howler monkeys, rhesus macaques, and bonnet macaques, the opposite preference prevailed: high- ranking females gave birth to female offspring, and low-ranking females give birth to male offspring. In the eighty births to twenty female Kenyan baboons studied by Jeanne Altmann of the University of Chicago, the effect was so pronounced that high-ranking females were twice as likely to have daughters as low-ranking ones. Subsequent studies have come to less clear conclusions, and a few scientists believe that the monkey results are explained by chance. But one intriguing hint suggests otherwise.'
Symington 's spider monkeys preferred sons when dominant, whereas the other monkeys preferred daughters. This may be no accident: In most monkeys (including howlers, baboons, and macaques) males leave the troop of their birth and join another at puberty—so-called male-exogamy: In spider monkeys the reverse applies: Females leave home. If a monkey leaves the troop it is born into, it has no chance to inherit its mother 's rank: Therefore, high-ranking females will have young of whatever gender stays at home in order to pass on the high rank to them. Low-ranking females will have young of whatever gender leaves the troop in order not to saddle the young with low rank. Thus high-ranking howlers, baboons, and macaques have daughters; high-ranking spider monkeys have sons.'
This is a highly modified Trivers-Willard effect, known in the trade as a local-resource competition model. 58 High rank leads to a sex bias in favor of the gender that does not leave at puberty.
Could it possibly apply to human beings?
DOMINANT WOMEN HAVE SONS?
