The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [58]
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.
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.51
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 for 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–Willard was right at least some of the time.52
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.53
Clutton-Brock’s result alerted primatologists, who had long been suspecting 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 babies born to lowest-ranked females, all were female; of eight born to highest-ranked females, six were male; those in middle ranks had an equal sex ratio.54
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.55
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;