Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [47]
Starting with work published by the evolutionary biologist W. D. Hamilton in 1967, scientists have worked out exactly what sex ratio a female parasitic wasp is expected to produce under a variety of circumstances, such as the number of other females laying eggs in the same host. And nature has been astoundingly obliging in supporting the predictions, down to the last egg. Much of the original research was done in the lab, where the wasps can be induced to lay eggs on fly larvae in dishes, but recent DNA analysis of over three thousand offspring from forty-seven mother wasps collected in the field in Europe confirmed that the equations can be used to predict life in the real world. The theory even applies to the one-celled organisms that cause malaria, which also come in two sexes. Take that, string theory.
A Good Year for Sons
ALTHOUGH on average the number of men and women is roughly equal, the availability of either sex depends on the situation. You will be more likely to find a single man in a bar in Alaska, more apt to meet women at a convention for nurses. As it happens, these concerns about the sex ratio among humans also affected the development of another major theory about sex ratio in nature itself.
It all started when Bob Trivers, the biologist who worked out the ratios of reproductive individuals expected within ant colonies that I discussed above, was a graduate student at Harvard. Trivers was a teaching assistant for a popular course in primate behavior, and as he relates in his collected papers, he had a mathematics student named Dan Willard who took the course as a way to meet women. The math graduate program at Harvard, as elsewhere, was rather like the aforementioned Alaskan bar in terms of its sex ratio (I assume, without really knowing, that the resemblance stops there), but the primate behavior class of nearly three hundred students was about two-thirds female. Trivers never divulges whether Willard's social hopes were fulfilled, but after a lecture about why the sex ratio is usually 50:50, Willard came up with an idea that the two of them later published as a highly influential paper in Science.
Like many keen insights, this one seems simple once you hear it. Having offspring is costly, in the sense that it requires energy from the mother to produce eggs or babies. And as pregnant women the world over are acutely aware, the condition of the mother affects the condition of the baby, often for a long time after birth. Even among insects, better-nourished mothers can often produce larger eggs that in turn develop into more robust larvae. Although of course it would be ideal to always be in the best shape possible and produce the highest quality offspring a mother can, the situation doesn't always allow mothers to be in that tiptop condition.
Trivers and Willard reasoned that the consequences of producing a baby that is of less than optimal condition will differ depending on the sex of that baby. Because males in many animal species compete vigorously for mates, only males of the highest quality are expected to be successful in fertilizing a female. Producing a weakling son is therefore unlikely to yield any reproductive payoff for the mother. On the other hand, if one's son is very successful in combat with other males, he can potentially sire many more offspring than a single female, even one in the best condition. Even a daughter in poor condition, however, will almost certainly find a mate and reproduce. So Trivers and Willard predicted that when circumstances keep the mother from pouring a lot of resources into her offspring, she would be more likely to have daughters than sons, and vice versa for mothers lucky enough to be at the top of their game.
The mechanism behind such a sex bias is not entirely clear. No one,