This sex change makes good sense from the fish' s point of view because there is a basic difference between the risks and rewards of being male or female: A large female fish can lay only a few more eggs than a small one, but a large male fish, by fighting GENETIC MUTINY AND GENDER
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for and winning a harem of females, can have a great many more offspring than a small male. Conversely, a small male does worse than a small female because he fails to win a mate at all. Therefore, among polygamists the following strategy often appears: If small, be female; if large, be male:"
There is a lot to be said for such stratagems. It is profitable to be a female while growing up and get some breeding done, and then change sex and hit the jackpot as a polygamist male once you are big enough to command a harem: Indeed, the surprise is that more mammals and birds do not adopt this system: Half-grown male deer spend years in a state of celibacy awaiting the chance to breed, while their sisters produce a fawn a year.
A second way of determining gender is to leave it to the environment. In some fish, shrimp, and reptiles, gender is determined by the temperature at which the egg is incubated. Among turtles, warm eggs hatch into females; among alligators, warm eggs hatch into males; among crocodiles, warm and cool eggs hatch into females, intermediate ones into males: (Reptiles are the most adventurous sex determiners of alclass="underline" Many lizards and snakes use genetic means, but whereas XY iguanas become male and XX
female, XY snakes become female and XX male:) Atlantic silverside fish are even more unusuaclass="underline" Those in the North Atlantic determine their gender by genes as we do; those farther south use the temperature of the water to set the gender of the embryo. p2
This environmental method seems a peculiar way of going about it: It means that unusually warm conditions can lead to too many male alligators and too few females. It leads to "intersexes,"
animals that are neither one thing nor the other: 43 Indeed, no biologist has a watertight explanation for why alligators, crocodiles, and turtles employ this technique: The best one is that it is all size related: The warm eggs hatch as larger babies than the cool ones. If being large is more of an advantage to males than females (true of crocodiles, in which males compete for females) or vice versa (true of turtles, in which large females lay more eggs than small ones, whereas small males are just as capable of fertilizing females as large ones), then it would pay to make warm eggs hatch
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The Red Queen
as the gender that most benefits from being large." A clearer example of the same phenomenon is the case of a nematode worm that lives inside an insect larva. Its size is set by the size of the insect; once it has eaten all of its home and host, it grows no more. But whereas a big female worm can lay more eggs, a big male worm cannot fertilize more females. So big worms tend to become female and small ones male.' :
A third way of determining gender is for the mother to choose the sex of each child. One way of achieving this is peculiar to monogonont rotifers, bees, and wasps: Their eggs become female only if fertilized: Unfertilized eggs hatch into males (which means that males are haploid and have only one set of genes to the females ' two. Again, this makes some sort of sense. It means that a female can found a dynasty even if she never meets a male. Since most wasps are parasites that live inside other insects, this may help a single female who happens on an insect host to start a colony without waiting for a male to arrive. But haplodiploidy is vulnerable to certain kinds of genetic mutiny. For example, in a wasp called Nasonia, there is a rare supernumerary chromosome called PSR, inherited through the male line, that causes any female egg in which it finds itself to become a male by the simple expedient of getting rid of all the father 's chromosomes except itself.
Reduced to just the haploid maternal complement of chromosomes, the egg develops into a male. PSR is found where females predominate and has the advantage that it is in the rare, and therefore sought-after, gender.'
This, briefly, is the theory of sex allocation: Animals choose the appropriate gender for their circumstances unless forced to rely on the genetic lottery of sex chromosomes: But in recent years biologists have begun to realize that the genetic lottery of sex chromosomes is not incompatible with sex allocation. If they could distinguish between X and Y sperm, even birds and mammals could bias the sex ratios of their offs pring, and they would be selected to do so in exactly the same way as crocodiles and nematodes—to produce more of the gender that most benefits from being bigger when the offspring are likely to be big."
GENETIC MUTINY AND GENDER
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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 on 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 his 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 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?"'
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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The Red Queen
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 alclass="underline" 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:'°