The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [56]
A second way of determining gender is to leave it to the environment. In some fish, shrimps 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 all. 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 peculiar. Those in the north Atlantic determine their gender by genes as we do; those further south use the temperature of the water to set the gender of the embryo.41
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.42 Indeed, no biologist has a watertight explanation of 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 and large ones win) 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 are), then it would pay to make warm eggs hatch as the gender that most benefits from being large.43 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.44
A third way of determining gender is for the mother to choose the sex of each child. The most impressive way of achieving this is peculiar to monogonont rotifers, bees and wasps. Their eggs become female only if fertilized. Unfertilized eggs hatch into males (this means that males are haploid and have only one set of genes to the females’ two). Again, this makes some 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, which 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 for itself that it is in the rare, and therefore sought-after, gender.45
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. For if they could distinguish between X and Y sperm, even birds and mammals could bias the sex ratios of their offspring and they would be selected to do so in exactly the same way as crocodiles and nematodes