The Flight of the Iguana by David Quammen Page A

Under the Charnov-Bull model, by contrast, sex determination is just an intermediate step on the long path toward what might be called (in California, anyway) self-actualization.
    â€œTo illustrate,” say Charnov and Bull, “if an individual finds itself in an environment where it can become a below average female or an above average male, selection will favor its becoming male because it can pass on more of its genes than if it were female.” And vice versa. “Selection may therefore favour ‘environmental sex determination’ (ESD) because of the control it allows an individual.” Fine, that seems reasonable—but several complicated assumptions lie behind this simple statement.
    One assumption is that the evolutionary fitness of an individual (as tallied, always, by how many offspring survive) is measured relative to other individuals of the same species and the same sex. In other words, a male alligator competes only against other male alligators for the right to leave a large share of descendants. Another assumption made by Charnov and Bull is that the environment in question is patchy, with some patches conferring heightened advantage on females, other patches conferring heightened advantage on males.
    From these two givens, Charnov and Bull argue that natural selection “should most strongly favour ESD when 1) the offspring enters an environment, away from the parent, which has a large effect on its lifetime fitness . . . and 2) the offspring and the parent have little control (or predictive ability) over which patch type the offspring enters. These conditions make it unfavourable to determine sex at conception because of the possibility that a male will enter a patch that is much more favourablefor a female, or the reverse.” So ESD allows an individual to match its own sexual identity with the particular patch of environment that will heighten its prospect under that identity—not by choosing the patch, but instead by choosing the sexual identity.
    Among alligators the patch differences seem to be a matter of temperature at which the eggs are incubated. Nests built on dry levee (according to that elaborate field study, done in Louisiana) tend to stay warmer than ninety-three degrees F. and to yield males. Nests built on wet marsh tend to stay cooler than eighty-six degrees and to yield females. In either case the environment during incubation may have a large effect on lifetime fitness, because that temperature difference translates to size of the individual at hatching, which in turn affects how soon the individual will reach breeding size. Females have a greater need than males do to reach breeding size quickly, because their allotted span of breeding years is much shorter. (That part applies also to humans, providing an evolutionary rationale for why prepubescent fifth-grade boys should find themselves mystified by their classmates’ burgeoning breasts.) The consequence of all this is that cool nest temperatures—which turn out larger baby alligators—give most females a head start toward breeding size. So if an individual alligator is relatively small at the time of hatching, it will fare better in the lifelong Darwinian competition as an average male than as a below average female. And, fortunately, ESD will have already matched its sex to its best prospects.
    But what about those ridiculous Bonellia? They seem to be covered equally well by the Charnov-Bull model. In this case the patchiness of the environment is a matter of presence or absence of female Bonellia. The larvae can’t control where they travel, and the adult females are sessile, traveling nowhere at all. When any one larva encounters a female, that fact has great effect on its lifetime fitness, because it means that the little devil will have an opportunity to mate. Otherwise he would be forced to take hisown chances as a female, with some jeopardy of permanent spinsterhood. ESD