The Flight of the Iguana by David Quammen

the lobed proboscis can be extruded out, three feet or more, to grope for passing morsels of food. But the proboscis of Bonellia collects more than nourishment; it also collects mates.
    When a tiny sexless larva of the same species comes into contact with this proboscis, the larva attaches itself there on the tube and (apparently in response to chemical signals) begins the process of turning into a dwarf Bonellia male. Eventually the mature male, still no larger than a caraway seed, will make his way up the proboscis and into the female’s gut, claiming a permanent home within the uterus. There he will live off her as a parasite, a feckless but useful gigolo, conveniently on hand to fertilize her eggs.
    In finding its female host, the Bonellia larva has found also its own sexual identity, its own selfhood. If the same larva had not blundered upon a female proboscis, it would (in most cases, though there are exceptions) eventually have settled down in a rocky cleft and grown into a large bulb-and-tube female itself. The presence or absence of a female proboscis is the crucial environmental fact, in the life of each young Bonellia, that settles the matter of sex.
    This is ESD at its most vivid, and back as early as 1920 Bonellia had already become quietly famous among zoologists as the leading exemplum of the phenomenon. But environmental sex determination seemed then just an oddity, an aberration, the kind of garish and mildly repugnant trick that one would expect from an obscure group of marine invertebrates like the spoon worms. Today we know better. ESD has been discovered also among orchids, nematode worms, crustaceans, lizards, at least one speciesof fish, four or five species of turtle, and the American alligator.
    In the case of the alligator, an elaborate set of experiments and field observations has recently proved that sex determination for this species involves little or no genetic component. Instead, the sex ratio in a litter of hatchling alligators seems to be completely dependent upon the temperature at which those eggs were incubated. An alligator nest maintained at eighty-six degrees F. or cooler will produce nothing but females. The same batch of eggs, if kept at ninety-three degrees or warmer, will hatch out as all males. Alligator eggs have an incubation period of about sixty-five days, but sex determination seems to occur during just the second and third weeks. At temperatures between the range of eighty-six and ninety-three degrees, the nest will yield a mix of males and females.
    Still, the interesting issue is why. Why has the alligator come to depend upon thermal signals, rather than genetic coding, to set the sexual identities of its offspring? Why has Bonellia evolved a system using social contact (or the lack of it) for the same purpose?
    And from that pair of questions derives another, even more puzzling: What could Bonellia and the American alligator have in common?
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    Writing in the journal Nature, Eric Charnov and James Bull have offered a conceptual model that might make evolutionary sense of the whole range of ESD cases. “We propose that labile sex determination (not fixed at conception) is favoured by natural selection when an individual’s fitness (as a male or female) is strongly influenced by environmental conditions and where the individual has little control over which environment it will experience.” Control is the key word. If an organism can’t completely control where it’s going (like a Bonellia larva, riding helplessly on the sea currents), then maybe there is compensatory value inretaining control over what sort of being (male or female) it will be when it gets there.
    This might sound like something from Alice in Wonderland, but in truth there is nothing illogical about it. The apparent reversal of logic merely goes against our preconceptions. We think of sexual identity as virtually a prerequisite to existence.