I might live to see gators from North Carolina colonize our local swamps and creeks. Warmer winters from climate change will entice them and torrid summers push them north. Or maybe not. After recovering from overhunting for making leather handbags, they may now face a stranger crisis. Too many males for available females.
That’s not just a dilemma for crocodilians. The Chinese government frets about a skewed sex-ratio from a rural preference for boys, access to illegal abortions, and the aggravating effect of the one-child policy (now relaxed). The primary sex-ratio presents a sterner challenge to change. It is set at fertilization according to the type of sperm cell, either male carrying the Y chromosome or female with an X. A single gene on the Y, called Sry, makes the difference.
Not so for alligators, crocodiles, turtles and some lizards. The temperature of eggs in the nest decides whether a pair of testes or ovaries form, although the downstream processes are comparable. When croc or gator eggs are incubated at 30° degrees C they turn out female, but mostly male at 34°. Any higher makes a few more females before the lethal zone. Their parents have minor control by shifting materials to make the nest warmer or cooler.
Some species have turned turtle. A higher temperature favors female turtle hatchlings. In a very narrow sense, global warming is welcome news for males seeking a mate and perhaps for population stability since females are arbiters of reproduction. The female sex hormone is evidently key because an injection of estrogen into eggs produces all females, whereas treatment has no such effect on mammalian fetuses. Pollution of waterways by estrogenic plasticizers (phthalates) could make a potential skew of the sexes even skewier.
It might seem logical that during evolution a species like ourselves would abandon a trigger that wouldn’t register because body temperature is constant. But that can’t be the reason because birds, snakes and some reptiles, most of which incubate outside the body, have adopted a genetic method too (though different genes).
The advantage of a genetic trigger is stability for balancing the sex ratio in an uncertain environment. Why, then, if it is so beneficial haven’t crocs et al switched? They have persisted with the supposed ancestral method because it still works for them, and the alternative does have at least one limitation. It might be called the ‘Shrinking Male’ (feminists love it).
Designating a chromosome for maleness runs into a problem when it gets inured to that role. It gathers genes for making sperm while other essential genes transfer to other chromosomes. Hence, it finds little to match on its partner chromosome for exchanging segments of DNA, the process of meiosis that preserves fitness. The Y consequently shrinks and accumulates mutations in genes affecting fertility (common in men). No longer being a married pair, the X chromosome drifts away to become an ‘ex’, and a time can be projected when the Y shrinks to oblivion. The process has gone farthest in dasyurid marsupials (think of Tasmanian devil), which have only one-fifth of the equivalent DNA in humans, and that mostly degenerate DNA babble.
This fuse burns slowly, so there is no risk of the human Y disappearing soon. Our species has devised much faster routes to extinction.
That last word brings me to dinosaurs. They likely had a thermal trigger to decide their sex, like crocs today, but died out from a cataclysmic bolide 65 million years ago. If they didn’t all perish immediately in a fireball, the following impact winter could have finished them off in a generation by skewing the sex-ratio to all-male or all-female. It’s pure speculation, and questionable, because crocs, who existed from long before, survived the impact and will likely again as temperatures rise, provided we preserve some habitat for them.