Which reptile's sex is set by nest temperature, not genes?

Crocodiles, alligators, most turtles and some lizards have no sex chromosomes at all. The temperature of the nest during a critical window of incubation decides the outcome. In many turtles, cooler nests give males and warmer ones give females; in crocodiles the pattern is more complicated, with males produced in a middle band and females at both extremes.

It appears to be an ancient arrangement rather than a quirk, and it has an obvious hazard attached. A species with genetic sex determination produces roughly equal numbers whatever the weather. A species relying on temperature does not, and a warming climate can tip whole populations.

That is already measurable. Green turtle rookeries in the northern Great Barrier Reef have been producing overwhelmingly female hatchlings - in one study over 99 per cent of juveniles from the northern population - which is a functioning population now and a demographic problem in a few decades. Some species can adjust by nesting earlier or in shadier sand, and whether that can keep pace is the open question.