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In turtles, the sand decides the sex

There are no sex chromosomes. There is a thermometer.

In turtles, the sand decides the sex
Photo: NPS Staff · Public domain · Wikimedia Commons

Most turtles, all crocodilians and many lizards have no sex chromosomes at all. Sex is determined by the temperature of the nest during a window in the middle third of incubation. In many sea turtles the pivotal temperature is around 29.5 degrees: cooler produces males, warmer produces females, and a swing of two degrees can produce an entire clutch of one sex.

Which is why this is no longer only a curiosity. Surveys of green turtle populations on the northern Great Barrier Reef have found juveniles running over 99 per cent female. The mechanism that let the species track its climate for a hundred million years now tracks a climate moving faster than it can.

Sources

Bull, Quarterly Review of Biology (1980); Jensen et al., Current Biology (2018)

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General information, not medical advice. Anything persistent or painful belongs with a doctor, who has heard it before.