In the 1980s, scientists trying to save sea turtles noticed something truly bizarre.
They thought they were doing something good: rescuing eggs from vulnerable beaches and keeping them warm in incubators until they were ready to swim out to sea.
But when the sea turtles were born, almost every single one of them was male. At that point, scientists had known for some 80 years that sex was determined by a creature’s chromosomes. It seemed crazy that you could skew a hatchling’s gender just by taking its egg out of the sand — just as crazy as saying that the gender of a baby depended on where its mother lived while she was pregnant.
And yet here were dozens of all-male sea turtle siblings wriggling in front of them, emphatically suggesting that sex wasn’t as straightforward as it seemed.
What those scientists encountered was temperature-dependent sex determination (TSD), a phenomenon found in a range of cold-blooded animals. Unlike mammals, birds and other creatures, whose sex is set by the chromosomes they get from their parents, the trigger that causes turtle embryos to develop into baby boys or girls comes from outside the egg.
Warmer ambient temperatures during incubation will make the hatchlings skew toward female. But keep the eggs just a few degrees cooler — as the scientists in the ’80s inadvertently did — and they’ll come out mostly male…