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Some lizards that begin developing as males will actually hatch as females if the egg is particularly warm – and now we know why. The heat triggers genes that override chromosomal sex determination.

In the 1960s, French scientists discovered that reptiles in Senegal would hatch as females when temperatures rose much above about 30°C. Since then, researchers have noted that the sex of many reptiles and some fish actually depends entirely on the temperature during their development.

In a few animals, like the central bearded dragons (Pogona vitticeps) of Australia, sex determination depends on both genetics and temperature. Males have two identical sex chromosomes – ZZ – and females have two different sex chromosomes – ZW. But male embryos will develop as females if the egg is warm enough. This means females may develop in one of two ways, but the mechanisms behind this phenomenon have eluded scientists for more than half a century.

To explore the mystery, Sarah Whiteley at the University of Canberra in Australia and her colleagues ran genetic sequencing on unhatched bearded dragons incubated either at 28°C – cool enough for ZZ embryos to hatch as males – or at 36°C – warm enough for ZZ embryos to hatch as females.

For the eggs at 36°C, the researchers found that ZW female embryos had “dramatically” different active genes during the major stages of sex development, compared with ZZ females, demonstrating there are two distinct sets of genes that can make a central bearded dragon female. In the ZZ females, the genes that “wanted” to code for male development were forcibly switched off, and those for female development were switched on.

“The sex chromosomes in the dragon are… more recently developed – on an evolutionary timescale – compared to [human] sex chromosomes,” Whiteley says. “So sex reversal might be a relic of temperature sensitivity [alone].”

Even so, two ZZ males in her study “resisted” sex reversal, Whiteley says. While exact numbers aren’t yet available, an estimated 1 per cent of ZZ embryos hatched as males even when the eggs were incubated at 36°C.

Why sex would be determined by temperature remains unknown. But a greater female-to-male ratio at higher temperatures might be helpful for the survival of the species as the climate warms, says Whiteley. “It’s better that it’s more females [hatching] than males because… females are the ones that determine the reproductive output of the population,” she says.