Hotter Temperatures Trigger Sex Change in Australian Lizards

Hotter temperatures are messing with the gender of Australia's bearded dragon lizards, a new study finds. Dragons that are genetically male hatch as females and give birth to other lizards. And the way the lizards' gender is determined is getting changed so much that the female sex chromosome may eventually disappear entirely, the study authors say.

"This is the first time we have proved that sex reversal happens in the wild in any reptile at all," said Clare Holleley of the University of Canberra, lead author of the study in the journal Nature Wednesday. The study, she said, "is showing that climate extremes can very rapidly fundamentally alter the biology of an organism."

Image: Bearded dragon lizard
Hotter temperatures are messing with the gender of Australia’s bearded dragon lizards, a new study finds. Arthur Georges / University of Canberra, Australia via AP

To understand what's happening, it helps to a have a quick lesson in the birds and the bees — and the bearded dragons and other reptiles. Some reptiles, like alligators and some turtles, have their genders determined not by sex chromosomes, like humans and other mammals, but by temperature during incubation. Until now, bearded dragons had their gender based on chromosomes. Like birds, their sex chromosomes are Z and W instead of X and Y. Males are ZZ. Females are ZW. In humans, everyone has an X and the presence of Y makes a person genetically male. In bearded dragons, everyone has a Z and the presence of a W makes them a genetic female.

In the past, scientists have shown in the lab that hot temperatures can switch that natural chromosome-based gender. Holleley and colleagues examined the genetic sex markers of 131 wild-caught bearded dragons in Queensland province and found that 11 of them were female outwardly — even having offspring — but had the ZZ chromosomes of a genetic male. Their sex determination was "switched into overdrive," Holleley said.

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Holleley concedes 11 dragons is a small sample size, so she and colleagues will continue and expand their research.

The genetic-male-into-female dragons not only laid eggs, but in a way were better mothers than genetically determined females, laying more eggs, said study co-author Arthur Georges, chief scientist for the Institute for Applied Ecology at the University of Canberra. The team also found that the offspring of these dragons no longer have their gender determined by chromosomes, but by temperature. "They're throwing away the equivalent of the Y, which we call the W, chromosome," Georges said. "If the climate warms not much more at all, the percentage of sex reversal will increase and the W chromosome will be lost from the population."