Like most animals, bearded dragons have a genetic sex based on chromosomes. Rather than mammals' X and Y chromosomes, these reptiles have Z and W—a ZZ chromosome pairing normally makes a male, and ZW a female. However, in a 2007 lab study, scientists found that temperature-induced changes—a rare but recognized feature of some reptiles—could override a lizard's genetic sex. At an incubation temperature over 32 °C (90 °F), all the eggs hatched female, even those that had ZZ chromosomes. It was unclear if this just happened to be a fluke associated with the laboratory environment, or if the same conditions would affect sex out in the wild.

A team of Australian researchers led by Clare E. Holleley set out to investigate this phenomenon for a paper published recently in Nature. With very little fear of humans, the lizards were easy to scoop up and analyze, with scientists checking for demonstrable sex, as well as chromosomal composition.

Of the 131 lizards the team studied, lab tests showed that 11 females had no W chromosome, meaning they were originally males whose sex had changed in response to temperature pressures. Surprisingly, these ZZ females were not only fertile, they laid almost twice as many eggs as ZW mothers. Having mated with ZZ males these extra fecund ZZ females laid eggs that had only Z chromosomes.

"They've completely lost a whole chromosome in one generation," Holleley told New Scientist. The Z generation are technically all genetically male but their genetically male mothers seem to have passed on a propensity for sex change. Their "sons" switched sex at even slightly lower temperatures than they themselves had.

This discovery represents the first known time that two forms of sex determination have coexisted in the wild. This particular moment in bearded dragon evolution is notable because with climate change creating ever warmer weather, temperature determination could change more and more males into ZZ females, driving the species towards extinction.