Gender is determined genetically in mammals and birds, but for many reptiles it is the temperature at which the eggs are incubated that governs whether they will hatch as males or females. For bearded dragons, though, it is neither one nor the other – or, rather, a bit of both.
These spiny Australian lizards start off genetically male or female. However, a genetic male incubated at a temperature above 32°C becomes a fully functional female. And according to new research, it gets even stranger.
These individuals resemble males in both appearance and behaviour – unlike ‘normal’ females, for example, they are particularly bold and active. In fact in many measures they are even more male-like than genetic males.
It is not clear whether these ‘transgender’ individuals are favoured by selection or a problematic side effect of a complex system of sex determination.
“I think the mechanism is likely adaptive,” said Richard Shine, a professor in evolutionary biology at the University of Sydney who led the research. “For some reason, it pays to be a female if you incubate in a very hot nest.”
He added that both types of female may persist because each is favoured in different circumstances.
“If running around more and being bold pays off – for example, if predators are scarce – then the sex-reversed girls will probably thrive. But in a world full of predators quick to detect a more active animal, sex-reversed girls may have short lives and unhappy final moments.
“I’d love to know about the ecology of these animals in the wild – how they fit into the normal social system, and their fitness, spatial ecology and foraging behaviour.”
There is some evidence that sex-reversed females are increasing, possibly in response to rising global temperatures.
Indeed, if transgender females do outcompete genetic females in certain conditions, it suggests a mechanism for why some reptiles have entirely lost a genetic basis for sex determination.