Sex is complicated, not just the act, but determining male and female. Bearded Dragons have taken this a step further with genetically male females. A new study has revealed the behavior and characteristics of these “non-concordant females”, potentially gaining insight into sex differences in other species.
Mammals, birds, and many reptiles determine sex genetically. Other reptiles, such as snapping turtles use temperature dependent sex selection (TDSS) instead. Eggs incubated at one temperature hatch females, at another, males.
In 2002, Professor Rick Shine of the University of Sydney blew minds with the discovery that Australian three-lined skink lizard use both. Five years later the Central Bearded Dragon was shown to do the same. Now, Shine has co-authored a paper in the Proceedings of the Royal Society B revealing that genetically male bearded dragon females have many characteristics more like males.
If you're confused, let's take this slow. Central Bearded Dragons (Pogona vitticeps) are Australian lizards. Like Discworld's dwarfs, both males and females have “beards”, actually a mass of spikes around the neck.
Normally their sex is determined genetically. Males have ZZ sex chromosomes, females ZW. However, when their eggs are incubated at temperatures above 32°C (90°F) some genetic males are born female. These females are fertile, sometimes producing more eggs than the ZW females.
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Shine and his co-authors saw an opportunity to learn whether differences between the sexes in Bearded Dragons are a product of hormones produced by the gonads, or if the chromosomes that normally determine sex control other differences.
ZZ females had tails as long as males, which are quite a bit longer than the females. Likewise, body temperature for both ZZs was higher than for ZX females.
A test for boldness produced something even more remarkable. Male dragons are quicker to leave a shelter to seek food, but ZZ females proved quicker still.
“One of the most interesting aspects is that, under natural conditions, we can see a process producing individuals with the bodies of females but, at least to some degree, with the brains of males,” Shine said in a statement.
Shine told IFLScience the discovery could explain how species shift between TDSS and genetic determination. “The ancestral reptile was probably temperature dependent, but evolutionary trees show frequent shifts both ways,” he said. Until now it was thought these shifts took thousands of years, but Shine says that if circumstances changed to favor the ZZ females a change could happen in a few generations.
Global warming will increase ZZ female numbers, and may reduce predators, allowing bold females to outcompete the more cautious ZWs. If a ZZ male and ZZ female mate, “all offspring would be ZZ,” Shine said, fixing TDSS within the population. Mothers of all dragons indeed. Reversal would require the appearance of a sex-selective mutation.
Shine explained to IFLScience the effect is not symmetric, as cold temperatures do not produce ZW females in the dragons. The skinks, however, are the mirror image, with genetical females converted to males in cold temperatures. Shine told IFLS he would be willing to bet some other reptile species also combine both sex-selection methods, but only detailed study will reveal which ones.