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Super-female bearded dragons 'more male than males'

When is a female a female? And when is a male a male? These are the questions that scientists continue to ponder after the latest research on an Australian lizard that reverses its sex when exposed to high incubation temperatures.

Sex-reversed lizards key points

Key points

  • Incubation temperature can create female lizards with male chromosomes
  • These lay more eggs than standard females
  • They are also bolder in their behaviour than the average male
  • This strange combination could provide them with an advantage

The study shows central bearded dragons (Pogona vitticeps) that are born with male chromosomes, but can lay eggs, have other strange characteristics.

Not only, as found previously, do they behave like super females, laying more eggs than standard females, but they are also bolder and more active than males.

This could give them an advantage in the arid and semi-arid areas of Australia they inhabit, said Professor Rick Shine of the University of Sydney, who led the research, published today in the Royal Society journal Proceedings B.

The study follows previous research by some of the same scientists that compared the genetic sex of 131 central bearded dragons, determined by blood samples taken from the tails, and their actual sex, as determined by their size, behaviour and gonads.

Male bearded dragons usually have two Z chromosomes and females usually have a Z and a W chromosome.

Eleven of the dragons studied previously were sex-reversed females, in other words, females with ZZ chromosomes. These looked more like males than females.

Back in the laboratory the researchers found that at lower incubation temperatures these ZZ females produced males, and at high temperatures they produced ZZ females.

Like their mothers, these baby ZZ females also had the potential to lay more eggs than standard females.

Without the production of standard ZW females, these super females could rapidly dominate the population.

The sex determination process in the dragons had changed from one governed by chromosomes to one governed by temperature, the researchers said.

"In the latest study I suggested we should find out what this actually meant in terms of behaviour," Professor Shine said.

"To do this my colleague Hong Li, a visiting scholar from China, got to work on standardised observational tests."

Dr Li studied 115 juvenile and hatchling dragons from eggs incubated at various temperatures.

He found that, when it came to activity and boldness, the 20 super female lizards in the group behaved in an even more male-like way than standard males.

Each lizard was housed in a cage with a container to shelter in and tested to see how active they were, and how quickly they emerged from their shelters (a standard sign of boldness).

"We found that sex-reversed females were much more active than both males and standard females, especially when they were presented with an unfamiliar object," Professor Shine said.

"In the boldness test, the sex-reversed females emerged much quicker from their shelters than the others as well.

Professor Shine and his team also found the super females had higher body temperatures than standard females - temperatures more similar to that of males.

"A bearded dragon can have male sex chromosomes but be a functioning female. Yet it can be more male-like in the way it looks, in its temperature and in its behaviour. They are more male-like than the actual males," Professor Shine said.

Because super female dragons lay more eggs and because standard females can be bred out of the population, then why aren't all central bearded dragons either males or super females?

"It's a phenomenon that happens at quite high temperatures - we don't know the exact temperature in the nest," Professor Shine said.

"Maybe being bold can be good in some situations. They could get more food. Sometimes it might mean they become easier prey. Either way, it's a strange animal indeed."

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