New research shows that baby dragons decide on their sex just before hatching

23 October 2018: Researchers from the University of Canberra’s Institute for Applied Ecology and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) have discovered that bearded dragon lizards take their time to decide whether they are male or female.

As published in Scientific Reports, researcher and PhD candidate at the University of Canberra Sarah Whiteley says that the findings buck trendsobserved over decades of mammalian research, and demonstrate that snakes and lizards might have a very different (and hitherto undiscovered) set of developmental events.

“One of the most important developmental decisions an embryo can make is its sex, which will influence how an organism looks, acts, reproduces and more,” she said.

“Until now, it was thought that sexual development was a pretty well-understood process – once the gonads (ovaries or testes) have differentiated, then all the other secondary sexual characteristics develop rapidly.”

But bearded dragons are apparently not so quick to decide on their sexes.

Using a suite of techniques including scanning electron microscopy and histology, the researchers revealed that in the egg, female dragons with ovaries also completely develop the male sexual organ, called a ‘hemipenis’.

“Amazingly, this male structure is retained until just before the females hatch out of the egg,” Ms Whiteley said. “This means that during approximately one third of the embryonic development, female bearded dragons present as half male.”

Another unique trait of the bearded dragon lizard is that chromosomal sex determination can be overridden by extreme temperatures.

Male dragon embryos incubated at high temperatures can develop as females. These individuals are known as ‘sex reversed’, with the chromosomes of a male, but the appearance of a female.

This new study has also identified the first-ever cellular markers of temperature sex reversal.

Senior Research Scientist at CSIRO, Dr Clare Holleley, said that at a very precise stage of development, the fight between male and female development is visually identifiable.

“In the gonad, both male and female cell-types are observed at the same time, at which point this organ is called an ‘ovotestis’,” Dr Holleley said.

“The battle between male and female development is only temporary, and one or the other quickly wins the fight.”

This research proves that understanding of how sexual development proceeds in the full diversity of non-mammalian organisms is still in its infancy.

“It also demonstrates how the environment can drastically alter what is usually considered an immutable trait,” Dr Holleley said. “For dragons, the line between what constitutes maleness and femaleness is more blurred than ever.”

Dr Clare Holleley and Ms Sarah Whiteley are available for interview.