“If it can be verified, this is an astonishing discovery,” the University of Canberra’s Professor Arthur Georges, one of Australia’s leading reptile experts, said in an email. “This is the first time there is any suggestion of sex reversal in an adult reptile.”

Sarah Whiteley, a researcher studying the sexual development of dragons at the University of Canberra, said “such an unusual occurrence would require very convincing data”.

“Without having seen any of the data myself, I must remain sceptical, though very intrigued,” she said.

The aquarium keepers have had the lizard examined by a vet, who confirmed the presence of male genitals. They hope to soon publish findings in an academic journal.

The small creature, known as Doris, has been with the aquarium for six years, living quietly in the tropical forest exhibit alongside two other dragons – a male and a female. Doris spent her time hiding on logs, eating bugs, laying a cache of eggs every month. Several hatched into baby forest dragons.

Late last year, the male forest dragon – known as Old Mate – died, and the keepers moved the two females into a new exhibit.

The first sign of change was food. Most female dragons won’t eat until it’s time to lay eggs, when they binge. Doris started eating all the time, like a male dragon.

“Not long after, the secondary sex characteristics started changing, like a brighter colour, a thicker crest, and she started getting quite a lot bigger,” said Tom Fair, the reptiles’ keeper at Sea Life Melbourne Aquarium.

At her monthly weigh-in, Doris’s weight had shot up, from 112 grams to 159 – “which is normal for a male”, said Mr Fair. “It’s a massive difference.”

He took the dragon to the aquarium’s vet, who gave it an ultrasound, revealing ovaries and undeveloped eggs. A month later, they brought in a team of specialist vets who performed a more thorough ultrasound.

There, unmistakable on the screen, was the evidence: No eggs. No ovaries. “They confirmed Doris now has testicles,” said Mr Fair.

Skinks and central bearded dragons have shown the ability to reverse sex but only in eggs before they hatch.

Among fish, male clownfish living in a school can change gender to female when the alpha female dies, a process known as sequential hermaphroditism.

Independent experts were fascinated but sceptical.

“The most simple explanation is an endocrine issue – disease or an environmental endocrine disruptor, but this would need to be significant,” said Dr Jane Melville, senior curator of terrestrial vertebrates at Museums Victoria.

Or perhaps the aquarium had misidentified Doris’s sex in the first place.

Mr Fair considered that as well. “But really, once we have the confirmation the testes are there, it is a definite shift over. I personally have observed that individual laying eggs many, many times.”

His theory: Doris – who has not yet been given a new name – is displaying social gender switching, similar to clownfish, making the switch due to the absence of a male.

“But that’s just guesswork. There are more questions than answers,” Mr Fair said.

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