SEA LIFE Aquarium Melbourne used to have one male and two female Boyd's forest dragons. Shortly after the male died, keepers noticed some unusual changes in one of the females. After careful observations they concluded it was changing sex, something apparently never witnessed before in adult reptiles.

Boyd's forest dragons are small arboreal lizards that live in the rainforests of north Queensland. The dragon that has changed sex had previously mated with the male and laid eggs, confirming it was female. However, now it has stopped producing eggs and has developed male physical characteristics, including testes. 

Keeper Tom Fair told IFLScience the two females had a strong rivalry for years, although one was definitely dominant. “The first thing that alerted us to a change was that the two started getting along, sitting on the same branch instead of being at opposite ends of the tank, glaring at each other,” he said.

That could have been the result of either the male's death or the shift to a new enclosure around the same time. However, the keepers next observed the dominant dragon had started growing again, having previously reached a typical female weight of around 100 grams (3.5 ounces). Moreover, her crest was thickening, a key observable difference between the males and females in this species.

The process happened slowly, but the unnamed dragon has now reached 160 grams (5.6 ounces), a typical male weight. “We performed an ultrasound examination to determine which reproductive organs were present,” Fair said in a statement. “Surprisingly, we discovered that there was no longer any ovarian tissue present and mature testes had developed.”

“We thought this was a first for a Boyd's forest dragon,” Fair told IFLScience, “We didn't expect to learn it was a first for reptiles as a whole.” Fair notes some bearded dragons have been observed changing sex in the egg, but as far as he has been able to learn, nothing similar has been recorded before in an adult reptile. Fair noted the aquarium is full of species, such as wrasse and clownfish, where changing sex is a normal part of life. Famously, sex changes in amphibians were a plot device to allow “life to find a way” in Jurassic Park. Fair knew the same thing was not common in reptiles, but discovering it was unprecedented was a shock.

The aquarium doesn't have the resources to investigate the dragon's genetics to see if they have a dragon with some unusual chromosomes, or if this is a wrasse-like evolved response for females finding themselves without males around. Fair told IFLScience they are talking to scientists about conducting such tests, but need to consider the ethical implications since “this is an ambassador animal, not a research specimen.” Similar concerns have prevented the addition of any other dragons to the tank.

Meanwhile, the remaining female is laying eggs frequently and the team is checking to see if any are fertile, as well as watching for mating behavior, beyond shared branch sitting. Neither has been observed so far.