Revealing the baby thylacine

In the century preceding the death of Australia’s last known Tasmanian tiger, in Hobart in 1936, more than 750 specimens were collected and ended up in museums across the world. Most were bones and skins, but 13 were joeys in various stages of development, taken from their dead mothers’ pouches and preserved in ethanol.

These “wet” specimens of Tasmanian tigers, or thylacines (Thylacinus cynocephalus), have proved to be a fantastic resource, not only allowing the sequencing of the creature’s genome – its entire DNA blueprint – which was published late last year, but now also allowing the creation of a series of 3D digital models of the young, from shortly following birth to first leaving the pouch around three months later.

Andrew Pask, from the University of Melbourne, Australia, and lead scientist behind both efforts, says the results “are giving us a much deeper picture now of what the thylacine was and how it grew and developed”.

As his team reveal in the journal Royal Society Open Science, they have created computed tomography (CT) scans of all 13 joeys preserved in ethanol, which are stored in museum collections in Hobart, Melbourne and Sydney in Australia, and Prague in the Czech Republic.

The scientists have been particularly keen to understand the biological basis of how this marsupial carnivore, related to the kangaroo, evolved into an animal that looked and behaved very much like a wolf, despite not having shared an ancestor with canids for 160 million years.

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Study co-author Christy Hipsley of Museums Victoria says that the scans “show in incredible detail how the Tasmanian tiger started its journey in life as a joey that looked very much like any other marsupial, with robust forearms so that it could climb into its mother’s pouch. But by the time it left the pouch after around 12 weeks to start independent life, it looked more like a dog or wolf, with longer hind limbs than forelimbs.”

Despite looking like tiny, pink jellybeans, all marsupials must have very well-developed front legs at birth – which allow them to climb from their mother’s urogenital opening into the pouch – and a well-developed head, so they can latch on to teat to drink milk. They then complete the majority of their development.

“We were able to show that the young started off looking pretty much like any other marsupial, and it wasn’t until really late in their pouch life that their bones started to grow really long in their hindlimbs to take on that cursorial predator appearance – the sort of running-and-hunting-type body form you see in the thylacine,” Pask adds.

“So, they didn’t get that puppy appearance until near the end of their time in the pouch.”

As there are so few thylacine specimens in ethanol, the museums have generally been very cautious about allowing researchers access to them – so the CT scan data, which is now freely available online, provides a great resource for further research.

Kathryn Medlock, curator at the Tasmanian Museums and Art Gallery (TMAG), said she and her colleagues had been asked many times over the years to allow the pouch young to be dissected, but had always turned down such requests.

“One of the major advantages of this new technology is that it has enabled us to do research and answer many questions without destruction of the sample specimens. This is a significant advancement,” she says.

The smallest specimens scanned are thought to have been less than two weeks old when they were preserved. They were rediscovered by accident in the collection of the Zoology Department of Charles University in Prague in 2011.

“They were a great find, and really important specimens because they show that the thylacine really started off looking very much like a [generic] little marsupial,” Pask says.

Intriguingly, the CT scans also revealed that two of the 13 specimens, long-labelled thylacines, are also more likely to be young quolls or Tasmanian devils.

John Hutchinson, a professor of evolutionary biomechanics at The Royal Veterinary College in Hatfield, UK, says the dataset is “tremendously valuable”, because of the rarity of the specimens and the great public interest in thylacines.

“It’s a fantastic resource and a rigorous scientific analysis and that is to be celebrated,” he says.

Hutchinson, who was not involved in the research, says the 3D models “are gorgeous illustrations of the anatomy that science will benefit from evermore”.

The findings, as regards the change of the limb bone length with size and development, make a lot of sense, he says: “Eventually the hind limbs catch up to the forelimbs in size, and the limbs overall change shape to be more like those of larger cursorial [fast-running] and long-legged modern carnivores.”

Pask says that making all the genome and CT scan data freely available online means this is now a permanent resource for future research. “Nobody will ever have to scan these specimens again,” he adds.

And that’s a good thing. Some of them are insured for up to $2 million each, and have never previously left their museum collections.