Australasian Science Magazine

Two Tasmanian tigers in Hobart Zoo prior to 1921. Photographer unknown.

By Marie Attard & Stephen Wroe

A new study of the biomechanics of the Tasmanian tiger’s skull debunks the hysteria behind the campaign that led to its extinction.

They were one of Australia’s great biological mysteries, a biscuit-coloured marsupial with a large head, bold dark stripes down its back and a reverse-facing pouch. To newly arrived European settlers, this elusive New World creature was a Tasmanian oddity that inevitably became a source of confusion, contempt and fear. Now, 75 years after the last known individual died in captivity at Hobart Zoo, the thylacine – or Tasmanian tiger – remains one of the least-understood of Australia’s native animals.

But modern research is beginning to lift the veil and reveal the tiger’s true nature. In our laboratory, for instance, advanced computer modelling of the Tasmanian tiger’s skull suggests that it was not well-adapted to tackle large prey. Its skull was big but lightly constructed, and more suitable for tackling wallabies and bandicoots.

Isotope-based research is beginning to provide direct evidence of the Tasmanian tiger’s diet. Such techniques will also help to gain a more thorough understanding of diet and lifestyle in increasingly rare Australian species.

The fossil history of Tasmanian tigers in Australasia dates back some 23 million years, and has revealed surprising diversity. Twelve fossil species from this family are now known.

The lineage was wiped out from mainland Australia around 3000 years ago, and possibly earlier in New Guinea. Aboriginal land use patterns, climate change and competition with dingoes have been linked to the Tasmanian tiger’s extinction on the continent's mainland. A small population of Tasmanian tigers persisted on the remote island of Tasmania, where there were no dingoes and Aboriginal land use differed from the mainland. They were by far the largest marsupial carnivore to survive up to recent times.

Over the past decade, scientists have tried to recover DNA from a preserved Tasmanian tiger pup in the hope of one day resurrecting the species. Some people are even convinced that they still exist, and extensive expeditions have sought to find recent traces of this near-mythical marsupial. This almost fabled figure ranks alongside the Loch Ness Monster and Bigfoot among cryptozoologists – but unlike these, of course, we know that Tasmanian tigers did exist.

A Tragic History
Tasmanian tigers were rarely sighted by early Europeans, so contemporary accounts of their behaviour are a mix of hearsay, fact and myth that remain difficult to tease apart. It was known by many names – Tasmanian wolf, opossum hyaena and tiger to name a few. Their close resemblance to other apex predators such as wolves fuelled assumptions that they were dangerous, and aroused the fear of settlers.

Marsupials such as the Tasmanian tiger were believed to be inferior to their placental counterparts and were seen as primitive dying races. Sir Ray Lankester, a well-known British zoologist, said: “When one watches the Tasmanian wolf, one comes to the conclusion that it is stupid and of much lower intelligence than the common wolf. Its appearance, ways and movements suggest the fancy that it is a kangaroo masquerading as a wolf, and not very successfully.”

During the 20th century, rumours spread that Tasmanian tigers fed like blood-sucking vampires, giving this nocturnal predator an almost supernatural status. Similar speculation plagued the placental grey wolf around the same time in the United States, with such hysteria further provoking campaigns to be rid of both species. These attitudes unwittingly shaped settlers’ regard of the Tasmanian tiger – and likely swayed political decisions to determine their fate.

Despite their rarity, from the start the species was increasingly blamed for attacks on sheep. In an attempt to reduce the alleged threat of Tasmanian tigers to the sheep industry, bounties were paid by land owners and farmers from as early as 1830.

Although people ultimately identified feral dogs as the more serious and pressing menace, the media continued to depict the Tasmanian tiger as a villain. In the 1880s Hobart’s The Mercury newspaper described the public perception of the animal as “cowardly, as stealing down on the sheep at night and wantonly killing many more than it could eat, as being worthless for its skin”.

Concerns were raised by The Mercury in 1886 about misconceptions and misinformation surrounding the so-called “tiger”: “It is not the ferocious brute the name implies, and under no circumstances would it attack even a child. On two occasions I have met with recently arrived immigrants who objected to leave town to secure work in the country for fear they or their children might be devoured.”

In 1888 the Tasmanian government paid a pound sterling for every dead adult Tasmanian tiger head. At the time, the award would have been equivalent to half a week’s wage. In all they paid out 2184 bounties, but many more were likely killed than was claimed in the bounty records.

An epidemic disease ruthlessly swept across the Tasmanian tiger population in the late 1800s, causing further devastation. The Tasmanian tiger preferred open forests and heathlands but persistent hunting and land clearing confined the fast-dwindling population to dense rainforests.

Despite its obvious decline, the species did not receive official protection from the Tasmanian government until 2 months before the last known captive animal died in 1936. Too little was done to protect them. In the end we were too late to save them.

Changing Perceptions
Our perceptions about the Tasmanian tiger continue to evolve as we learn more about their behaviour through new scientific approaches. Recent scientific findings have added to a complex picture of how the Tasmanian tiger lived and why it went extinct after millions of years of successful survival in Australia.

Whether or not Tasmanian tigers were capable of taking down large prey like kangaroos, emus or adult sheep has been a contentious issue. To answer this question requires further knowledge about the mechanical limitations of their skull. For this, our research team from the University of NSW has recently analysed the mechanical performance of the Tasmanian tiger’s skull relative to two living marsupial predators – the Tasmanian devil and spotted-tailed quoll.

In the past, 2D modelling routinely formed the basis of studies to establish relationships between form and function in biological structures. With exponential progress in computer hardware and the development of new software and protocols, we are now able to create more realistic simulations using 3D models.

The process we apply is called finite element analysis, which was originally developed for the aerospace industry but has increasingly been used to predict the mechanical behaviour of biological structures.

The first step is to scan a skull from each species using CT, which stitches together many X-ray images to create a 3D digital representation of the skull. The material properties of bone are then assigned to the model.

Reliable results depend on getting accurate predictions of the forces that would have been applied by the animal in life. For fossil species, dissections from related living species provide a guide to where the jaw-closing muscles attached to the skull.

The model we generate from this process is used to simulate different bites and generate predictions of how stress and strain would be distributed through the skull under different feeding or biting behaviours observed in living carnivore species.

For example, when a predator bites down on prey it obviously uses its jaw muscles, but killing and dismembering prey may also include “thrashing” and “ripping” behaviours in which the predator shakes its head sideways to rip the prey apart. Other predators will twist their head or, alternatively, pull backwards against the prey using their neck.

We have simulated these behaviours for each marsupial carnivore in our study to see how they compare.

From the results of the finite element analysis we can assess the magnitude and distribution of stresses in a skull in response to biting down on or resisting struggling prey. The colour images created of the skull highlight areas in cool-blue where stresses are low and hot-red to white where stresses are relatively high.

We were surprised to find that the Tasmanian tiger’s skull had more stress “hot spot” zones than other large marsupial carnivores like the Tasmanian devil and spotted-tailed quoll, which hunt animals larger than themselves. Our results suggest that, in contrast, the jaws of the Tasmanian tiger were probably better suited to catching small- to medium-sized animals such as bandicoots, wallabies and possums.

Most accounts from the 19th and early 20th century describe Tasmanian tigers as solitary hunters, although they may have hunted in small groups. This may have consisted of two adults or a mated pair with up to four young. With the rapid decrease in the Tasmanian tiger population as a result of hunting by Europeans, accounts of group hunting became rare. The limitation of only catching small prey by solitary Tasmanian tigers may have placed additional pressure on this large carnivore.

The diet of Tasmanian tigers likely overlapped with that of the Tasmanian devil and spotted-tailed quoll, leaving it vulnerable to competition. Moreover, the morphology of the Tasmanian tiger’s teeth suggests that it was almost entirely restricted to eating meat, further restricting its ability to adapt to changing circumstances. Limited prey options potentially increased their susceptibility to ecosystem disruptions, such as those caused by European land use practices.

To provide further clues about the Tasmanian tiger’s likely hunting style, Borja Figueirido and Christine Janis of Brown University have taken a closer look at the anatomy of the elbow joint. Based on an analysis of the Tasmanian tiger’s elbow structure they predicted that the hunting strategy of Tasmanian tigers more closely resembled cats than dogs. Cats and Tasmanian tigers both have more flexible elbows to better manipulate and grapple with prey, features typical of short distance ambush predators.

In contrast the elbow joint of wolves are far less flexible, being more fixed in a “palm” down position, making them more effective long distance runners that exhausted their prey.

The seemingly disproportionate and even stumpy legs of the Tasmanian tiger do not appear to be suitable to long distance pursuit.

Other short-legged marsupial carnivores typically use short pursuits to capture prey. For example, the stocky Tasmanian devil will use a combination of ambush and short, moderate-speed pursuits when hunting.

However, some historical accounts do describe Tasmanian tigers doggedly following prey for long periods of time.

To really flesh out the diet of this extinct carnivore, we are presently analysing the chemical components of Tasmanian tiger pelts, skulls and skeletons in collaboration with Tracey Rogers’ lab at the University of NSW. The supporting idea is that your body tissues originate from the food you eat. The stable isotope signature of the food becomes assimilated into your body tissues, giving you a very distinctive signature of what you have been eating. Basically, you are what you eat.

To retrace the diet of a predator, we also need to know the isotopic values of potential prey species. In collaboration with 20 museums worldwide we have collected tissues from Tasmanian tigers and potential prey species dating as far back as the 1830s. By comparing preserved tissue from both predator and potential prey species, we will be able to get a better picture of what Tasmanian tigers were really hunting.

New Beginnings
What role does research have in shaping our perception of the species?

Since the extinction of the Tasmanian tiger there has been a new wave of understanding of their behaviour. We have come a long way from the closed views of past centuries. Research can show us the way forward with improved and more informed conservation strategies, but strategies are meaningless without action and the resources needed to put them into effect.

Much of the Australian fauna is now imperilled, and Australia has the very dubious distinction of having achieved the world’s highest mammalian extinction rate. For example, grave fears are held that Christmas Island’s pipistrelle bats may now be extinct, with no sightings of the species since 2009.

Tasmanian devils may be facing a similar fate. An aggressive facial tumour has wiped out more than 90% of Tasmanian devils in high-density areas, and the disease continues to spread. The recent introduction of the red fox places this Tasmanian icon under further pressure.

The list of Australian mammals in danger of extinction continues – with several species of wombats, bandicoots, wallabies, quolls and two whales – the blue whale and southern right whale – at risk of extinction. Without awareness of these issues, and even more importantly action, we may lose much of the precious diversity our country has to offer.

What can we do to help? There are many volunteer programs available through organisations such as WIRES, WWF and Australian zoos and fauna parks. These organisations also accept donations towards conservation.

You can make a difference to conserve Australia’s unique wildlife.