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Why did the Tasmanian tiger go extinct?

Australia accounts for one-third of all contemporary mammal extinctions worldwide. At least ten species and six subspecies of Australian marsupials have become extinct following European settlement, and…

Alb Quarrell holding his prized thylacine kill, 1921. Courtesty Tasmanian Museum and Art Gallery

Australia accounts for one-third of all contemporary mammal extinctions worldwide. At least ten species and six subspecies of Australian marsupials have become extinct following European settlement, and many more are now at a high risk of extinction.

We need to identify the factors involved in historical declines and extinctions to help guard against future biodiversity loss.

One of Australia’s most fabled species, the Tasmanian tiger, also known as the thylacine, went extinct on the continent’s mainland around 2000 years ago. A small population of thylacines persisted on Tasmania when Europeans arrived in Australia.

The species was rapidly viewed as a pest and a dangerous threat to livestock, though many of these claims were highly exaggerated. Over 2,000 bounties were paid by the government between 1888 to 1909 to eradicate the species. A sudden decline in the thylacine population was reported in the early 1900s, and the species was declared extinct in 1936.

The government bounty may seem to be the obvious extinction culprit. But growing scientific evidence reveals a complex tapestry of forces involved in their decline. Among these are competition with dogs, habitat loss and changing fire regimes leading to population fragmentation, and an epidemic disease that spread through the population in the 1920s.

A mounting body of evidence reveal that larger bodied species are at greater risk of extinction than smaller bodied species. As a large-bodied predator, relying on small prey would have been energetically constraining for thylacines: their food may have been inadequate to support them unless small prey were abundant.

Thylacine skeleton from the Tasmanian Museum and Art Gallery Christopher Hammang

Whether thyacines were capable of taking down large prey species like kangaroos, emus or adult sheep remains a contentious subject. Based on their teeth and jaw, it is almost certain that they were meat specialists. They don’t have the dental features associated with bone consumption and scavenging. Anecdotal evidence suggests thylacines may have taken large prey up to 30kg, such as kangaroos and emus. But few naturalists were present to record its foraging behaviour and many accounts are derived from unreliable or biased sources.

Morphological features, such as their extremely long snout and very low rates of canine tooth wear and fracture suggest they relied on small prey, though their wide gape may have allowed them to catch larger species.

Thylacines were thought to use caves as lairs, and have been associated with prey found in sub-fossil cave deposits. These ranged in size from 1 to 5kg. As prey brought back to a den may be the smaller species killed, they may not represent the full range of prey species killed by thylacines.

With the advancement of new techniques, it may be possible to conclusively evaluate the diet of the thylacine. The mechanical performance of the thylacine skull may provide clues into their mode of hunting and disclose limitations in the size of prey they could catch.

An engineering technique called “finite element analysis” was used to digitally construct the skull of the thylacine and two living relatives – the Tasmanian devil and spotted-tailed quoll. These are both capable of hunting large prey relative to their own body size. If thylacines were able to take down large prey, we would expect their skulls to perform similarly under different feeding simulations.

Computed tomography (CT) scans of each skull were digitised to create a three-dimensional model. Beams were attached to the skull to simulate the different jaw-closing muscles that act on the skull during chewing. We also tested common killing behaviours – including shaking, pulling and chomping down on a prey item. We obtained stress data from each simulation, which act as a good measure of failure in ductile materials such as bone.

Three-dimensional computer model of thylacine skull Marie Attard

To our surprise, we found that thylacines performed poorly compared to other marsupial carnivore in all simulations, and showed peak levels of stress at their snout. The long narrow snout suggests thylacines hunting alone were more suited to catching small-sized prey, such as bandicoots and possums. The findings of this study were published in the Journal of Zoology.

Family groups containing mature young may have been able to take down larger animals. In 1941, G. Stevenson described co-operative hunting of thyalcines: “they…jump on it. Kangaroos are killed by standing on them and biting through the short rib into the body cavity and ripping the rib cage open”.

For some predator species, group-hunting can reduce physical disadvantages, letting them kill larger prey. However, persecution by humans would have made co-operative hunting very difficult, and likely lead to an increase of solo hunters favouring small-sized prey.

Intensive competition for small prey by invasive species such as feral cats and dogs would have directly influenced the thylacine’s survival.

With improved understanding of the diet and movement of living and extinct marsupial carnivores, we can improve management strategies and help conserve our unique wildlife.

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15 Comments sorted by

  1. John Newlands

    tree changer

    By attributing the extinction to multiple interacting factors this article seems to contradict yesterday's statement from Adelaide Uni that humans were the sole cause. One of AU assumptions was that hunting reduced wallaby populations. Not true in my observation so long as they have nearby refuges they can outbreed huge levels of hunting.

    As it happens I live about 12 km from where the picture was taken. I think the thylacine can be compared to the Swift parrot in that they may have died out anyway with or without humans. The difference is political sensitivities differ between periods so no the Swift Parrot will be saved from itself. My guess is the last wild thylacine died around 1950. Even with earlier protection I doubt whether the slow moving thylacine could have survived road kill and attacks by domestic dogs. Perhaps it could be said humans hastened the extinction and things would only have gotten worse.

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    1. Mike Swinbourne

      logged in via Facebook

      In reply to John Newlands

      I think I understand what you are saying John, but you appear to be concentrating on direct human influences, and ignoring indirect influences. Road kill and attacks by domestic dogs are human influences - it is not just shooting.

      Evidence suggests that it was human influences which caused (or largely contributed to) the thylacine to go extinct on the mainland long before Europeans arrived on the scence. Europeans just completed what the Aboriginals had started centuries before.

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  2. John Newlands

    tree changer

    It should be pointed out that other living members of the Dasyurid family are struggling, notably the Tasmanian devil and several species of quoll. Ironically human intervention is keeping them going. They each have some kind of self limiting vulnerability. For the devil that may be fighting while for the northern quoll it may be eating cane toads.

    While h. saps beats itself up about extinctions the irony may be that we are now preventing them when they may have happened without us. The Madagascan Dodo bird was slow moving and delicious. I'd say it always had a use-by date whether it was hunters, cars, dogs, disease or habitat removal. My advice to thylacine nostalgists is to get over it.

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    1. Don Gibbons

      Clerk

      In reply to John Newlands

      Hi John

      While humans may not be the sole factor in the extinction of an accelerating number of species over the last few tens of thousands of years, I think there is a fair amount of evidence that we have been significant contributors to the shortening of the "use by dates" of many species. Northern quolls wouldn't have a problem with cane toads if we hadn't have introduced them here. Every species has a use by date, even H.saps, unless we figure out a way to muddle through the heat death of the…

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    2. Chris Owens

      Professional

      In reply to John Newlands

      John, I think it is more than a stretch to suggest we have assisted the species to survive. Curiously early Dasyurids and Thylacines have been found at Riversleigh from deposits approximately 25M years old. So they can survive and radiate for millions of years, yet they are in trouble in a geological instant after we arrive. Incidently work on samples from Riversleigh suggest a minimum divergence time between Dasyurids and Thylacines of at least 25M years and possibly up to 40M years http://www.naturalworlds.org/thylacine/mrp/cloning/cloning_3.htm

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    3. John Newlands

      tree changer

      In reply to Chris Owens

      My neighbour makes a plausible sounding case that the herbicide Atrazine was the trigger for devil facial tumour disease.

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    4. Mike Jubow

      forestry nurseryman

      In reply to John Newlands

      John, evidence for the assumption please. Was Atrizine in common use where the facial tumor disease was first discovered? Is Atrazine in common use throughout the areas where the disease is common? Is it likely that a disease caused by a herbicide is transmissable to other individuals? I would like to see some evidence for the suggestion.

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    5. John Newlands

      tree changer

      In reply to Mike Jubow

      The lore among old timers is that devils became ill in several separate locations where Atrazine was in use. Interestingly they don't blame animal poisons like 1080. They don't say ongoing Atrazine exposure is needed for transmission. I don't know if it is still used in Tasmania though I note fish farms may be having problems with upstream use of Brush Off, again anecdotal evidence only. To pursue the DFTD angle rigorously would need an inquiry with invited submissions followed by lab experiments. I suspect the result would be inconclusive.

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    6. Mike Jubow

      forestry nurseryman

      In reply to Chris Owens

      Aw, c'mon Chris, that's a pretty dubious website. Certainly not mainstream science. In the meantime, hearsay and wacko-science websites are not contributing to the resolution of the problem. Mainstream scientists have been working their butt off to resolve the facial tumour disease in Tassie Devils and have made some progress but not a cure as I understand it. The last thing we need is for wacko science to step in with spurious claims. It is as helpful as the fools who would deny the other major achievements of science such as vacination etc.

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    7. Mike Jubow

      forestry nurseryman

      In reply to John Newlands

      John, I hope you are not relating Brush Off to Atrazine. There is no relationship and Wickipedia says of Brush Off -- "Metsulfuron-methyl is a residual sulfonylurea herbicide that kills broadleaf weeds and some annual grasses. It is a systemic compound with foliar and soil activity, that inhibits cell division in shoots and roots. It has residual activity in soils, allowing it to be used infrequently but requiring up to 22 months before planting certain crops (sunflowers, flax, corn, or safflower…

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    8. Chris Owens

      Professional

      In reply to Mike Jubow

      Mike,

      $105M payout by the manufacturer to communities in the US due to contamination of a catchment and the chemical has been banned in Europe since 2001. Wacko science?

      Atrazine in water tied to menstrual irregularities, low hormones
      http://www.environmentalhealthnews.org/ehs/news/2011/2011-1123atrazine-tied-to-menstrual-irregularities

      Atrazine interferes with endocrine hormones http://www.sciencedaily.com/releases/2010/03/100301151927.htm

      The relationship between Atrazine and increased risks of non-Hodgkin lymphoma, ovarian and prostate cancers http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3230407/

      US National Center for Biotechnology Information - wacko science?

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    9. John Newlands

      tree changer

      In reply to Mike Jubow

      Not confusing Brush Off and Atrazine. When somebody sprays Brush Off on blackberries then days later thousands of salmon float to the surface in a pond fed by the same stream that looks suss. I agree these things need thorough testing. However like Agent Gibbs in NCIS nor do I believe in coincidences.

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    10. John Holmes

      Agronomist - semi retired consultant

      In reply to John Newlands

      Just how much wetter or crop oil was added to the Brush Off? The literature does not seem to highlight many problems with Metsulfuron-methyl or similar sulfonylurea herbicides and fish. Plants can be very sensitive.

      In one of my trials, an aquatic weed (parrot feather) was killed with a solution of 0.005 ppm in pots, while the common level LD50 for fish is of the order of about 150 ppm. The concentrations can be estimated if the water flow and volumes are roughly known and area sprayed…

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    11. John Holmes

      Agronomist - semi retired consultant

      In reply to Mike Jubow

      The persistence of this group of chemicals is very highly correlated to soil pH. In Acid soils quite short, sometimes less than 2-3 months of moist soil and clovers will grow OK. Seen that in trials In alkaline soils or with alkaline sub soils, sulfonylureas will persist for long periods. Clovers not growing for up to 3 years. Root pruning is observed in some situations. Seek local advice.

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