The Tasmanian devil is endangered, not extinct, though the distinction has felt uncomfortably thin at times. Once found across mainland Australia, this stocky marsupial carnivore now survives only on the island state of Tasmania, where a bizarre transmissible cancer has killed the majority of the wild population since the mid-1990s. The picture is more hopeful than the headlines of two decades ago suggested, but the species remains in serious trouble.
Gone From the Mainland, Holding On in Tasmania
Tasmanian devils vanished from mainland Australia roughly 3,200 years ago. Fossil dating supports a synchronized extinction of both devils and their larger relative, the thylacine, on the mainland during the Late Holocene, with estimates placing the disappearance between about 3,179 and 3,227 years before present.1PubMed Central. High-quality fossil dates support a synchronous, Late Holocene extinction of devils and thylacines in mainland Australia The causes remain debated. Climate shifts linked to El Niño–Southern Oscillation activity affected both the mainland and Tasmania, but on the mainland, additional pressures from humans and possibly dingoes tipped the balance.2Journal of Biogeography. Ancient DNA tracks the mainland extinction and island survival of the Tasmanian devil Tasmania, which dingoes never reached, served as a refuge.
The population that persisted in Tasmania was not thriving in some unbroken golden age. Genetic analyses show extensive population declines across Tasmania correlating with environmental changes around the last glacial maximum and subsequent unstable climate.3PubMed Central. Extensive population decline in the Tasmanian devil predates European settlement and devil facial tumour disease By the time Europeans arrived, genetic diversity was already low, a bottleneck dating back centuries. That limited diversity would later matter enormously.
The Transmissible Cancer That Changed Everything
In the mid-1990s, researchers noticed devils in northeastern Tasmania developing grotesque facial tumors. What they were dealing with turned out to be one of the rarest phenomena in biology: a contagious cancer. Devil facial tumor disease, or DFTD, is not caused by a virus. The cancer cells themselves are the infectious agent, passing from one devil to another during the biting that commonly occurs when devils feed together or mate.4PubMed Central. A Devil of a Transmissible Cancer The tumors grow rapidly on the face and mouth, eventually preventing the animal from eating. Most infected devils die within months.
The original cancer lineage, now called DFT1, arose from Schwann cells, a type of cell that normally insulates nerves. Then in 2014 and 2015, researchers in southern Tasmania found devils with facial tumors that looked the same on the outside but were genetically distinct. This second lineage, DFT2, arose independently, carries a Y chromosome (DFT1 is of female origin), and shares no detectable cytogenetic similarity with DFT1.5PubMed Central. A second transmissible cancer in Tasmanian devils The emergence of two separate transmissible cancers in the same species was startling, and it raised questions about whether something particular about devils makes them prone to this kind of disease.
Why Devils Are So Vulnerable
The answer involves their immune systems. Normally, when foreign cells enter a body, immune cells recognize molecular flags on the surface of those cells and attack. DFTD gets around this because the tumor cells have lost the expression of those surface molecules, effectively becoming invisible to the host’s immune defenses.6PubMed Central. How the devil facial tumor disease escapes host immune responses
This immune evasion is compounded by the devils’ own genetics. Their immune diversity was already remarkably low before the disease arrived. Studies of museum specimens and ancient DNA show that limited diversity in the genes governing immune recognition has been a feature of devil populations since at least the mid-Holocene, well before European settlement.7PubMed Central. Low major histocompatibility complex diversity in the Tasmanian devil predates European settlement and may explain susceptibility to disease epidemics Analyses of mitochondrial genomes and nuclear markers across current and museum specimens confirm the low diversity preceded DFTD by at least a century.8PubMed Central. Genetic diversity and population structure of the endangered marsupial Sarcophilus harrisii (Tasmanian devil) In practical terms, the devils look too genetically similar to each other, which makes it easier for foreign tumor cells to slip past immune surveillance. It is the same principle that makes organ transplants between identical twins unlikely to be rejected, except here the “transplant” is cancer.
How the Disease Has Reshaped Devil Populations
DFTD has caused severe population declines across much of Tasmania, and those declines have rippled through devil behavior and life history. Affected populations show earlier breeding, younger age structures, and reduced social interactions.9PubMed Central. Isotopic niche variation in Tasmanian devils Sarcophilus harrisii with progression of devil facial tumor disease When adults are being killed off by disease, younger animals breed sooner. This compressed life cycle keeps the population going but changes its character.
The disease has not hit every population equally, though. At a well-studied site in western Tasmania called West Pencil Pine, researchers found no rapid increase in disease prevalence and no evidence of population decline four years after DFTD was first detected there, a stark contrast to eastern sites where the disease devastated populations quickly.10PubMed. Reduced effect of Tasmanian devil facial tumor disease at the disease front Lower population densities in the west may slow transmission, since devils encounter each other less often.
From Epidemic to Endemic
Early models painted a bleak picture. Some projections suggested DFTD could drive devils to extinction within decades. But the disease has not followed that trajectory. Using phylodynamic analysis, researchers tracked how DFTD’s transmission rate changed over time and found a sharp decline: from roughly 3.5 secondary infections per infected individual down to about 1.11PubMed. A transmissible cancer shifts from emergence to endemism in Tasmanian devils A transmission rate around 1 means the cancer is replacing itself but no longer exploding through the population. The shift looks like a transition from an epidemic phase to an endemic one, where the disease persists at lower levels rather than burning through and wiping out its host.12PubMed Central. Two Decades of the Impact of Tasmanian Devil Facial Tumor Disease
Why the slowdown? Part of it is straightforward population dynamics: as devil numbers dropped, infected animals simply encountered fewer potential victims. But there is growing evidence that biology is also at work, specifically, that devils are evolving resistance.
Devils Are Fighting Back
Genomic studies have found parallel evolutionary changes across multiple devil populations in regions linked to immune function and cancer risk. The same genomic signatures appeared independently in three separate populations, a strong signal that natural selection is acting on resistance to the disease.13Nature Communications. Rapid evolutionary response to a transmissible cancer in Tasmanian devils This happened over just a handful of generations, which is remarkably fast for a mammal. Devils typically breed at two years old, so the selective pressure has been intense enough to produce measurable genetic shifts within a couple of decades.
Modeling work that incorporates both ecological and evolutionary dynamics supports a more optimistic outlook than earlier projections. A coevolutionary model parameterized with nearly two decades of demographic data, disease epidemiology, and genome-wide association studies found a high probability of devil persistence over 50 generations (roughly 100 years) and predicted that devils and DFTD are likely to coexist, with greater devil recovery than earlier ecological-only models suggested.14PubMed. Coevolution promotes the coexistence of Tasmanian devils and a fatal, transmissible cancer That is a very different story from “extinction within decades.”
The tumor, too, may be evolving. When researchers tested whether introducing immune-recognition molecules back onto tumor cells could trigger a devil’s immune response, they found that artificially restoring those molecular flags did make the cells visible again.15PubMed Central. Class II transactivator induces expression of MHC-I and MHC-II in transmissible Tasmanian devil facial tumours But the tumor has its own countermoves. In at least one case, a tumor that arose after immunotherapy showed upregulation of immune checkpoint molecules, the same trick human cancers use to shut down immune attacks even after being detected.16PubMed. Mesenchymal plasticity of devil facial tumour cells during in vivo vaccine and immunotherapy trials It is an arms race, and the outcome is not guaranteed.
The Insurance Population
While wild devils and the disease sort out their coevolutionary future, conservationists have not been standing by. In 2006, the Save the Tasmanian Devil Program established an insurance metapopulation: a network of captive and semi-wild populations held in zoos, wildlife parks, and fenced natural areas, designed to preserve genetic diversity and natural behaviors in case the wild population collapsed entirely.17PubMed Central. No evidence of inbreeding depression in a Tasmanian devil insurance population despite significant variation in inbreeding The program spans a range of settings, from intensive zoo-based enclosures to larger, more naturalistic environments.18PubMed. A Tasmanian devil breeding program to support wild recovery
One of the key concerns with any captive breeding effort is whether the backup population actually represents the genetic range of the wild species, or whether it becomes a genetically narrow slice. Genetic analysis of the insurance metapopulation and the introduced population on Maria Island, a disease-free island off Tasmania’s east coast, found that both were representative of range-wide genetic diversity and contained a mix of the genetic clusters present across wild populations.19PubMed Central. Restoring faith in conservation action: Maintaining wild genetic diversity through the Tasmanian devil insurance program That is an encouraging sign for any future reintroduction effort.
Vaccines and Immunization Trials
Developing a vaccine against a contagious cancer is not like developing a vaccine against a virus, but researchers have been trying. In two immunization trials conducted on cohorts of devils before their release into the wild, 95% of vaccinated animals developed antibodies against DFTD. The vaccine used tumor cells that had been manipulated to re-express the surface recognition molecules that DFTD normally hides.20PubMed Central. Immunization Strategies Producing a Humoral IgG Immune Response against Devil Facial Tumor Disease in the Majority of Tasmanian Devils Destined for Wild Release
Follow-up on 33 vaccinated devils released in 2016 showed that anti-DFTD antibodies persisted for up to two years after vaccination. More telling was what happened when some of those vaccinated devils did develop tumors. Biopsies revealed immune cells infiltrating the tumors, something almost never seen in unvaccinated devils. Among 20 unvaccinated incumbent devils with DFTD at the release site, only one had any sign of immune-cell infiltration, compared to three of four biopsied vaccinated animals.21Wildlife Research. Post-release immune responses of Tasmanian devils vaccinated with an experimental devil facial tumour disease vaccine The vaccine did not prevent all infections, but it appeared to wake the immune system up enough to mount a fight. Whether that translates into longer survival or lower transmission in the wild remains an open question.
What Happens When Devils Disappear From an Ecosystem
The decline of devils has given researchers an unplanned experiment in what happens when a top predator and scavenger is removed from an ecosystem. Devils are Tasmania’s largest surviving mammalian carnivore, and their role in scavenging carrion turns out to be surprisingly hard to replace. Carcasses persisted roughly 2.6 times longer in areas where devil populations had declined compared to areas where they remained healthy.22PubMed Central. Top carnivore decline has cascading effects on scavengers and carrion persistence
With fewer devils cleaning up carcasses, invertebrate scavengers have stepped in. At sites with the lowest devil densities, the amount of carcass removed within five days was about 3.6 times lower than at higher-density sites, and carrion beetle and blowfly larvae were roughly twice as abundant.23Ecology. Decline of an apex vertebrate scavenger increases carrion use by invertebrates A world with fewer devils is, in a very tangible sense, a world with more maggots and beetles on carcasses, with potential knock-on effects for nutrient cycling and disease transmission.
The relationship between devil decline and feral cat populations has been more contentious. One study found that feral cat occurrence was significantly and negatively associated with devil presence, suggesting that as devils declined, cats increased.24PubMed. Trophic cascades following the disease-induced decline of an apex predator, the Tasmanian devil But another investigation looking specifically at the link between devil decline and the collapse of the eastern quoll found no evidence of a negative relationship between devil and cat abundance, and no confirmation that higher cat numbers followed devil losses.25PubMed Central. Devil declines and catastrophic cascades: is mesopredator release of feral cats inhibiting recovery of the eastern quoll? The mesopredator release story — the idea that removing the top predator automatically unleashes the next one down — is appealing in its simplicity but does not always play out cleanly.
Could Devils Return to Mainland Australia?
A more radical conservation idea has been floated: reintroducing Tasmanian devils to the Australian mainland, where they have been absent for over three millennia. Modeling work suggests that devils could fill some of the ecological role that dingoes currently play in parts of Australia where dingoes have been removed by farmers. Devil introduction was linked in models to lower abundances of introduced mesopredators and herbivores, and to increases in small and medium-sized native mammals and understorey vegetation complexity.26Biological Conservation. Reintroduction of Tasmanian devils to mainland Australia can restore top-down control in ecosystems where dingoes have been extirpated The effects were predicted to be weaker than those of dingoes, and threatened species most vulnerable to fox predation benefited little from the simulated devil introduction. Still, the idea gets at a broader point: devils are not just charismatic animals worth saving for their own sake, they are ecosystem engineers whose presence or absence reshapes the landscape around them.
Any mainland reintroduction would face enormous practical hurdles, from disease management to interactions with species that have had thousands of years to fill the niche devils left behind. But the existence of the insurance population, the vaccine work, and the growing evidence of natural resistance all make the idea less far-fetched than it would have sounded even a decade ago.
What DFTD Teaches About Transmissible Cancer
Transmissible cancers are exceptionally rare in nature. Only a handful are known: DFTD’s two lineages in devils, canine transmissible venereal tumor in dogs, and a few contagious cancers found in bivalves. The fact that devils spawned two independent transmissible cancer lineages has reshaped the scientific understanding of how often this can happen, suggesting that transmissible cancers may arise more frequently in nature than previously assumed.5PubMed Central. A second transmissible cancer in Tasmanian devils
DFTD has also drawn attention from cancer researchers working on human disease. The immune evasion strategies the tumor uses, particularly the downregulation of surface recognition molecules and the upregulation of immune checkpoint molecules, mirror strategies employed by human cancers. The pathogenesis of DFTD has parallels with certain human scenarios, including the rare transmission of cancer between organ-transplant donors and recipients, and certain malignancies at the maternal-fetal interface during pregnancy.27PubMed. Tasmanian devil facial tumor disease: insights into reduced tumor surveillance from an unusual malignancy Studying how a contagious cancer evolves in real time in a wild population offers insights that are difficult to get from any other system.