Almost everything people know about the Tasmanian devil comes from cartoons: a creature that spins like a tornado and devours whatever it finds. The real animal is smaller, quieter and eats mostly carrion. And it is going through something that in biology was close to a theoretical impossibility.
A contagious cancer. Not a virus that causes cancer, of which there are several known examples, but the tumour itself jumping from one individual to another and carrying on its life in the new body.
01 A tumour that should not be contagious
Cancer, by definition, dies with its carrier: they are the body's own cells multiplying unchecked and disappearing when that body disappears. For tumour cells to survive the jump into another organism and go on proliferating there is extremely rare. Very few cases are known in the whole animal kingdom.
This is one of them. It was first described in 1996, in the island's northeast, and estimates place its appearance around 1986. The tumours grow on the face and inside the mouth, and end up preventing the animal from feeding.
02 Why it happened precisely here
Two conditions are needed and Tasmania has both. The first is behaviour: devils bite each other constantly. Competing over carrion, mating, disputing a spot. Those bites to the face are exactly the vehicle the tumour needs.
The second is genetics, and here the island's geography comes in. This is a population isolated from the mainland since sea level rose and flooded Bass Strait, thousands of years ago: two hundred and forty kilometres of water and no exchange with the outside.
The result is low genetic diversity. And when two individuals are genetically very similar, the second one's immune system may not recognise the first one's cells as foreign. Instead of rejecting them, it lets them grow.
03 80% in twenty years
The disease spread from the northeast to the rest of the island, and the figures are hard: in two decades the total population fell by around 80%, and in some specific areas the drop exceeded 90%.
A collapse like that does not stay confined to one species. The devil is the largest living carnivorous marsupial and acts as the ecosystem's cleaning service: it removes dead animals before they rot. Without it, that role falls vacant and other predators move into the gap.
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04 The backup copy
The response was to build what is called an insurance population: healthy animals, kept apart from the sick ones, the way you keep a backup of a file that is corrupting. By June 2012 that group reached five hundred individuals.
And from November 2012 devils began to be moved to an island off the east coast, a natural enclosure without the disease and with no way for it to get in. An island within an island: the same logic of isolation that had caused the problem, this time used in its favour.
05 Evolution faster than expected
The unexpected part came from the wild populations. By 2016 it was detected that in the worst-hit areas certain genetic variants associated with resistance were becoming more frequent. The survivors were not simply the lucky ones.
This is natural selection happening over a handful of generations, something usually described on far longer timescales. It does not mean the species is safe, but it changes the outlook: total extinction is no longer the outcome that was taken for granted twenty years ago.
06 Where to see them and how to behave
Seeing them in the wild is unlikely: they are nocturnal, scarce and move through scrub. The realistic option is the sanctuaries and conservation centres spread around the island, several of them tied to the insurance programme.
And there is a way of helping that costs nothing: driving slowly on rural roads at night. The devil feeds on carrion, including what is left on the tarmac, so the road draws it precisely to the place where it can be killed.
One idea is worth taking away. The same barrier of water that saved this species when it vanished from the mainland is what left it with so little genetic variety that a tumour could pass from mouth to mouth. Isolation protected it first and exposed it later.
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