Tasmanian Devils are learning to live with the cancer that was pushing them to extinction

Tasmanian devils have been battling Devil Facial Tumor Disease (DFTD), a rare form of transmissible cancer, for over 20 years, and many scientists have been concerned that the disease could push them to extinction.

But hope has appeared recently, as the tough little predators appear to be developing genetic adaptations that could allow them to live with the disease.

Devil Facial Tumour Disease

Tasmanian devils are the world's largest carnivorous marsupials. They're confined to the island of Tasmania, off the south coast of Australia.

The squat, low-slung predators, about as big as a medium-sized dog, have a vicious bite and a frightening scream, which helped earn them the nickname 'the devil.'

Between 1996 to 2015, more than 80 per cent of the devil population was wiped out by DFTD.

"It's one of those unfortunate tricks of biology in which cancer cells become immortal and transmissible," said Rodrigo Hamede from the School of Natural Sciences at the University of Tasmania, in an interview with Bob McDonald on Quirks & Quarks. Hamede has been studying the devils since 2004.

"[Transmissible] cancers are not very common around the world. There are only eight of them that we know of, and two of them are in Tasmanian devils, so they're incredibly unlucky."

DFTD has spread through the devil population through facial bites, which occur commonly as devils socialize and assert dominance.

The bites end up transferring cancer cells into healthy animals. Solid tumours then quickly grow around the facial area, and can break teeth and jaws, go through nostrils and eyes, and produce ugly and disfiguring wounds. The tumours can eventually kill by metastasizing into internal organs or by preventing the animals from feeding.

"It's a very graphic disease," said Hamede. Most devils die within a year or year-and-a-half of contracting DFTD.

A more than twenty year battle

During the first 10 years of the epidemic from 1996 to early 2000s, the species went from abundant in Tasmania to being listed as endangered.

Hamede, who's been studying the interplay between the devils and the disease, watched the epidemic work its ugly course.

"The population decline was catastrophic," said Hamede. "It's unusual in such a short time span to have a single disease that could affect a population so deeply."

But despair about the future of the devils has slowly been turning into hope in recent years, as many devils were observed to be surviving longer with the cancer.

"Over the last five to six years, we've become a little more optimistic," said Hamede. "We have seen signatures in the devil and the tumour that may have resulted in the devils learning to coexist with the tumour." Natural selection seemed to be working by promoting the spread of genes that helped prolong the animals' survival.

Researchers saw changes in key regions of the devil's genome, known to be associated with cancer and immune function in humans, after six to eight generations, or about 12 to 15 years. 

The devils were surviving long enough to add another litter to the population, and female devils were reaching sexual maturity in their first year instead of second, allowing them to reproduce before the disease killed them.

The researchers found evidence that some devils were even able to fight off the tumours, said Hamede. "These are animals who get infected, and somehow months later, the tumours are not there anymore or they're smaller."

An evolutionary arms race may become a negotiated truce

While the devils are adapting to the cancer, the cancer is changing as well. The host and pathogens are both trying to adapt to each other, explained Hamede.

"Tumour is a living entity and there's been changes in the genome of the tumour in some instances that have led it to become more harmful," said Hamede. But he thinks there will be evolutionary pressure on the tumour to become less harmful as well, since ultimately it depends on the survival of its host for its own survival and propagation. After all, if the devils go extinct, so will the tumour.  

The ultimate outcome is still uncertain. But in the long term, given what's seen so far, Hamede thinks the two will evolve to coexist. 

"If the devils can learn to live longer with the tumours, they'll be able to reproduce and survive as a species."

But the cancer is not the only threat to the devil's survival. Things like habitat fragmentation and deaths from vehicle traffic become much bigger threats when the animals are already in a vulnerable state.

If the devils were to be lost, Hamede says it would be devastating for the Tasmanian ecosystem because they're a top predator.

"If we lose the devils, we lose a lot more than the value of a species. We lose balance in the ecology, in the system of Tasmania."