Tasmanian devils must evolve to be less aggressive if they are to avoid becoming extinct, suggests new research.
The study sheds new light on an infectious cancer threatening to wipe out the species' wild population, which only exists on the Australian island of Tasmania. The tumours caused by the devastating disease, known as devil facial tumour disease (DFTD), interfere with feeding and affected animals often starve to death.
Rodrigo Hamede and his team at the University of Tasmania investigated the connection between the infection of DFTD, which is spread when one animal bites another, and the number of bites that an animal received.
They found that devils with fewer bites – the more aggressive ones – were more likely to develop the disease. "Our results, that devils with fewer bites are more likely to develop DFTD, were very surprising and counterintuitive," said Hamede. "In most infectious diseases there are so-called super-spreaders, a few individuals responsible for most of the transmission. But we found the more aggressive devils, rather than being super-spreaders, are super-receivers.
"This means that more aggressive devils do not get bitten as often, but they bite the tumours of the less aggressive devils and become infected," said Hamede.
They hypothesise that evolution should favour less aggressive types of devil, which could reduce the rate of transmission of the disease.
"We are hoping that there will be some sort of co-evolution towards coexistence between the devil and the disease," said Hamede, whose work was published in the Journal of Animal Ecology on Monday. "There may be some indication that devils can somehow learn to behave in a way that makes them less susceptible to acquiring the disease."
The team caught devils using traps over a period of three years and recorded any pattern of injuries and any tumours they could observe.
Understanding the behavioural and ecological circumstances associated with the transmission of DFTD is the key to controlling its spread in devil populations. There is no treatment for, or vaccine against, DFTD.
Since it was first identified in 1996, sightings of the Tasmanian devil have declined by more than 80%. As of 2009, the Tasmanian devil has been listed as endangered on the IUCN red list, the benchmark for the global conservation status of plant and animal species.
DFTD is one of only two infectious cancers in the world, the other one is the canine transmissible venereal tumour in domestic dogs. DFTD, however, is the only one to be considered 100% fatal. It is a clonal disease which means that the same cancerous cell line developed in a single Tasmanian devil 15-20 years ago and has survived by direct transmission between the devils.