Survival of the ‘not-so-fit’?

Strong, healthy Tasmanian devils could be at greater risk of contracting a deadly, transmittable form of cancer than weaker devils, a new study suggests.

The research, conducted by scientists from Australia and the US, and published in Ecology Letters, showed that Tasmanian devils who contracted devil facial tumor disease (DFTD) tended to have higher survival rates, prior to disease-induced death, than devils who didn’t contract the disease. Infected devils also appeared to show higher reproductive rates than their non-infected counterparts, with more annual breeding attempts and larger litter sizes.

“It’s an important finding,” said Dr. Konstans Wells, head of the study and a researcher at Griffith University’s Environmental Futures Research Insititute. “It indicates that the fittest devils, which are the ones typically engaging in mating or aggressive behavior, are at highest risk to acquire tumors.”

DFTD is transmittable cancer, first observed in 1996, which is most often spread between Tasmanian devils when they bite each other. It results in the appearance of large tumors around the face and mouth and often leads to death within about 6 months. The disease has had a serious impact on the Tasmanian devil population, and they’re now considered an endangered species.

The study is the result of 10 years of data collection, obtained by Dr. Rodrigo Hamede and Associate Professor Menna Jones from the University of Tasmania, who kept tabs on a population of devils in western Tasmania.

Interestingly, the data appears to contradict the commonly held idea that infectious diseases are most often spread through the infection of fewer fit individuals. Professor Hamish McCallum, a co-author of the paper, described the situation: “It’s more commonly thought that infectious disease attacks the weakest members of a population – for example, our annual flu epidemics are primarily a problem for the old and otherwise sick. But this disease is a bit different. It shows that animals who are otherwise very ‘fit’ (in the evolutionary sense) are exactly the ones the disease takes out.”

According to Professor McCallum, an explanation for the increased likelihood of ‘fitter’ Tasmanian devils developing DFTD could be the way by which the disease is spread. “A good hypothesis is that what’s going on is that it’s the most socially dominant animals that are most likely to contract the disease because they initiate most of the aggressive and mating encounters and we know the disease is spread by biting,” he said.

The study also revealed some promising news for the Tasmanian devils; the data most recently collected seemed to show a decline in force of infection and transmission rate of DFTD. This could potentially be because the number of socially dominant devils, who could be responsible for DFTD transmission through biting, are falling – but it could also be an indication that the devils’ immune response is becoming more adept at protecting from the disease’s deadly effects.

“Our results show a recent decline in the likelihood that the devils become infected,” said Professor Andrew Storfer from Washington State University. “This could indicate some evolving resistance of devils to cancer.”