Tasmanian demons should be extinct by now. With a deadly, highly contagious face cancer plaguing demon populations, predictions from the last decade more or less spelled out for the iconic marsupial.
Only 25,000 devils (Sarcophilus harrisii) remain, below the 150,000 of the 1990s, but a new analysis offers hope. The devil’s facial tumor disease has become much less transmissible since the height of the epidemic, suggesting it won’t end the species, researchers report in the December 11 Science issue.
Instead, the disease may remain at lower levels or “the tumor itself may disappear,” says Andrew Storfer, an evolutionary geneticist at Washington State University in Pullman.
Storfer and colleagues reconstructed the history of tumor spread by analyzing changes in tumor genes that evolve on a regular, clockwise basis. Samples from 51 tumors dating back to 2003 helped calibrate this timeline.
Although the disease was discovered in 1996 (SN: 3/11/13), the study found that it probably originated years earlier, in the 1980s, circulating slowly at first. At its peak in the late 1990s, each affected demon infected another 3.5 demons, on average, usually by bite. Recently, that number has dropped to one, suggesting that the epidemic may disappear.
The slowdown may be due to population decline: fewer demos means fewer opportunities for transmission for a disease that is spreading faster within dense groups. In addition, the tumor itself could become less transmissible; researchers have identified some genes that could underlie this change. Finally, the demons themselves appear to have evolved disease resistance (SN: 30/08/16).
But demons are still in danger and some experts want to introduce captive-bred animals to increase numbers. That could happen again, Storfer says, by allowing the disease to take off again. "It sounds boring, but doing nothing may be the best option for the demons."