Contagious cancer has been sweeping through populations of Tasmanian devils, killing most of the adults. Now, young devils are breeding earlier than ever before. But is this a case of rapid evolution or just a fleeting response to a changing environment?
Tasmanian devil facial tumor disease surfaced in Tasmania about 10 years ago and now affects the majority of devils on the island (Science, 18 February 2005, p. 1035). Lesions around the animal's mouth can grow as large as Ping-Pong balls and spread over the face; unable to eat, the devils die of starvation within months of the cancer's appearance. The disease is highly contagious, and adults are particularly susceptible, possibly because the tumor cells are often spread during sexual contact. The problem has proved so devastating that Tasmanian devils were declared an endangered species in May.
Zoologist Menna Jones of the University of Tasmania has also noticed a sharp increase in pregnancies among 1-year-old devils. Normally, the animals, which live about 6 years, don't breed before age 2. But when Jones and colleagues gauged the age and reproductive status of devils--via tooth erosion and sex organ development--they found evidence for breeding several months to a year earlier than normal at four out of five sites studied. The proportion of early breeding females reached a surprising 83% at one location, the team reports this week in the Proceedings of the National Academy of Sciences.
The shift toward early breeding could be an evolutionary response, says Jones. By killing any adults that breed after age 2, the cancer could be genetically selecting for younger breeding females, she notes. If so, the findings would make the devil the first known mammal to rapidly evolve its reproductive patterns in response to a disease.
Evolutionary biologist Nelson Hairston of Cornell University is skeptical. The young devils may have started breeding early simply because they have better access to food and mates, now that fewer adults are on the scene. To truly make the case for genetic evolution, he says, the team would have to show that devils taken from zoos, for example, would not adopt early breeding in response to more food and more mates.
Hairston also notes that scientists have shown rapid evolution in mammals such as rabbits and sheep, though not in response to disease (Science, 16 March 2007, p. 1571). So the Tasmanian story wouldn't prompt a sea change in our understanding of evolution. "But it would add a sexy example [of the phenomenon]," he says.
With reporting by Lauren Cahoon.