July 15, 2008 -- An infectious, fast-spreading cancer is so prevalent among Tasmanian devils that the entire life history and breeding process for these carnivorous marsupials has changed in recent years, causing females to breed as teenagers instead of as adults, concludes a study published in the latest Proceedings of the National Academy of Sciences.
The finding represents the first known case of an infectious disease leading to earlier reproduction in a mammal.
Tasmanian devils remain on the brink of extinction, with some populations reduced by as much as 89 percent due to devil facial tumor disease (DFTD). The disease results in disfiguring facial tumors and causes victims to die within months. Humans may have inadvertently caused it.
Lead author Menna Jones, a University of Tasmania zoologist, explained to Discovery News that "exposure to a carcinogen or a cocktail of chemicals, such as accidental poisoning" or some kind of natural, genetic change could have led to the disease, which scientists first detected in the mid 1990's.
"Once one individual has developed the tumors, they are passed on as a rogue cell line through direct transfer of tumor cells through biting," Jones said, adding that most penetrating biting injuries occur among adult males and females in the mating season.
"The cells then grow as an allograft -- like a tissue graft -- assisted by the very low genetic diversity of devils and subsequent failure of the immune system to recognize them as non-self," she said.
Jones and her team studied data on breeding and population dynamics from five Tasmanian devil sites within Tasmania, an island and state off the southeastern coast of Australia.
They determined that before the disease struck, females would begin breeding as adults at the age of two, and would then continue to produce a litter annually for three years, with death generally occurring when they turned five or six. Now, 16 times more females than before start to breed at the age of one, when they are still considered to be "teenagers," and they may not even survive long enough to rear their young.
Both females and males are promiscuous, with up to four fathers per litter of four pups. The pups live in their mother's pouch for four months and are then suckled by their mother in a den for another five months.
"If the mother gets a DFTD tumor, she will only live for about six months," Jones said. "Whether she can raise her young depends on how old they were when she got the tumor. They might survive if she died one to two months prior to weaning, but probably not if she died earlier."
The researchers theorize that since Tasmanian devil populations have been so dramatically reduced, the surviving pups may enjoy greater food supplies and reduced competition from adults, which could permit the earlier breeding. While the fate of the species remains unknown, this series of events may represent an evolved mechanism to stave off extinction.
Stephen O'Brien, chief of the Laboratory of Genomic Diversity at the U.S. National Cancer Institute, told Discovery News that he was surprised "to see the almost instantaneous adaptation/selection" for younger animals to reproduce.
"Watching an extinction event is a very scary and harrowing process, yet this is what the Tazzy devil scientists are seeing close up," he added.
Chris Johnson, a James Cook University professor who specializes in both Australian marsupials and the biology of extinction, expressed deep concern for not only Tasmanian devils, but also for other animals in their ecosystem that may suffer.
He predicts that since Tasmanian devils are now maturing, breeding and dying at younger ages, they will decrease in physical size, "making them less capable of competing effectively with feral cats, which are widespread in Tasmania, and red foxes."
"Both foxes and cats have proved to be extremely destructive predators of mainland Australian wildlife," Johnson added. "I hope that the diminution of the Tasmanian devil, both in abundance and size, will not release further cat and fox impacts in Tasmania, but I am afraid that this will happen."
Related Links:
Jennifer Viegas' Blog: Born Animal
San Diego Zoo Tasmanian Devil Page
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