Science & Nature

Tasmanian devilry

I learned many months ago about devil facial tumor disease, or DFTD, a infectious cancer that is wiping out the Tasmanian Devil population so rampantly and dramatically that in only 12 years, the devil population has shrunk anywhere from 62% to 95%. That’s roughly 170,000 scrappy little devils in the early 1990s to anywhere from 40,000-70,000 today. The infectious cancer is thought to be so virulent that devils could be extinct in little more than a decade.

As sexy evolutionary biologist Olivia Judson points out in her NY Times science column, there is room for concern (not of the shrill apocalyptic sort, but of the “wow, there’s so much we don’t understand” sort).

Devils are a homogeneous species, having evolved on remote Tasmania after the sea claimed the land bridge that connected it to Australia. While it shared that island for a (modern) time with the Tasmanian Tiger (Thylacine), it survived while the Tiger went extinct, probably in the first half of the 20th century (none of these gorgeous Tigers — marsupials, actually — has been spotted since the 1930s).

Judson is concerned primarily with the reasons for and implications of transmissible cancers, at both the devil, human, and entire biological levels. As she points out, devils transmit their cancer to each other in a way that is not possible in humans (with the rare exceptions of in utero transmission or instances of sexually transmitted human papilloma virus). She uses the alarming example of kissing a person with throat cancer and then getting throat cancer.

What interests me, however, is the micro-evolutionary adjustment devils seem to be making to counteract the cataclysmic plunge in their survival rates. Of course, they don’t make this adjustment consciously, nor can the behavior I’m about to describe technically be called evolution since evolution, by definition, occurs at glacial paces across mostly unfathomable spans of time.

In a nutshell, devil females breed at ages 3-4. Since many infected females aren’t living that long, thanks to the cancer, they are now starting to breed at 1 year of age. Natural selection favors adaptation. As Richard Dawkins points out in The Blind Watchmaker, the cheetah who evolves to run faster meets the antelope who evolves to more quickly escape him. Antelopes that do not increase their speed over time perish and their slow genes do not survive the selection process.

The question is, are we witnessing a sped-up version of adaptation in Tasmanian devils? In only 12 years, they have modified their reproductive behaviors to account for their retarded life spans. Twelve years! In evolutionary time, that’s an angstrom-wide appointment on a mile-wide calendar.

Whether this is evolution in accelerated action or not, the more interesting question is whether it will work. More species have gone extinct in Earth’s history than all the combined extant species multiplied by a large number. We don’t have a lot of modern empirical data to clue us into what’s happening to the devils — many of the species we watch disappear do so related to changes or reductions in their physical environment. To watch a species expire because of an infectious disease apparently is rare.

Is it possible that certain homogeneous animals, gripped by a fatal mutation, experience accelerated evolution as a sort of death throe? How many times has this happened before? And if it’s happened before, did the radical changes to reproductive behavior ever succeed? Could the devils save themselves simply by breeding more quickly and in greater numbers in the same way we “chicken-soup” ourselves through a devastating cold? Although there’s science behind it, can you really explain why you feel better, when sick, after a hot bowl of chicken soup? You are acculturated to accept it on instinct that chicken soup helps get you past the misery.

Extrapolate this out to the devils and wonder if a confluence of devil disposition, immunological reaction, and genetic imperatives for survival are making a mad dash to cross the finish line before the death rate trumps the statistical likelihood that the Tasmanian devil can avoid extinction.

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