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September 03, 2012

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Tasmanian devil may be saved by a bit less devilishness

Sept. 3, 2012
Courtesy of the British Ecological Society
and World Science staff

E­volv­ing to be­come less ag­gres­sive could be key to sav­ing the fa­mously fe­ro­cious Tas­ma­ni­an dev­il from ex­tinc­tion, re­search sug­gests. 

The over-the-top snarling and mu­tu­al vi­cious­ness among the small Aus­tral­ian mam­mals is leg­end­ary and makes for in­ter­est­ing watch­ing. But their fre­quent bit­ing al­so spreads Dev­il Fa­cial Tu­mor Dis­ease, or DFTD—a can­cer that’s wip­ing the un­usu­al crit­ters out.

A male Tasmanian devil (Image © Sarah Peck)
The new study found that sur­pris­ing­ly, dev­ils that get bit­ten more of­ten are less likely to be­come in­fected. This means that quite lit­er­al­ly, the meeker mem­bers of the spe­cies might be the ones that among their kind even­tu­ally in­her­it the Earth, as ma­lig­nan­cies grad­u­ally pick off the most fierce in­di­vid­u­als.

The find­ings are “sur­pris­ing and coun­ter-in­tui­tive,” said Ro­drigo Ha­m­ede, lead au­thor of the stu­dy, pub­lished Sept. 3 in the Jour­nal of An­i­mal Ecol­o­gy on­line. “In most in­fec­tious dis­eases there are so-called super-spreaders, a few in­di­vid­u­als re­spon­si­ble for most of the trans­mis­sion. But we found the more ag­gres­sive dev­ils, rath­er than be­ing super-spreaders, are super-receivers.”

Hamede, of the Uni­vers­ity of Tas­ma­nia in Aus­tral­ia, and col­leagues set up doz­ens of traps at two sites for 10-day pe­ri­ods eve­ry three months be­tween 2006 and 2010. They then recorded the pat­tern of in­ju­ries in the dev­ils, and iden­ti­fied any tu­mors. One of the sites, called West Pen­cil Pine, was cho­sen be­cause dev­ils there seem to be less badly hit by the dis­ease.

The re­search­ers found that the lev­el of bites was si­m­i­lar at both sites; dev­ils with few­er bites were sig­nif­i­cantly more likely to de­vel­op the dis­ease; and most tu­mours oc­curred in the mouths. “This means that more ag­gres­sive dev­ils do not get bit­ten as of­ten, but they bite the tu­mours of the less ag­gres­sive dev­ils and be­come in­fected,” said Hamede.

Be­cause there is no treat­ment or vac­cine for the can­cer, the find­ings and the next stage of the re­search have im­por­tant im­plica­t­ions for sav­ing the spe­cies from ex­tinc­tion, he added. “Our next step is fas­ci­nat­ing. First we need to ex­plore the ge­net­ic dif­fer­ences that might be less­en­ing the im­pact of DFTD in the West Pen­cil Pine dev­il popula­t­ion. Sec­ond, we need more de­tailed da­ta on dev­il be­hav­iour to de­fine ‘shy’ or ‘bold’ types. We could then use this in­forma­t­ion to de­vel­op a man­age­ment strat­e­gy to re­duce the spread of the dis­ease by boost­ing nat­u­ral se­lec­tion of less ag­gres­sive, and there­fore more re­sil­ient, dev­ils.”

Nat­u­ral se­lec­tion is an ev­o­lu­tion­ary pro­cess in which en­vi­ron­men­tal pres­sures kill off or in­hi­bit re­prod­uc­tion am­ong crea­tures that are ge­net­ic­ally less suit­ed for a par­tic­u­lar nat­u­ral set­ting. As a re­sult, their more well-a­dapt­ed peers grad­u­ally take over the gene pool.

Un­der­stand­ing how in­fec­tious dis­eases spread is key to con­trol­ling them, but stu­dying dis­ease trans­mis­sion in wild an­i­mals is of­ten very dif­fi­cult, Hamede said. And in DFTD, ecol­o­gists al­so need a bet­ter un­der­stand­ing of dev­il be­hav­iour. Dev­ils are sol­i­tary yet so­cial an­i­mals. They don’t live in groups but meet of­ten, ei­ther dur­ing mat­ing, es­tab­lish­ing so­cial hi­er­ar­chies or when feed­ing around car­cass­es—all oc­ca­sions when they bite each oth­er.

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Evolving to become less aggressive could be key to saving the famously ferocious Tasmanian devil from extinction, research suggests. The over-the-top snarling and mutual viciousness among the small Australian mammals is legendary and makes for interesting watching. But their frequent biting also spreads Devil Facial Tumor Disease, or DFTD—a cancer that’s wiping the unusual critters out. The new study found that surprisingly, devils that get bitten more often are less likely to become infected. This means that quite literally, the meeker members of the species might be the ones that among their kind eventually inherit the Earth, as malignancies gradually pick off the most fierce individuals. The findings are “surprising and counter-intuitive,” said Rodrigo Hamede of the University of Tasmania, the lead author. “In most infectious diseases there are so-called super-spreaders, a few individuals responsible for most of the transmission. But we found the more aggressive devils, rather than being super-spreaders, are super-receivers.” The findings are published Sept. 3 in the Journal of Animal Ecology online. Hamede and colleagues set up dozens of traps at two sites for 10-day periods every three months between 2006 and 2010. They then recorded the pattern of injuries in the devils, and identified any tumors. One of the sites, called West Pencil Pine, was chosen because devils there seem to be less badly hit by the disease. The researchers found that the level of bites was similar at both sites; devils with fewer bites were significantly more likely to develop the disease; and most tumours occurred in the mouths. “This means that more aggressive devils do not get bitten as often, but they bite the tumours of the less aggressive devils and become infected,” said Hamede. Because there is no treatment or vaccine for the cancer, the findings and the next stage of the research have important implications for saving the species from extinction, he added. “Our next step is fascinating. First we need to explore the genetic differences that might be lessening the impact of DFTD in the West Pencil Pine devil population. Second, we need more detailed data on devil behaviour to define ‘shy’ or ‘bold’ types. We could then use this information to develop a management strategy to reduce the spread of the disease by boosting natural selection of less aggressive, and therefore more resilient, devils.” Natural selection is an evolutionary process in which environmental pressures kill off animals that are genetically less suited for a particular natural setting. As a result, their more well-adapted peers gradually take over the gene pool. Understanding how infectious diseases spread is key to controlling them, but studying disease transmission in wild animals is often very difficult, Hamede said. And in DFTD, ecologists also need a better understanding of devil behaviour. Devils are solitary yet social animals. They don’t live in groups but meet often, either during mating, establishing social hierarchies or when feeding around carcasses—all occasions when they bite each other.