Australia's "Road Warrior" Toads Get Arthritis

Stephanie Peatling in Sydney, Australia
for National Geographic News
October 15, 2007
For cane toads the scourge of Australia's vast north, spreading farther and faster is literally backbreaking work, a new study shows.

The toads that jump—and thus populate new territory—the fastest are more likely to be larger and have longer legs. But this advantage also has a drawback: up to 10 percent of the biggest invasive toads suffer from arthritis, a new study says.

The large, yellow toads, native to South and Central America, were introduced into the northeastern state of Queensland in northern Australia in 1935 in an attempt to stop cane beetles from devastating sugarcane crops.

Now up to 200 million of the poisonous toads exist in the country, and they are rapidly spreading west through the state of Northern Territory at a rate of up to 37 miles (60 kilometers) a year. (Related news: "Poison Toads Leap Across Australia" [November 29, 2004].)

The toads haves severely impacted ecosystems in Australia. Predators—and sometimes pets—that eat toads die immediately from their poison, and the toads themselves feast on any animal they can fit in their mouths. Cane toads also compete with native frogs for habitat.

Evolution of the Fastest

Study co-author Rick Shine is a professor in evolutionary biology at the University of Sydney.

"There has been lots of attention [given] to the impacts of invading species on natural ecosystems, but less thought has been given to the challenges that the invaders face themselves," he said.

Shine and colleagues studied nearly 500 preserved toads from three populations: one from Queensland and two from the Northern Territory. (See a map of Australia.)

The Northern Territory toads that Shine's team studied were very different from their Queensland relatives.

"They are very active, sprinting down the highway and breeding as they go. They are moving as fast and as far as you can imagine," Shine said.

The fastest toads travel nearly half-a-mile (one kilometer) a night, he added.

"Toads with longer legs are moving faster and traveling longer distances, while the others are being left behind," he said.

"It's a classic evolutionary phenomenon, the evolution of the fastest." (Related news: "Toxic Toads Evolve Longer Legs, Study Says" [February 15, 2006].)

Shine and his research colleagues believe the toads leading the charge are the offspring of bigger and faster toads.

But the speed and strength comes at a cost: arthritis of the spine due to constant wear and tear and a heightened susceptibility to a soil-based bacteria that contributes to the degenerative joint disease.

"Toads aren't built to be road warriors," Shine said. "They are built to sit around a swamp. They are pushing themselves so hard against the constraints of their bodies that they are putting too much pressure on the spine and immune system."

Frogs and toads generally do not get arthritis, he added.

David Skelly is an ecology professor at Yale University in New Haven, Connecticut.

Skelly said that Shine and colleagues have asked a question few invasion biologists have ever bothered to ask: "What is the effect of invasion on the invader?"

"Invasion biology is a big field and these folks, by asking an uncommon question, will have a big influence on how scientists will study invasion in the future," Skelly said by email.

For instance, understanding illness in invaders may be useful in identifying potential methods of controlling them, he added.

The study appears this week in the journal Proceedings of the National Academy of Sciences.

Halting Progress

Graeme Sawyer is the founder of the Northern Territory community organization FrogWatch, which monitors and tries to halt the progress of cane toads.

The fact that cane toads are still moving quickly west and wreaking havoc on native wildlife as they go is important, not their leg length, he said.

He also said he is not convinced by the research, pointing out the toads' expansion may be more to do with seasonal conditions.

A good wet season means the toads move faster and farther while a drier season slows them down, he said.

Sawyer and his team of volunteers have just finished a trial project that gives them hope for eradicating the pesky amphibians.

The team set up a fence around waterholes to prevent the toads from drinking at night.

"If we knew where all the waterholes [were] we could destroy the population," Sawyer said.

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