"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.
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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