Arthritic cane toads hobbling across Australia
Cane toads are paying a debilitating price for their invasion of Australia, researchers show. It seems the stress of constant movement has left many of the toxic pests vulnerable to severe arthritis.
Professor Rick Shine and his colleagues at the University of Sydney have been following the reviled pests as they spread into the Northern Territory.
Their latest research, published online today in the journal Proceedings of the National Academy of Sciences, shows that roughly one in 10 of the largest toads are suffering severe spinal arthritis.
The arthritis is linked to infection with the common soil bacteria Ochrobactrum anthropi.
In part, the situation seems to have arisen because the toads have evolved longer legs to adapt to the invasion, Shine says.
"Frogs' bodies are not well designed for long distance athletic challenges.
"But because of the invasion process, the cane toads have evolved into these road warriors that are travelling every night during the wet season."
During that season, the invasion front advances an average 130 metres each day, he says. For the toads, that means an extraordinary amount of physical wear and tear.
"There's a huge ball of bony tissue that's grown up around the joints," says Shine. "It substantially affects their ability to hop around."
Although radio tracking of the arthritic toads shows they keep moving, the researchers have found that if you chase them down a track, they begin taking shorter hops and show signs of stress after just a few metres.
"It certainly looks like they are extremely uncomfortable," he says.
Immune system run down
The fact that the toads are suffering in this way suggests their immune systems are worn down, which could prove to be a good thing for efforts to control the spread of the hopping pests.
"I think it's telling us it's not easy being an invader and that may well give us an Achilles heel when we start to talk about how to control toads. These guys at the invasion front are doing it tough."
The arthritis might also act as a brake on the accelerating spread of the species, Shine suggests.
"It may be that we're not going to see a great deal more acceleration of the toad front because they're really reaching the point that that's about as much as you can get a frog's body to do."