Published online 15 October 2007 | Nature | doi:10.1038/news.2007.165

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Invading cane toads suffer from arthritis

Health problems could be Achilles' heel for Australia’s fast-hopping toads.

cane toadThe faster they travel, the more likely their joints are to hurt.M. Greenlees / PNAS

Cane toads rampaging across Australia have for years been portrayed as an unstoppable invasive force of super-amphibians. But it seems their march across the country is costing them dearly — it is making the invaders prone to arthritis.

The findings hint that invading animals may be more vulnerable to infections designed to stop their spread.

Cane toads (Bufo marinus) were introduced into northeastern Australia in 1935 to control crop-eating insects. But they spread quickly, and have been blamed for declines in native animals in colonized areas. Today, ecologists and farmers are doing their best to keep the invading hoards from expanding any further.

Now Rick Shine and his colleagues have found that toads at the front of the invading population are suffering arthritis as a result of their fast-hopping travels1.

This is likely to be a big shake-up to those studying invasive species: the idea that an introduced animal capable of establishing itself and decimating local wildlife is nonetheless under severe stress is a novel one.

"In my experience this is a totally unprecedented observation," says Dave Skelly, an amphibian expert at Yale University in Connecticut. "Thinking about the costs of invasions is something that deserves our attention. This is going to change how researchers in the field think about their systems."

Long legged

Shine, an evolutionary biologist at the University of Sydney, Australia, had previously shown that toads at the invasion front tend to have longer legs than those trailing behind, allowing them to conquer more territory more quickly2.

While conducting this research, Shine had also noticed a high number of spinal abnormalities in the advancing cane toads. By examining preserved specimens taken from invading and established toad populations, Shine's group found that about 10% of large invading adults had severe spinal arthritis — which in toads manifests as bony growths fusing the joints between vertebrae.

And by looking at live populations, Shine and his colleagues determined that the faster, larger toads were more likely to have arthritis, they report in the Proceedings of the National Academy of Sciences1.

Cultures from affected joints point the finger of blame at Ochrobactrum anthropi, a usually harmless and common soil bacteria that seems to be infecting toads made vulnerable by the stress of travel.

"Basically, the body plan of frogs [and hence toads] has evolved to support a fairly sedentary lifestyle: sit beside the swamp, catch bugs, et cetera," says Shine. "The invasion process has put very different pressures onto cane toads, and has resulted in the evolution of an animal that, based on radiotracking, moves long distances and does so every night." This gives the toads access to more food, but isn't doing their joints any favours.

Racing invalids

To see whether arthritis hinders the toads' invasion, Shine raced arthritic toads against healthy ones. On a 10-metre runway, the arthritic toads quickly slowed down compared with their healthy brethren. However, when toads were fitted with radio transmitters and released into the wild, the arthritic toads seemed to move just as far as the healthy critters.

"Toads are incredibly stoic animals," says Shine. "We confidently expected that the arthritic toads would travel less far under field conditions than same-sized non-afflicted toads. Instead, the arthritic toads simply keep on going. They can’t keep moving rapidly for long distances, and are forced to take shorter hops. But they keep going nonetheless."

But the work could still lead to ways to control the toads.

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Shine has been investigating the possibility of using lungworm parasites, which are known to kill some toads, to control the advancing hoards. The new research suggests that invasion-front toads have weakened immune systems, so they may be particularly vulnerable to such parasites.

"The toads may be vulnerable to other immune challenges," says Shine. "The high levels of stress they experience may well prove an Achilles' heel that we can exploit to help control these troublesome invaders." 

  • References

    1. Brown, G. et al. Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0705057104 (2007).
    2. Phillips, B. L. et al. Nature 439, 803 (2006). | Article | ISI | ChemPort |
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