Cane toads were introduced to Australia in 1935

The toads (Bufo marinus) were only introduced in the 1930s but have already overwhelmed the local wildlife in Queensland with their rapid reproduction and toxic flesh, which kills many predators foolish enough to make them a meal.

But for two species of snake, at least, natural selection has produced a defence: the snakes have developed relatively smaller heads and longer bodies.

In essence, the reduced gape of the animals limits their ability to eat the toads likely to do them the most damage.

It's nice to see that Nature's looking after itself Dr Ben Phillips, University of Sydney

"We've got large lizards, such as monitor lizards, that seem to die after eating cane toads; a lot of our snakes after eating them will die," explained Dr Ben Phillips, of the University of Sydney.

"All the native frog-eating creatures in Australia, and the native cat that we have, are disappearing quite dramatically from areas where cane toads are turning up," he told the BBC World Service's Science In Action programme.

"Basically, large predators that would normally eat frogs are succumbing to cane toads quite dramatically."

Quick adaptation

The cane toad was introduced in 1935 to help control a crop pest, but has since become a nuisance itself.

Its range in Queensland has steadily expanded and the toad is now moving into New South Wales and the Northern Territory.

The way the two species of snake have adapted to cope with this challenge has been described as a classic example of "contemporary evolution".

The red-bellied black snake (Pseudechis porphyriacus) and the green tree snake (Dendrelaphis punctulatus) are highly susceptible to toad toxins.

And the presence of Bufo marinus has imposed an immense selection pressure on their populations.

"One of the ways the snakes seem to be fighting back is by changing their body shape. Basically, their heads have got smaller relative to their bodies (or their bodies have got bigger relative to their heads; whichever way you want to think about it)," said Dr Phillips.

"If a snake's got a small head, it's going to be able to eat a much smaller prey item."

"What that means is that because snakes eat their prey whole and the size of meal is entirely dependent on the size of its head; if a snake's got a small head, it's only going to be able to eat a small prey item.

"Thus, it's going to be able to poison itself a lot less effectively on a cane toad - which is probably a good thing, given that they seem to be a little bit silly about eating things that taste bad."

Natural selection ensures these are the snakes that prosper and reproduce; their head-body traits come to dominate populations.

Alien response

What seems remarkable is that this adaptation has occurred in just 70 years. But Dr Phillips says it should not be too surprising since snakes breed comparatively quickly.

"We need to remember that snakes have a generation time of two or three years; so basically that means a time of 20 to 25 generations has passed since the cane toads arrived in some areas," he said.

"That's a reasonable amount of time, evolutionarily speaking."

The University of Sydney researcher commented that it was encouraging to see that ecosystems could respond to problems imposed by invasive species.

"I think it's a bad idea to leave species around the place - it's almost impossible to know what kind of impact they will have," he said.

"But the upside of what we have found is that while it's all doom and gloom about the environment - we hear a lot of bad news - it's nice to see that Nature's looking after itself."

Dr Phillips and his colleague Dr Richard Shine published their work in a recent edition of the journal Proceedings of the National Academy of Sciences.