Cane toads ahead of competition

2006-02-18 12:22

Sydney - Australia's pesky cane toads are evolving longer legs so they can be first to the juicy bugs in virgin areas, a researcher said on Thursday.

New arrivals in wetlands have legs that are closer to half the length of their bodies compared with just a third in stay-at-home populations of the toxic amphibians.

"These supertoads are sprinting along like trained athletes," Sydney University biologist Rick Shine told Australia's ABC Radio. "Short of them developing jet propulsion, it's hard to imagine them going all that much faster."

Cane toads are a pest because they decimate native animals in the semi-tropical far north as they spread across the continent from their east coast haven.

They were introduced to Queensland from Hawaii in 1935 to gobble up the cane beetles savaging the sugar cane crop and have since become a hated invader in all points north, south and west of the cane fields. They ooze a toxic substance from their backs that kills potential predators - even animals the size of wild dogs and kangaroos.

Advantages of being ahead of the game

Professor Shine is deeply impressed with the warty cane toad that can grow to the size of a dinner plate.

"Conservation biologists need to think again about evolution," he said. "We tend to imagine these are very stable systems - a toad is a toad is a toad - but we forget that they are very dynamic and toads are changing."

It's the pace of change that thrills Professor Shine most.

In the 1940s and 1950s, when biologists started studying the tide of cane toads sweeping across the continent, the front was advancing at a rate of 10km a year. Now, the rate is more like 50km or 60km a year.

He explains the benefits of natural selection: "If you're an invading species, there are advantages in being at the front - lots of juicy bugs, no competition. If that's the case then any toads that happens to be born with longer legs is the one likely to get to the invasion front."

Bigger brains

These supertoads give birth to more supertoads, who in turn leapfrog the stragglers and race to the front line.

As well as longer limbs, the new breed of supertoads seem to have bigger brains too. Just as homing pigeons in England have learned to use the motorway system as a quick way of finding the way back to their roosts, cane toads have taken to the tarmac to travel faster.

Shine found that convoys of cane toads were travelling at night up the Stuart Highway towards the Northern Territory's famed Kakadu National Park.

"We've been very kind to the toads by putting in roads," he said. "We're basically hanging out a welcome sign, asking them to invade as rapidly as they can." The convoys were covering up to 1km a night on the Alice Springs-Darwin route. -Sapa-dpa

- SAPA