Solution to the cane toad problem: let them eat each other

Australians have tried trapping them, tricking them and bludgeoning them to death. Now a pair of researchers believe they may have found a solution to the cane toad problem: let them eat each other out of existence.

While studying captive toads, the Sydney University scientists discovered what they believe to be cannibalistic tendencies. They noticed that adults often wiggle their toes when around young toads, prompting the latter to hop towards them, apparently mistaking the toes for a juicy insect - their main diet. The adults then gobble up the youngsters.

Professor Rick Shine said the toad-eat-toad behaviour could be exploited to reduce the number of the creatures, that have colonised Australia and become the country's worst pest. With adaptation of their habitat, "we might end up with a group of toads that were very good at eating smaller toads,"he told the Australian Broadcasting Corporation. Males are more likely to be cannibals, as they loiter around ponds, where females come to lay their eggs. But females present a greater danger to native animals, as they roam more widely, looking for food.

The toads, which secrete a deadly toxin when threatened, have ravaged populations of indigenous fauna, including kookaburras, snakes, goannas (a type of sand monitor) and quolls (native marsupial cats). Their skin remains poisonous after death, and even dingoes and freshwater crocodiles have died after eating them.

Professor Shine and a PhD student, Mattias Hagman, set up a laboratory experiment to explore cane toad behaviour. Toads were separated by clear glass, and not allowed to eat each other, as that would have contravened ethical guidelines.

The pair noticed that baby toads only approached adults that wiggled their toes - always the middle toes on their hind feet. Professor Shine said: "[The toe waving] seems to be beautifully designed to arouse the feeding responses of a small cane toad. They presumably think it looks like a small insect." The researchers also used freeze-dried toads, to which they attached mechanically controlled fake toes. They recorded how far the young toads moved towards the toes as they wiggled them at different speeds and even changed their colour. They found that nature could not be improved on.

"We showed that the way that the real toads do it is in fact the best possible colour and speed ... to lure baby toads in," said Professor Shine. He suggested that physical adaptation of ponds, to encourage toads to breed in the same place, could increase cannibalism. If more young were eaten, there would be a drop in the number of toads mature enough to breed.

His team also discovered that male toads often drown females when copulating with them in water. If females were targeted through culling so they were outnumbered by males it would lead to more being drowned.

There are also plans to use a parasite found on native frogs, and discovered to be effective in killing cane toads or stunting growth.