Australasian Science: Australia's authority on science since 1938

Cane Toads Are Their Own Worst Enemy

By Stephen Luntz

An alarm pheromone released by cane toads could be the key to their control.

So far cane toads have survived everything that humans and the Australian environment have thrown at them in their quest to conquer the continent’s north. However, it seems one creature can subjugate the invader: the cane toads themselves.

“Cane toads are different from native Australian frogs in their possession of a sophisticated communication system used by tadpoles,” says Prof Rick Shine of the University of Sydney.

Shine helped to discover an alarm pheromone that indicates the presence of a threat, causing toad tadpoles to decamp for safer territory. However, repeated exposure is harmful. “If they experience this chemical frequently while developing, many of them die, apparently from stress,” Shine says.

Having isolated and synthesised the active ingredient in the tadpoles’ alarm response, Shine may have a mechanism for stunting their growth. However, an even more useful discovery arose when Shine’s team realised that tadpoles can home in on the smell of freshly laid cane toad eggs, which they eat.

“The big benefit is the removal of future competitors, because a cane toad is another cane toad’s worst enemy,” Shine says. “There’s probably been a 30-million-year arms race, with the eggs under pressure not to be detectable and the tadpoles under pressure to detect.”

A third chemical may explain how detection has triumphed. Female cane toads avoid laying eggs in a water body in which eggs already exist, so in the normal course of events the eggs laid by astute mothers can signal their presence in safety.

But the cane toad arms race goes further still. When a tadpole detects the presence of eggs but cannot reach them, it emits a chemical that kills or stunts younger rivals. Shine says that his team have not identified this suppression pheromone yet, but “merely the presence of older cane toad tadpoles in the water surrounding the eggs is enough to wreck the development of the tadpoles which emerge from those new eggs.”

Dosing water bodies with such a pheromone offers obvious benefits, but Shine also says that the chemicals emitted by developing eggs may be used to lure tadpoles into traps “without attracting the native tadpoles”.

“The Kimberley is a large place, and we won’t be spraying the chemicals from low-flying helicopters,” Shine says. “However, they are well-designed for deployment by community groups, so we may be able to suppress the toads near population centres or in areas with pockets of endangered native species.”