Toads Caned by Own Poison

By Stephen Luntz

Cane toad poison can be trap the toad’s tadpoles so effectively that it may be possible to preserve high value conservation areas from this amphibious menace.

Last year Prof Rick Shine of the University of Sydney’s School of Biological Sciences outlined a number of ways in which the sensitivity of the toads to chemicals produced by their own species might be used against them (AS, Jan/Feb 2012, p.9). Now research published in the Proceedings of the Royal Society B has revealed just how effective one of these approaches has been.

Shine’s team pondered the fact that toad tadpoles are attracted to dead toads. After testing a number of possible chemicals, team members discovered that the poison itself is a powerful attractant to cane toad tadpoles while repelling those of most native species. “A few species just ignore it,” Shine says.

Funnel traps baited with secretions from the shoulders of adult toads attracted so many tadpoles that bodies of water in which the traps were placed were clear of toads afterwards. This is a stunning finding given the difficulties of removing toads using other techniques.

Shine notes that a single female toad can lay 30,000 eggs. “This means that even if you catch and kill 99% of the adult toads in an area, the few that are left can produce so many offspring that before you know it you are back to where you started – just as many cane toads as ever.”

Toads have colonised such large areas of Australia that Shine considers eradication impossible. However, by placing traps in areas with high concentrations of threatened species, Shine believes it may be possible to avert the worst of Rhinella marina’s damage, particularly in parts of Western Australia that are currently toad-free.

“In continuing work with our collaborators at the University of Queensland we are developing an even stronger, safer and easier-to-use bait,” Shine says. “A purified version, possibly in tablet form, would last longer and be easier to handle.”

Shine also believes that the shape of the funnel trap may be improved to catch more tadpoles. However, after trapping 40,000 tadpoles in a single natural pond in less than a week, it is clear the traps, and poison, are well on the way to widespread use.

While being drawn to adult toads appears a risky move for tadpoles in a cannibalistic species, Shine explains: “The worst enemy of a toad tadpole is another tadpole. They’ve learned to recognise eggs and eat them quickly to destroy them.

“We realised the chemical in the eggs the tadpole picks up on is the same one in the adult poison. They put a lot of poison into the eggs to protect against other predators, but it makes them vulnerable to their own species.”

Other potential toad chemicals, such as one that deters females from laying eggs in waters where other toads have been, are now on the back burner.