Paracelsus' poison

Attack of the Killer Tadpoles

The Cane Toad. Young Bufo marinus

I’ve written before about the toxins secreted by the Cane Toad. The toad’s venom is a complex mix of chemicals. The most important is related to digoxin, the toxin found in the Foxglove plant, and works the same way. It changes the excitability of the heart muscle, causing the heart to contract wildly and ineffectually, or stop completely.

Structure of Marinobufagin. Ian Musgrave

This brew of poisons is what makes an animal that would normally be a crunchy treat into a last meal for native lizards, local marsupial carnivores and dogs. The surviving predators tend to give the toxic toads a wide berth.

Yet, ironically, this very toxin that protects the toad may be its undoing, according to some clever research reported in The Conversation today.

Because it’s not just the adults that are poisonous. The toads eggs also produce some of the toxins that adults produce, and secrete them into the water.

Lots of animals use chemical signals to find food (think sharks following traces of blood) or mates (pheromones secreted by moths and other insects that drift on the wind) where mere visual identification would not be effective. These chemical signals can also be used as a warning (don’t eat me, I’m poisonous).

The wafts of toad egg toxin drifting through the water deter predators from eating the eggs (water beetles for example, rather than the dogs and lizards the adults face). This generally works well, except for one problem.

The cane toad tadpoles are cannibals.

Any pond where toads lay their eggs will have a finite amount of food. If all the Cane Toad eggs in a pond hatched, the tadpoles would rapidly run out of food. So it pays the early hatching tadpoles to hunt down and eat the Cane Toad eggs that have not yet hatched.

But if Cane Toad eggs are hidden in murky waters in the breeding ponds, how does a Cane Toad tadpole efficiently hunt down the eggs? By following chemical cues. And what chemical cue is unique to the Cane Toad eggs? The very toxin that protects them from other predators! No other animal in Australia produces these toxins.

Of course, Cane Toad tadpoles are resistant to the toxins that the Cane Toad eggs secrete, so the toxin is not a deterrent to them snacking down on the eggs.

We know this because of the research by Rick Shines’ team, reported in the Proceedings of the Royal Society B (full free article here).

Now here’s the clever bit: the researchers used this information to devise a way to clear out Cane Toads from their breeding areas.

By smearing a small amount of Cane Toad toxin in a trap, and letting the toxin leak out into the water of a pond, the hungry Cane Toad tadpoles seeking to consume their as yet unhatched brethren sniff out the poison, swarm up into the traps and are caught.

It’s like the pheromone traps we use to catch insect pests, but instead of attracting them with a potential mate, we attract the tadpoles with a potential meal.

Excitingly, it looks like the traps were able to virtually clear out the ponds of Cane Toad tadpoles with minimal effects on other fauna. Probably because the toxin that attracts the Cane Toad tadpoles repels native fish and native frogs.

Currently our methods of control are backbreakingly laborious, either hand collecting adults (like in of Toad Day Out, when North Queenslanders compete to gather as many of the warty creatures as possible for mass extermination), or traps for adults, which require frequent emptying.

We can see how well that has worked, by the relentless spread of the pest across northern Australia. You only need a few pregnant females to escape and your work is undone.

The toxin traps have the potential to clear breeding ponds efficiently, and to be easily transportable to wild terrain.

Of course, the Cane Toad is spread over such a wide area and has penetrated into such difficult to reach landscapes that any control method will be facing heroic challenges.

Still, it would be gently ironic if the very toxin that makes Cane Toads such a devastator of native wildlife were to contribute to their removal or control.