Tricking freshwater crocodiles in Western Australia believe that poisonous cane toads cause food poisoning can prevent the crocodiles from dying during future infestations of the plague, a new study has found, yielding dramatic reductions in mortality.
The practice, known as taste aversion training, involves removing the poisonous parts from hundreds of dead cane toads and injecting them with a nausea-inducing chemical – in this case, lithium chloride, a strong salt.
Researchers then put the mature frogs in water and the bait is eaten by unsuspecting crocodiles, which are a protected species. Scientists hope the temporary illnesses will prevent the crocodiles from eating marauding frogs and dying during an actual invasion. The results so far are promising.
“[It’s like] a bad late night takeaway,” said lead author and wildlife biologist Dr Georgia Ward-Fear. “You remember that taste and you often swear off that food for a very long time, so it’s a relatable experience.”
Cane toad venoms are strong enough to kill freshwater crocodiles and most native animals that eat frogs or frog eggs.
Scientists from Macquarie University worked with Bunuba Indigenous rangers and Western Australia’s Department of Biodiversity, Conservation and Attractions as part of the trialin response to cane toads spreading unabated across northern Australia.
Between 2019 and 2022, the team carried out bait trials across four large gorge systems in the Kimberley. This included the Dan͟ggu Geikie Gorge, where cane toads have been present for two years, plus three gorges where invasions of cane toads were expected for the first time.
The bait trials were conducted in five-day intervals. Frog bait was rigged every day and replenished in the afternoon. Control baits with chicken – without any nausea-inducing additives – were also set up on the first and last day to see if the crocodiles avoided only the frogs, or both.
In all four canyon systems, by the end of the fifth day, the crocodiles had not eaten as many cane toads, and in fact showed a clear aversion to them. But what really amazed scientists was what happened when the predicted cane toad invasions actually arrived.
“We didn’t see any deaths in those canyon systems, which was really exciting,” Ward-Fear said. “We expected large death rates based on what we saw in nearby areas.”
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In the gorge system where cane toads were present for two years, the death rate decreased by 95% after the bait.
Cane toads were first introduced to Australia in the 1930s as a biocontrol measure against sugarcane beetles. The plan failed – and more than 90 years later, the frogs are killing native predator species that eat them.
Ranger co-ordinator Paul Bin Busu said losing freshwater crocodiles to cane toads would mean bottom feeders in the rivers would eat all the bait, “leaving no fish for the barramundi and stingray to eat”. The flow-through effect to the food chain and the entire local ecosystem will be noticeable, he said.
The solution, at least according to this study, is to prepare specific populations at risk of being invaded by cane toads, rather than trying to eliminate all toads from the environment – which has so far proven impossible.
