Tricking freshwater crocodiles in Western Australia into believing toxic cane toads cause food poisoning could stop the crocs dying during future infestations of the pest, a new study which produced dramatic cuts in fatalities has found.
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 set the laced toads in water and the bait is eaten by unsuspecting crocodiles, which are a protected species. Scientists hope the temporary sicknesses will prevent the crocodiles from eating marauding toads during a real invasion and dying. The results so far are promising.
“[It’s like] a bad late night takeaway,” lead author and wildlife biologist Dr Georgia Ward-Fear said. “You remember that taste and you often swear off that food for a very long time, so it’s a relatable experience.”
Cane toad toxins 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 the department of biodiversity, conservation and attractions in Western Australia as part of the trial, in response to cane toads spreading unabated across northern Australia.
Between 2019 and 2022, the team conducted baiting 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 baiting trials were conducted in five-day stints. Toad baits were strung up each day and replenished in the afternoon. Control baits using chicken – without any nausea-inducing additives – were also set up on the first and last day, to see if the crocodiles avoided just the toads, or both.
In all four gorge systems, the crocodiles did not eat as many cane toads by the end of the fifth day, and in fact displayed a distinct aversion to them. But what really astonished scientists was what happened when the predicted cane toad invasions actually arrived.
“We saw no mortalities in those gorge systems, which was really exciting,” Ward-Fear said. “We were expecting large rates of mortality based on what we were seeing in nearby areas.”
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In the gorge system where cane toads had been present for two years, the mortality rate decreased by 95% after the baiting.
Cane toads were first introduced to Australia in the 1930s as a bio control measure against sugar cane beetles. The plan failed – and more than 90 years later the toads are killing native predator species that eat them.
Ranger coordinator, 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-on effect to the food chain, and entire local ecosystem, would be noticeable, he said.
The solution, at least according to this study, is preparing 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.
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