How crocodiles were taught to stop eating deadly poisonous cane toads

As wild crocodiles in Australia continue to die from eating poisonous cane toads, scientists have trained the animals to avoid the deadly meal by giving them a memorable dose of food poisoning.

Cane toad (Rainella Marina) was introduced to Australia in the 1930s to control pests for the sugar cane industry, but has spread relentlessly across the continent, wreaking havoc on native wildlife and becoming a devastating threat to the environment.

Native predators are unaware of the threat posed by the toads’ poison glands, which secrete compounds called bufotoxins. Eating these toads is almost always fatal, says Georgia Ward-Fear of Macquarie University in Australia. “There’s no chance of a safe encounter and learning a lesson by not eating them,” she says.

That is certainly true for freshwater crocodiles (Crocodylus johnstoni), and in some parts of northern Australia, animal populations fell by more than 70 percent during the first wave of toad infestations.

Researchers have had some success training other affected Australian species, such as monitor lizards and quolls, not to eat toads: they remove the poisonous glands from the toads and instead inject them with a nausea-inducing chemical that makes predators reluctant to eat them in the future.

Now Ward-Fire and her colleagues are testing the technique on freshwater crocodiles. The team monitored crocodile populations in four target areas in the Fitzroy Valley region of northwest Western Australia as the toads approached in September 2021.

The researchers set out about 2,400 baited toad carcasses, which they then injected with lithium chloride, a substance known to detoxify and induce non-fatal nausea in reptiles, and unbaited chicken necks as controls.

Initially, the baited toads and unbaited chicken necks were nearly all eaten, but within five days, when food poisoning symptoms spread to four local crocodile populations, the predators began to wise up and stopped eating the toads, but continued to eat the chickens.

The crocodiles also appeared to learn to avoid newly arrived live cane toads. In areas where toads had recently arrived before training, crocodile mortality dropped by 95 percent, and in areas where toads arrived after aversion training, no deaths from cane toad poisoning were recorded. The team repeated the feeding program in 2022 and found that the crocodiles were still averse to eating the baited toads. “We were kind of surprised that this worked,” Ward-Fear says.