How crocodiles were made to stop eating deadly, poisonous cane toads

Wild crocodiles in Australia continue to die from eating poisonous cane toads, so scientists have trained them to avoid the deadly meal by giving them a significant amount of food poisoning.

Cane toads (Rhinella Marina) were introduced to Australia in the 1930s to control pests in the sugar cane industry, but they have now become a devastating environmental threat in their own right, wreaking havoc on native wildlife as they spread inexorably across the continent.

Native predators are unaware of the threat posed by the toads’ poisonous glands, which secrete bufotoxin, a compound that can even kill humans. Eating these toads is almost always fatal, says Georgia Ward-Fear of Macquarie University in Australia. “There’s no way to have a non-fatal encounter with them and learn not to eat them,” she says.

This certainly applies to freshwater crocodiles (Crocodylus johnstoni), with populations of the animals plunging by more than 70 percent in some areas of northern Australia as the first waves of the toads moved through the area.

Researchers have successfully persuaded other affected Australian species, including monitor lizards and quolls, not to eat the toads. They do this by removing the toads’ poisonous glands and instead giving them chemicals that cause nausea and deter the predators from eating them in the future.

Now Ward-Fear and her colleagues have tried the method 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.

They laid out nearly 2,400 baits by removing the poison from toad carcasses and adding lithium chloride, which has been shown to cause non-fatal nausea in reptiles. The team also laid out unbaited chicken necks as a control sample.

Initially, almost all of the toads and chicken necks used as bait were eaten. But within five days, symptoms of food poisoning spread throughout the four local crocodile populations. The predators became alert and stopped eating the toads, but continued to use the chickens.

The crocodiles also appear to have learned to avoid newly arrived live cane toads. In areas where the toads had recently arrived, crocodile mortality dropped by 95 percent, and in areas where they arrived after aversion training, no deaths from cane toad poisoning were recorded. The team repeated the baiting program in 2022 and found that the crocodiles were still averse to eating the baited toads. “It was quite surprising how well this worked,” says Ward-Fear.