How crocodiles were taught to stop eating deadly toxic cane toads

Wild crocodiles in Australia keep dying from eating toxic cane toads, so scientists have trained them to avoid the deadly meal by giving them a memorable dose of food poisoning.

Cane toads (Rhinella marina) were introduced to Australia in the 1930s to control agricultural pests in the sugar cane industry, but have themselves become a devastating environmental threat, wreaking havoc with native wildlife as they have spread relentlessly across the continent.

Native predators are naïve to the threat posed by the toads’ toxic glands, which secrete a compound called bufotoxin. Eating these toads is almost always deadly, says Georgia Ward-Fear at Macquarie University, Australia. “There is no opportunity to have a non-fatal encounter and learn a lesson not to eat them,” she says.

That is certainly the case for freshwater crocodiles (Crocodylus johnstoni), with populations of the animals in some areas of northern Australia collapsing by more than 70 per cent as the first waves of the toads swept through.

Researchers have had some success training some other affected Australian species, including monitor lizards and quolls, not to eat the toads. They do this by removing the toxic glands from the toads and instead infusing them with chemicals that cause nausea, making the predators averse to eating them in the future.

Now, Ward-Fear and her colleagues have attempted the technique on freshwater crocodiles. The team monitored crocodile populations in four target areas in the Fitzroy Valley region of north-west Western Australia, as the toads approached in September 2021.

They set nearly 2400 baits – removing the toxin from toad carcasses and infusing them with lithium chloride, which is known to induce non-fatal nausea in reptiles. The team also set unbaited chicken necks as a control.

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

The crocodiles also seem to have learned to avoid newly arrived live cane toads. In areas where the toads had recently arrived prior to the training, crocodile mortality fell by 95 per cent, and in areas where they arrived after the aversion training there were no recorded deaths from cane toad poisoning. The team repeated the baiting programme in 2022 and found the crocodiles were still averse to eating the baited toads. “It has been kind of surprising how well this has worked,” says Ward-Fear.