Nasty-tasting cane toads teach crocodiles a lifesaving lesson

All it takes is one miserable night after a bad dinner or drink to make humans avoid an ingredient for life. To teach freshwater crocodiles in Australia to avoid a lethally poisonous toad, all it takes is one very, very nasty toad butt.

Freshwater crocodiles who chowed down on lithium chloride-laced carcasses of invasive cane toads were far less likely to eat live toads when the poisonous amphibians came hopping along. The result could help prevent the predator die-offs that occur as cane toads make their way across the Australian continent, researchers report August 14 in Proceedings of the Royal Society B. 

Cane toads (Rhinella marina) were brought to Australia in the 1930s, and quickly began to wreak havoc on native species (SN: 2/3/14). Their harm comes not from what they eat, but from what eats them. The toads have large glands on their shoulders that contain potent poison. As cane toads spread across the continent, freshwater crocodiles (Crocodylus johnstoni) fall for the tempting snack. In 2008, researchers found that croc population densities along the Victoria River crashed by as much as 77 percent immediately following cane toad invasion.

But what if the crocs could be taught to avoid cane toads before the amphibians arrived?

Georgia Ward-Fear, a conservation scientist at Macquarie University in Sydney, has been testing a technique called conditioned taste aversion on several species of native Australian predators. The predators get exposed to a small taste of toad and live to regret it. In previous work, Ward-Fear and her colleagues released “teacher toads” — tadpoles, eggs or very young cane toads who don’t yet carry enough poison to be deadly — before the invasion front of adults arrived. Large monitor lizards (Varanus panoptes) ate the small toads and got sick, but lived and learned to avoid the lethal adult versions.  

Freshwater crocodiles don’t eat like monitor lizards. “Crocodiles ambush prey and eat whole if they can,” Ward-Fear says, and baby toads are prey far beneath the notice of crocodiles.

To train the crocs, Ward-Fear and her colleagues worked with the Bunuba Rangers, members of an Australian Indigenous group who see freshwater crocodiles as an important part of their dreamtime stories. They collected about 2,400 dead toads and removed the heads, poisonous glands and internal organs. The researchers laced the remaining toad butt with lithium chloride, a chemical that produces powerful nausea. They then dangled the dead toads along with motion capture cameras over the edge of the water across four gorges in the central Kimberley region of Australia, with unlaced chicken serving as a control.

After the live toads arrived, Ward-Fear and colleagues headed back out to find out if the crocodiles had learned from the baits that toads were not to be touched. They searched for any dead crocodiles and cut them open to see if a toxic toad was the cause of death.

Crocodiles learned quickly that once was enough when it came to dead toad butts. The researchers found remarkably few carcasses of crocs who’d dined on live cane toads. At one site — Danggu Geikie Gorge National Park, where toads had already arrived, rangers found 63 dead crocodiles in 2020. But after the team baited the group of crocs that came to the same area in 2021, only three died from eating toads. At another site, Bandlingan National Park, no crocodiles tasted toads after their training. In contrast, a nearby control area saw between 20 and 40 percent of their crocodiles die from eating newly arrived toads.

“This is a wonderfully innovative and effective example of the tremendous potential of conditioned taste aversion as a tool for conservation,” says Colleen St. Clair, a conservation biologist at the University of Alberta in Edmonton, Canada, who was not involved in the study. Many scientists have been leery of conditioned taste aversion since early trials in mammals failed in the 1970s, she says. “I expect this will be a landmark study, showing that a very old [vomit-inducing substance], lithium chloride, is effective for ambush predators like crocodiles.”

Before the study, rangers and scientists alike would find dying crocodiles thrashing helplessly in the water after eating deadly toads. It’s “very distressing for us as wildlife biologists and Indigenous rangers,” Ward-Fear says. The result is both an environmental and emotional success.