By Victoria Gill
Science reporter, BBC News
Quolls became locally extinct in Kakadu National Park when cane toads invaded
Scientists in Australia have designed a cane toad "sausage" that could help protect vulnerable predators from the poisonous toads.
The researchers developed the sausage as a bait that could help train animals to avoid eating the large toads.
They employed "taste aversion learning" - adding a nausea-inducing drug to cane toad meat.
This, the scientists say, caused animals to associate the smell of the toads with feeling sick.
Jonathan Webb from the University of Sydney, the senior member of the research team, explained: "It's a really powerful form of learning.
"Many people might have experienced it when they get food poisoning and then associate the taste or the smell of whatever food it was that made them ill with feeling sick."
His team focused on quolls - small carnivorous marsupials that used to be very abundant in northern Australia.
Their numbers have seriously declined in the last 20 years.
"These animals are a real icon in northern Australia," said Dr Webb. "They're very cute and have lots of personality.
It is not entirely clear why the quolls' numbers have declined so much, but the arrival of the invasive cane toads seemed rapidly to make their situation even worse.
"When the toads came along, suddenly the quolls became extinct in Kakadu National Park," said Dr Webb.
"What we were interested in doing was coming up with a practical solution to deal with this population crash when the toads invade."
The challenge, explained Dr Webb, was that the toads have very large toxin glands in their shoulders, primarily containing chemicals called bufadienolides, which can very quickly induce a cardiac arrest.
"The quolls see the toad as a big frog," he explained.
"It looks good to eat, so they just pounce on it and get a fatal dose of toxin. There's no chance they can learn from the encounter."
Catch and release
During the time when he was puzzling over this, he read a story to his children.
"It was a modern version of Little Red Riding Hood," Dr Webb recalled. "And at the end, the grandma, to get her own back, puts a bag of onions in the wolf's tummy so that he wakes up feeling sick.
"At that point I thought: what if we added a nausea inducing chemical to the toads?"
This unusual approach seems to work.
Cane toads have large toxin glands in their shoulders
Dr Webb's University of Sydney colleague, Stephanie O'Donnell, trained 30 quolls - feeding them pieces of dead toad that were laced with a nausea-inducing drug.
"After they ate it, they started to get a little bit crook (ill)," he said.
"The animals didn't vomit - just pawed at their faces for a while and then got back to normal. But the next time they were offered a toad they ignored it."
Dr Webb and his colleagues then released the quolls into the wild with radio collars so they could monitor them.
"In the wild, they did encounter big toads and they ignored them," said Dr Webb.
"You could see they were interested in the toads because they were big and they were hopping around. Some of them followed the toad for a while. But most of them just sniffed it, and then thought - yuck, you're no good to eat - and walked away."
Based on these results, the team is trialling a toad sausage containing minced toad legs with the nausea-inducing chemicals.
"We've had some promising results," said Dr Webb. "We had camera traps and have been able to see that the quolls would eat the sausages.
"What we haven't done yet is see if the quolls that eat the sausages then avoid the real cane toads.
"But it's such a powerful form of learning that the taste and smell of the prey can really override the other cues [such as the visual cue of the sight of the toad]."
Wildlife managers have previously developed bait sausages designed to control feral cats. So if the approach works well, it could be easily deployed - by laying the sausages in habitats to train predators before the toads invade.
Dr Webb said: "We're trialling this with monitor lizards and with bluetongue lizards too - they are also badly affected by the toads."