Imma Perfetto
Cosmos science journalist
Scientists have successfully saved freshwater crocodiles from toxic cane toads invading northern Australia with an unusual new tactic – doctored cane toad carcases.
By teaching freshwater crocodiles (Crocodylus johnstoni) to associate cane toads (Rhinella marina) with a bout of food poisoning, they reduced death rates by at least 95%.
Across the dry season (May to October) between 2019 and 2022, Macquarie University scientists worked on the project with Bunuba Indigenous rangers and the Department of Biodiversity, Conservation and Attractions (DBCA) in Western Australia.
They collected cane toads, removed the poisonous parts, and injected the bodies with a nausea-inducing chemical that caused the crocs eating them to feel temporarily sick.
It’s a behavioural ecology method known as conditioned taste aversion, and it worked remarkably well.
“The first three days we noticed the crocodiles were taking the cane toads, then they would go away,” says Bunuba ranger coordinator Paul Bin Busu, whose team set up hundreds of bait stations across 4 large gorge systems in the Kimberley region of north-western Australia.
The doctored cane toads were deployed alongside chicken meat control baits to monitor the effectiveness of the training.
“Then we noticed they would smell the cane toad before eating, and on the last day we noticed that it was mostly the chicken necks getting eaten,” says Bin Busu.
The team used nocturnal ‘spotlighting’ surveys and remotely triggered wildlife cameras to monitor crocodile and toad numbers following the intervention.
“Our baiting completely prevented deaths in areas where cane toads were arriving and decreased deaths by 95% in areas where toads had been for a couple of years,” says Macquarie’s Dr Georgia Ward-Fear, who is lead author of the report detailing the findings in Proceedings of the Royal Society B.
Ward-Fear says these effects continued in the years following.
Some populations of freshwater crocodiles in tropical Australia have fallen by more than 70% due to ingesting cane toads.
“Freshwater crocs can be heavily impacted as their river systems dry out during the late dry season,” says Ward-Fear.
“They end up congregating in large numbers with very little food, and as toads begin to use these waterbodies for rehydration, the two come into contact and we see large numbers of crocodile deaths over a few months.”
Now, conditioned taste aversion interventions can be planned both ahead of and behind the cane toad invasion front in areas with similar ecology.
