Researchers are seeking ways to encourage freshwater crocodiles to avoid eating toxic cane toads, thereby reducing their mortality rates.
A team of scientists from Macquarie University is working with the Bunuba Indigenous forestry officers and the Department of Biodiversity, Conservation and Attractions (DBCA) in Western Australia to test new measures aimed at protecting freshwater crocodiles from the invasive and dangerous cane toads that are spreading across northern Australia, as reported by Phys.org on August 13. The freshwater crocodile (Crocodylus johnstoni) is culturally significant in the region, and the disappearance of this predator would disrupt the fragile balance of the local ecosystem.
Dead cane toads used to lure freshwater crocodiles. (Photo: Phys.org).
A study published on August 14 in the journal Proceedings of the Royal Society B indicates a method that significantly reduces the mortality rate of freshwater crocodiles in areas first invaded by cane toads through teaching them to associate cane toads with food poisoning. The research team, led by Dr. Georgia Ward-Fear from the School of Natural Sciences at Macquarie University, along with her colleague, Professor Rick Shine, documented the successful implementation of a behavioral ecological method known as conditioned taste aversion (CTA).
Bunuba forestry coordinator Paul Bin Busu, along with forestry officers Kristen Andrews and Karl Bin Busu, reported notable changes in crocodile behavior for the program. After a successful intervention trial, Sara McAllister from the DBCA’s cane toad control program can plan future expansions in areas with similar ecosystems.
After cane toads were imported from the Americas in the 1930s, they caused the death of many native animals in northern Australia, wiping out several local predator species such as goannas, snakes, and freshwater crocodiles. These species are key food sources for local communities and play an essential role in the environment but lack natural defenses against the toxic cane toads.
The research primarily took place from May to October, during the dry season in northern Australia, when river systems can shrink into isolated ponds, supporting fewer shrimp, fish, and amphibians in the typical diet of freshwater crocodiles. According to Dr. Ward-Fear, freshwater crocodiles can be severely affected when river systems dry up at the end of the dry season. They gather in large numbers with very little food. As cane toads begin to utilize these ponds to rehydrate, the two species come into contact with each other, leading the research team to witness a significant number of crocodile deaths within a few months.
From 2019 to 2022, Dr. Ward-Fear and Professor Shine collaborated with local forestry officers and DBCA staff to distribute cane toad carcasses across four major gorges in the Kimberley region of northwestern Australia. Bunuba and DBCA collected hundreds of cane toads, removed their toxic glands, and injected the carcasses with enough emetic compounds so that crocodiles would feel temporarily ill but not die after consuming the bait.
Chicken bait without added emetic agents was used to monitor the effectiveness of the conditioned taste aversion training. According to Paul Bin Busu, who directed the forestry team placing hundreds of baits along the riverbanks, they observed that in the first three days, crocodiles chose cane toads and then swam away. Subsequently, they sniffed the cane toads before eating them. On the final day, they noticed that the crocodiles predominantly chose to eat chicken necks.
Using night surveys and remote-triggered cameras to monitor the number of crocodiles and cane toads, the research team found that areas where taste aversion trials were conducted had significantly lower crocodile mortality rates compared to other locations.
