Can Taste Aversion Protect Australia's Wildlife from Cane Toads?

Cane toad

Australia is home to some of the most fascinating wildlife seen nowhere else in the world. However, with the advent of European colonization, the country had seen an influx of alien species that have been colonizing its native wildlife for decades. One of these invasive conquerors was a foreign import known as the cane toad. Also called the marine toad and giant neotropical toad, this New World native was brought to Australia in 1935 to help eradicate the country's beetle population which was devastating the sugarcane crops. Unfortunately, the beetles took refuge on top of the sugarcane stalks and the toads were too heavy to climb up. Instead, the toads began multiplying exponentially and spread across Australia competing against the country's native amphibians and other wildlife for food, water, and space. They also posed a significant threat to carnivorous animals thanks to their toxic defense which when ingested, causes hallucinations to a would-be predator with death resulting from cardiac arrest. It is no wonder Australian people have been coming up with techniques to keep the cane toad numbers in check.

A freshwater crocodile investigating a freshly set bait. The compound-injected cane toad is dangling from a stake on the left, while a chicken neck suspended on the right is used as a control.  

Recently, in a study published in Proceedings of the Royal Society B, scientists from Macquarie University have reported to have implemented a method which has helped mitigate Australia's freshwater crocodile mortality rate by as much as 95 percent. The new technique involves feeding the crocodiles dead altered cane toads to cause food poisoning. While the crocodiles do not end up being intoxicated, they do experience a temporary unpleasant feeling which makes them less likely to eat the toxic toad in the future. According to conservation scientist one of the authors of the study, Georgia Ward-Fear, encounters between the crocodiles and toads occur couple of months a year during the dry season when rivers and gorges in northern Australia dry out. This results in small pools of water which attracts both animals, and ends with tens of hundreds of crocodiles dying within a period of six to eight weeks at the end of the dry season. Ward-Fear and her team, which included the Department of Biodiversity, Conservation and Attractions and a group of Bunuba rangers, came up with a strategy to warn the crocodiles of the toads. The team would first cut off the top part of the toads' bodies and remove most of the poisonous parts. They would then inject a chemical compound in the toads that brings out nausea in the crocodiles when eaten. The bait would be hung from stakes at the water's edge, along with suspended chicken carcasses as a control. After eating the bait injected with the compound, the crocodiles become sick with food poisoning but do not die. Instead, the resulting taste aversion would be deeply encrypted in the crocodiles' brains and they would learn not to eat the toads. By using this technique, Ward-Fear and her team were able to decrease the crocodile mortality rate by 95 percent in Danggu Geikie Gorge National Park where the toads had turned up two years ago.

The endangered northern quoll is one of many native carnivores threatened by the cane toad.

I find it really impressive and helpful that this method of using taste aversion is being implemented to protect Australia's crocodiles from cane toads. This technique has even been applauded by IUCN's bear conservationist Dr. Dave Garshelis, who utilized taste aversion more than twenty years ago to prevent black bears from eating military rations. However, after one year, the association between bears and tainted rations diminished. According to Dr. Garshelis, same situation may happen with the crocodiles and would mean the technique would probably be repeated over and over again. I agree with Dr. Garshelis that taste aversion doesn't have a long-term effect in preventing crocodiles from eating cane toads. Just like in the case of black bears, crocodiles would learn to distinguish pure toxic cane toads from dead nausea-inducing ones and this might result in more cases of poisoning. This is why I believe more research needs to be done in implementing other techniques to mitigate cane toad numbers in Australia. For example, how to identify and properly destroy cane toad spawn without harming the native wildlife. These poisonous amphibians have been terrorizing Australia's wildlife for nearly ninety years with an estimated 200 million hopping amok in the country. They have killed a wide-range of carnivorous animals which, besides crocodiles, include snakes, goannas, and quolls that see them as a potential meal. They have also shown up in towns and cities, where cats and dogs have been poisoned from biting or licking the toads. It is absolutely crucial to work in an effort to stem the tide of cane toads from further devastating Australia's ecosystems and other places where they have been introduced.