At dusk, crocodiles lurking in the Leonard River pop up and salivate over the carcass of a toad hanging from a metal boom.
As everyone knows, this is a bait deliberately set by scientists, and this is still a toad with ingredients, not an ordinary salty "seasoning", but a "seasoning" that can make crocodiles disgusting.
"Bait". Image source: Reference[1]
Why do scientists do this?
It all started more than 80 years ago. In 1935, 102 cane toads were introduced from Hawaii to sugarcane growing regions in Queensland and released into sugarcane fields. Toads were introduced with the intention of using them to prey on the larvae and adults of sugarcane beetles.
Toad. Image source: Wikipedia
Cane toads multiply and spread rapidly in new environments. They adapted to Australia's climate and environment and began to quickly occupy new habitats. With no natural predators and no competitors, the population of these toads increased rapidly, while the pests that were intended to be controlled were not affected at all, as sugarcane toads are mainly active on the ground, while these pests live underground.
However, cane toads pose a significant threat to Australia's native invertebrates, amphibians, reptiles and small mammals. They compete for food resources and prey on many native species, with the most serious problem being that the cane toad's skin and glands contain toxins that are deadly to many native Australia predators.
In the 90s of the 20th century, the spread of the cane toad gradually extended northward, approaching the border between the Northern Territory and Western Australia. In the early 2000s, cane toads began to invade northern Australia's river systems in large numbers, and crocodile habitats were the first to be poisoned and killed by eating cane toads. At this time, the mortality rate of the freshwater crocodile population begins to rise, and the river is often filled with belly-turning crocodile carcasses.
Crocodile carcass. Image source: ABC, Ref. [2]
After dissecting the crocodile's carcass, scientists found that their stomachs still had the remnants of the Last Supper: toad meat. Toad toxin works by affecting the heart and nervous system of animals, causing cardiac arrest or neurological disorders.
Why do you eat it when it's poisonous? Because this toxin is not like an irritant like capsaicin, which can cause intense discomfort or pain immediately in the mouth, crocodiles generally have nothing to fear.
Crocodiles are also very quick to attack and devour their prey, rarely chewing or judging the texture and taste of their prey when they eat it. The sugarcane toad, its slow, seemingly harmless prey, is easily mistaken by crocodiles for a common frog or other non-venomous amphibian, so crocodiles will quickly devour it.
Does taste aversion training really work?
In order to reduce the death of crocodiles and prevent them from eating poisonous toads, scientists came up with a solution: feed the toad to make it unpalatable. Technically known as conditioned taste aversion learning (CTA), this experiment is a learning method based on classical conditioning in which animals experience unpleasant physical reactions (e.g., nausea, vomiting) after ingesting a certain food to develop an aversion to that food and avoid eating it again in the future.
Starting in 2021, scientists began a three-year study in the Kimberley region of northwestern Australia, home to Northern Australia's largest freshwater predator: the Australian freshwater crocodile (Crocodylus johnstoni).
The research team deployed 2,395 bait points in four canyon systems in northwestern Australia, at each bait point, two metal piles were set up and bait was hung between the piles. There are two kinds of bait, one is the experimental group, which is the carcass of treated toads, which are detoxified and injected with 5ml of lithium chloride (LiCl) at a concentration of 0.5mol, while the other metal column is hung with a chicken neck.
A crocodile that bites the bait. Image source: Reference[1]
Chicken necks were used as non-treatment bait to help researchers determine whether crocodiles' disgust responses were specific to those of cane toads. If the crocodile does not show an aversive response to the chicken neck, but only to the cane toad, this indicates that the "CTA training" was successful, and that the crocodile's aversive response is directed at the specific food of the cane toad, rather than a general response to all foods.
Lithium chloride was chosen as a "seasoning" not just because it was salty, but because it can produce a nausea reaction. When an animal ingests lithium chloride, it can cause gastrointestinal upset, which in turn triggers nausea and vomiting reactions. Although the specific mechanism may vary from species to species, it is often related to its metabolism in the body and its effects on the central nervous system.
Historically, lithium chloride has been briefly used as a substitute for table salt (NaCl). In the aftermath of World War II, scientists began looking for low-sodium alternatives to help control salt intake as the association between high-sodium diets and high blood pressure became increasingly recognized. Lithium chloride was considered a possible substitute at the time because it was similar in chemical structure and taste to sodium chloride and had a salty taste.
In the late 40s of the 20th century, lithium chloride was used in certain low-sodium salt products, mainly for people with high blood pressure and those who needed a low-sodium diet. However, the use of lithium chloride quickly shows serious side effects, that is, gastrointestinal discomfort symptoms such as nausea, vomiting, diarrhea, etc. may occur.
Due to the toxic reaction of lithium chloride, related products were quickly removed from the shelves, and the use of lithium chloride as a salt substitute was discontinued in the early 50s of the 20th century. Although lithium chloride is no longer used in food, it still plays a role in medicine and animal experiments (e.g. "CTA training" in research). After the "CTA training", the effect was particularly noticeable in the short term, with a feeding rate of 92% on the first day and a drop to 51% on the fifth day.
Throughout the experimental period, the number of crocodiles remained stable and there were no mass deaths. In one national park, for example, in the first two years (2019 and 2020) after the cane toad invasion, the crocodile mortality rate in the area was 33% and 30%, respectively. After the implementation of "CTA training" (2021), the mortality rate dropped sharply to 1.5%, and in 2022 it rose to 11%.
In control areas that were not treated with "CTA training", the mortality rate of crocodiles was significantly increased. In 2020, 2021 and 2022, crocodile mortality rates in these regions were 20%, 40% and 36%, respectively. To put it simply, the method of feeding the cane toad is useful, and no crocodile likes to eat food that will make him sick to his stomach.
Alligator. Image courtesy of rainforest
However, "CTA training" may not be a long-term strategy, and although "CTA training" can be effective in getting animals to avoid specific foods in the short term, it is still questionable whether this aversion response can persist in the long term. Studies have shown that in some cases, animals may gradually return to aversion to aversion over time, especially when other food sources are lacking.
Also in Australia, for example, researchers used CTA to train northern mink rats to avoid eating poisonous cane toads. Although initial training was successful and minks were able to avoid feeding on cane toads, some minks waned their aversion response over time, especially in areas where cane toads were widespread. When food is scarce, mink rats may risk trying to feed the cane toad again, causing the training to be less effective.
Therefore, it is really difficult to engrave the memory of hating toads in the genes, and it may not be solved by two or three years of experiments.
bibliography
[1] Ward-Fear G, Bruny M, Rangers B, et al. Taste aversion training can educate free-ranging crocodiles against toxic invaders[J]. Proceedings of the Royal Society B, 2024, 291(2028): 20232507.
[2] Snijders L, Thierij N M, Appleby R, et al. Conditioned taste aversion as a tool for mitigating human-wildlife conflicts[J]. Frontiers in Conservation Science, 2021, 2: 744704.
[3] HANLON L W, ROMAINE M, GILROY F J, et al. Lithium chloride as a substitute for sodium chloride in the diet: Observations on its toxicity[J]. Journal of the American Medical Association, 1949, 139(11): 688-692.
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Produced by丨Popular Science China
Author丨Su Chengyu is a popular science creator
Producer丨China Science Expo
Editor-in-charge丨Dong Nana
Reviewer丨Xu Lai Linlin