The list of ‘deadly animals in Australia’ just got a little weirder. The cane toad, a toxic, invasive species notorious for devouring anything it can fit in its mouth — household rubbish, small rodents and even birds — has become highly cannibalistic in the 86 years since it was introduced to the continent, according to a new study. Its counterpart in South America, where cane toads originated, is far less cannibalistic.
The discovery could help researchers to understand the evolutionary underpinnings of how this uncommon and extreme behaviour emerges. Scientists have seen cannibalism evolve in species before, says Volker Rudolf, a community ecologist at Rice University in Texas, who studies the phenomenon. But what’s exciting about this work, he says, is that the researchers are almost seeing it “develop in front of their eyes”, given that the behaviour arose in less than a hundred years — the blink of an eye by evolutionary standards.
“These toads have gotten to the point where their own worst enemy is themselves,” says Jayna DeVore, an invasive-species biologist at Tetiaroa Society, a non-profit organization in French Polynesia, and a co-author of the study, which was published on 23 August in Proceedings of the National Academy of Sciences of the United States of America1. Scientists estimate that there are well over 200 million of the amphibians in Australia. They have become so abundant, says DeVore, that they face more evolutionary pressure from each other, as they compete for resources, than from anything else in Australia.
Tadpole terror
Farmers first introduced about 100 cane toads (Rhinella marina) to Australia from their native range in South America in 1935 to control cane beetles (Dermolepida albohirtum), which were wreaking havoc on sugarcane plantations. The giant toads failed to knock down the beetle populations, but they succeeded in epically multiplying. Because of their highly poisonous skin, which is coated in bufotoxins, they had no natural predators and went on to invade large swaths of the northern and eastern parts of the country.
Although adult cane toads are fearsome — they grow up to 25 centimetres in length — it’s their tadpoles that are usually the cannibals. Multiple tadpoles together can gobble more than 99% of the hatchlings that emerge from the tens of thousands of eggs in a single clutch2.
DeVore and colleagues were curious to see whether the cannibalistic behaviour was common across all cane toads, or if it was due to how invasive the Australian ones are. So they collected cane toads from Australia and from French Guiana, and bred them to produce hatchlings and older tadpoles. The team then exposed a single tadpole to 10 hatchlings from its group — either from Australia or South America — hundreds of times and found that invasive Australian tadpoles were 2.6 times as likely to cannibalize hatchlings as native South American ones.
Researchers have long known that the Australian tadpoles are attracted to the hatchlings because of the scent of the younger animals’ toxic skin. “You’ll get this huge avalanche of thousands and thousands of tiny cane-toad tadpoles coming toward this chemical,” says Rick Shine, an evolutionary biologist at the University of Sydney in Australia, and a co-author of the study. DeVore, Shine and co-workers saw this play out in their experiments: the Australian tadpoles were nearly 30 times as likely to swim towards a trap containing hatchlings as an empty trap, and the South American tadpoles showed no preference for either.
Although the speed with which the toads evolved this behaviour is impressive, the team was even more surprised by how fast the animals evolved a defence to protect against it. The researchers found that when invasive Australian hatchlings shared a tank with caged, older tadpoles from the same group, the hatchlings were more likely to have a shorter developmental period than that of the South American hatchlings. Older tadpoles don’t tend to eat older tadpoles — so the toads might have evolved to speed up their hatchling phase, the researchers found. This would limit the amount of time they spend vulnerable to cannibalism, even if the adaptation eventually stunts their growth, says DeVore.
Roshan Vijendravarma, an evolutionary biologist at the Curie Institute in Paris, who has studied cannibalism in fruit flies, says the differences between the invasive and native toads’ behaviour probably have a genetic basis, given how extreme they are and how quickly they evolved over relatively few generations of toads.
Shine and his colleagues think this idea is worth exploring and are studying it now. Although there are still mysteries around the cane toads’ cannibalistic tendencies, one thing is for certain, says Shine: “The cane toads that are currently hopping across Australia are extraordinarily different animals from the ones that were first taken out of the native range.”