Cannibal toads reveal ‘fast-moving evolution’, study says

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In Australia, poisonous toads have become their worst enemies.

For decades, scientists have witnessed cane toad tadpoles devouring their young parents in the puddles and ponds they share. The cause of the cannibalistic behavior was a mystery until now.

A new studypublished this month in the journal Ecology and Evolution, found that cane toad tadpoles in Australia develop an insatiable appetite when exposed to a toxin found in cane toad eggs, the same toxin that makes poisonous toads.

Cane toads, native to South and Central America, were introduced to Australia in 1935 by scientists who hoped they would reduce the number of cane beetles, which were causing problems for growers. Australian sugar cane. With plenty of prey and no predators able to resist their poison, the toads quickly swelled into the tens of millions, becoming an invasive pest that drove native amphibians from habitats in Australia.

But something changed when they settled into their Australian homes. Such cannibalism in cane toads had not been observed in the toad’s natural range. It only began to be seen throughout Australia in recent decades, suggesting that this behavior has evolved rapidly in the Australian population.

“This is a unique case where evolution is extremely rapid and we can see it happening in real time,” said Jayna DeVore, postdoctoral researcher at the University of Sydney and author of the study.

Just over a decade ago, Australian scientists, including Michael Crossland, a researcher at the University of Sydney and also an author of the study, were studying the impacts of cane toads on native frogs when they discovered that cane toad tadpoles had a ferocious appetite for hatching cane toad eggs, flocking to baited traps with them even when offered other types of amphibian eggs.

This led Dr. Crossland to conduct a series of experiments to better understand this phenomenon. Last year, he and his colleague Richard Shine, a biologist at Macquarie University, proved that cane toad tadpoles are attracted to chemical compounds associated with cane toad eggs and hatchlings. These eggs are chemically similar to those of other amphibians, but there is one important difference: they contain bufadienolide toxin, the same chemical that makes cane toads poisonous and protects them from predators. The researchers suspected that it was this chemical that prompted the tadpoles to feed on the youngest members of their species.

In their latest study, the researchers reared wild cane toads, put their tadpoles in tanks with different amounts of bufadienolide toxin in the water, and presented them with cane toad eggs as well as frog species eggs. australian. Tadpoles that were not exposed to bufadienolide toxin barely nibbled their toad and frog eggs. However, tadpoles that had been exposed to bufadienolide toxin consumed both native frog eggs and eggs of their own species with enthusiasm.

The researchers also offered tadpole eggs as they hatched. They found that the hatching process caused the tadpoles to exhibit the same cannibalistic hunger as when the toxin bufadienolide was added to their water. This suggests that the toxins in the eggs are released into the water when hatchlings emerge.

“We’ve known for some time that they’re very cannibalistic, but that explains the mechanism that seems to be driving this cannibalism,” said Matthew Greenlees, cane toad expert and postdoctoral researcher at Monash University, who did not participate in the study. .

The study authors argue that cane toads in Australia most likely evolved this response to their own toxins to reduce the number of other cane toads in their habitat.

“It is well known that toad tadpoles in Australia compete very strongly with each other,” Dr Crossland said. “The density of cane toads in Australia is so much greater than in their native range and under high density conditions cannibalism is likely to evolve. They are basically working on a way to weed out future competitors .

The fact that cane toads were able to develop this cannibalistic behavior in such a short time is “incredible”, Dr Crossland said. “Toads didn’t come to Australia until 1935. It’s an accelerated evolution.”

Invasive species tend to evolve faster than native species, in part because they multiply rapidly. This allows scientists to see evolution taking place over decades, as opposed to centuries or millennia.

Researchers believe that cane toads in Australia are not done evolving. For their next study, they plan to examine how cane toad hatchlings evolve to defend themselves against their cannibalistic elders. “It’s really an arms race between increasingly cannibalistic tadpoles and hatchlings,” Dr. DeVore said.

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