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The invading cannibalistic toad tadpoles in Australia eat so much of their young hatchlings that they have found themselves locked in an evolving and accelerating arms race.
The cane toad (Rhinla Marina) hatchlings, born by the thousands in small ponds in Australia, have no natural predators in Australia, but they do have to contend with older toad tadpoles feasting on helpless young amphibians. This causes newborns to develop at a breakneck rate into tadpoles themselves, according to a new study in the Proceedings of the National Academy of Sciences of the United States of America, the tadpoles in turn become even more aggressive cannibals.
Cane toads are not native to Australia, but rather South America. According to our sister site LiveScience, the cane toad was first introduced to Australia in the 1930s by sugarcane growers who thought the toad would be a great pest control solution to their beetle problem. From just 102 toads in the 1930s, the population has exploded to over 200 million.
This is due to both the absence of natural predators and natural proliferation, with female toads capable of laying over 10,000 eggs at one time. As subsequent generations are born in the same pool, the helpless toads are at the mercy of the survivors of the previous generation, who have just developed the ability to eat on their own and whose metabolism leads to a ravenous appetite.
“When these eggs first hatch, the young cannot swim or eat yet, so they can pretty much just sit there at the bottom of the pond until they develop into tadpoles,” he said. said Jayna DeVore, biologist at the Tetiaroa Society, a non-profit association in French Polynesia, specializing in invasive species and co-author of the study.
“Once the hatchlings turn into tadpoles,” DeVore explained, “they are too big and mobile for other tadpoles to eat them, so the cannibals have to work quickly if they are to consume them all. “
“These toads have gotten to the point where their own worst enemy is themselves,” DeVore told Nature.
The invasive nature of cane toads makes their evolution into frenzied cannibals easier to identify
“When I first saw this behavior in the wild, I was amazed at how voraciously the toad tadpoles searched for the toad hatchlings and ate them,” DeVore said.
To see how the cannibalistic behavior of toads has developed over time, DeVore’s team turned to the source: the toads of South America. Although they are still the same species, the 86 years the invasive toad group spent in Australia has clearly changed them.
While some cannibalism has been observed in native South American toads, their Australian counterparts have engaged in cannibalism 2.6 times more often in 500 experiments with different individual tadpoles. Australian tadpoles were also much more attracted to hatchlings than their South American counterparts. When setting up two traps, one with hatchlings and one without, Australian tadpoles were 30 times more likely to enter the hatchling trap.
“[South American] tadpoles were not attracted to hatchlings; they were just as likely to enter the empty trap as the hatchling trap, ”DeVore said. “This demonstrated that this strong attraction to the vulnerable hatchling stage, which helps cannibalistic tadpoles detect and locate their victims in Australia, is not present in the native range.
To combat this, newborns responded by speeding up their developmental cycle, spending less time in the egg and newborn stages. “We found that Australian toad broods grew faster; they reached the invulnerable tadpole stage in about four days, while native broods took about five days,” DeVore said.
Even more incredible, Australian hatchlings seem to have developed a sense when a cannibalistic tadpole threatens their brood, speeding up their own development in response. This comes at a cost, however, as this faster maturation in the early stages of eggs and hatchlings appears to lead to slower development in the tadpole stage.
“The good news is that cannibalism can control population growth,” DeVore said. “So, while cane toads are unlikely to become extinct, these cannibalistic behaviors may help regulate their abundance after the invasion.”
Analysis: invasive species are no joke
Invasive species are a major problem in the world, as reckless humans spread different species everywhere we go, usually wreaking havoc on a local ecosystem.
While the Australian toad is one of the most famous examples of invasive species, it is not the only one that Australia has had to contend with. Famously, rabbits were introduced to Australia in the 1850s and began to breed like rabbits, posing a huge threat to land and agriculture.
Australia is not alone either. The best-known case – the Colombian exchange, when European colonizers reached the Americas – introduced into the Americas and brought back to Europe countless species of plants, animals and contagious pathogens that have had contagious diseases. profound and lasting effects on the ecosystems of three continents.
Fortunately, our knowledge of these interactions has improved over the past century and we are much more aware of invasive species, but as the world becomes more and more interconnected, invasive species will only proliferate with unknowable consequences. for the future.
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