“You don’t ever see them attacking each other like that.”
Dr DeVore and her colleagues gathered samples of cane toad tadpoles from all parts of Australia, from NSW to locations across Queensland, the Northern Territory and WA.
They also took samples of tadpoles from the original cane toad populations in South America that were used to introduce the species to Queensland in the 1930s in a doomed attempt to eradicate the cane beetle.
The researchers found that the South American tadpoles lived peacefully with each other, while the tadpoles from across Australia tried to eat each other.
Dr DeVore said what appeared to have happened was the competition for resources in the more challenging Australian environment had led the Australian toad tadpoles to try to kill off their competition.
“These toads are so abundant in Australia, there’s just an incredible density of them, and they’re all trying to breed in these limited number of ponds that are available to them, many of which dry up at certain times of the year,” she said.
“So when other toads come and put their eggs in your pond, you know these are tadpoles you’re going to have to compete with, and so the advantage for them is if they eat those eggs they improve their own chances.”
Because the behaviour is found across Australia where the toads are found, the researchers believe it must have developed shortly after the toads were introduced.
The researchers discovered the tadpoles were specifically attracted to the poison a mother toad will secrete to cover the eggs, which make cane toad eggs so poisonous to other animals.
Other researchers have discovered that the tadpoles are attracted to the poison and have used that fact to develop traps which lure huge amounts of cane toad tadpoles.
The researchers had theorised about why the poison was signalling the tadpoles, but Dr DeVore and her colleagues believe they have the explanation.
She said the cannibalistic behaviour suggested Australian cane toads, long considered a pest, might be trying to regulate their population.
“This is very much a novel behaviour that has developed just in Australian toads, and this could be an example of a species naturally controlling population density,” she said.
“They won’t wipe themselves out because the cannibals will still be there, but it does mean that it will restrict population growth, and possibly the cane toad will become its own biggest predator.”
The research has been published in the journal PNAS.