Why poisonous Australian toad tadpoles have evolved into ravenous cannibals
Scientists say the invasive species may have adapted to regulate its own population
The same toxins that protect Australian cane toads from predators are turning their babies into insatiable little cannibals, new research has found.
Cane toads produce toxins that make them poisonous to most predators in Australia. But those same toxins, when emitted by fresh hatchlings, seems to propel older tadpoles to gobble up their helpless younger kin, the study says.
"It is exactly like a feeding frenzy," lead author Jayna DeVore, a postdoctoral researcher at the University of Sydney, told As It Happens guest host Gillian Findlay.
"They're these tiny little things when they first hatch, kind of like a little grain of rice. They're just laying there on the ground and they can't defend themselves in any way. And that's the moment when these cannibals can really smell them … and that's when they strike."
The findings were published earlier this month in the journal Ecology and Evolution.
Tadpoles will eat "anything' when the toxin is present
While cane toads have been observed eating their own kind in other parts of the world, DeVore and her colleagues wanted to explain why cannibalism in the Australian population is much more likely — and frenzied.
As part of their research, DeVore and her team exposed cane toad tadpoles to cane toad eggs and hatchlings, as well as frog eggs and hatchlings.
The tadpoles voraciously fed on their own kind, while mostly leaving the newly hatched frogs alone.
The most obvious difference between the two species, DeVore said, is the toxins. So the researchers tried adding toad toxins to water containing different types of hatchlings to see what the tadpoles would do.
"We found that whenever you added a toad toxin to the water, it would inspire this feeding frenzy and the tadpoles would just start eating anything that was in there."
Eating the competition
Eating your own kind may seem counterproductive, but DeVore says it actually makes sense from an evolutionary perspective.
That's because cane toads are an invasive species in Australia, first brought to the country from South and Central America in the '30s to curb the population of beetles feeding on sugar cane crops. They thrived in their new home, where they have no natural predators, and their booming populations have since wreaked havoc on the country's natural ecosystem, displacing native amphibians.
That means the cane toad's biggest competitors in Australia are other members of their own species. And there are a lot of them. After all, cane toads lay some 30,000 eggs at a time.
"So 30,000 tadpoles in the pond is just you and your siblings, right? And then imagine that more toads come and they start putting more and more eggs in your pond. And there's going to be all these additional tadpoles, hundreds of thousands of tadpoles. And if you don't reach metamorphosis and exit the pond before the pond dries up, you and your 30,000 siblings, you're all going to die," DeVore said.
"These cannibalistic tadpoles are highly motivated, I guess you would say, to make sure there's no additional competitors like eating their algae in their pond."
In fact, DeVore and her colleagues published a study last year that found cane toad tadpoles in their natural habitats don't resort to the same level of cannibalism.
Leslie Anthony, a Whistler, B.C., biologist and science writer, says DeVore's findings are an interesting look at how an invasive species can evolve to regulate its own population.
"Because cannibalism is a known feature of many amphibian life-history strategies and novel adaptation a feature of invasive species, it's not that surprising," he said in an email.
Anthony, who has studied cannibalism among salamander larvae and wrote a book about how invasive species are transforming the planet, pointed to the American bullfrog as a key example.
In their native habitat of eastern North America, the bullfrogs occasionally eat their own young, he said. But on Vancouver Island, where they are invasive and have no natural predators, "some bullfrogs subsist primarily on other bullfrogs."
"A friend in the B.C Ministry of the Environment once told me, 'Out here, bullfrogs are their own food chain; which, while propelling the problem, also fortuitously offers some intrinsic level of population control," he said.
DeVore says the cane toad's adaptive cannibalism could also be a double-edged sword. On the one hand, the toads seem to be regulating their own populations. On the other hands, the toads that survive the gruesome process will only be better and stronger, helping them to out-compete native species.
"So on the whole, it might limit the number of toads entering the environment, but those particular toads might be better off than if they hadn't cannibalized the other eggs in their pond," she said.
Written by Sheena Goodyear. Interview with Jayna DeVore produced by Katie Geleff.
