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CAMBRIDGE: Cane toad populations in Australia could be controlled with chemicals that tadpoles use to compete with one another, say researchers.
A study published in the Journal of the Royal Society Biology Letters today shows that cane toad tadpoles can reduce the viability of younger generations with waterborne chemical cues that suppress the growth and survival of developing embryos.
The researchers say that this mechanism could be exploited to halt the spread of cane toads in Australia.
"Here we've got an invasive species that's very hard to control," said Richard Shine, who led the research at the University of Sydney. "We seem to have discovered a mechanism ... for killing cane toads that the cane toads themselves have evolved, and we can take advantage of that."
Second weapon in tadpole competition
When the population is dense, cane toad tadpoles eat unhatched cane toad eggs to reduce competition for resources and ensure their own survival. But it may not be possible to find all of these eggs.
The new research shows that tadpoles have a second weapon to gain an advantage. It shows that they can exploit their chemical communication systems - which use pheromones as attractants and warnings - by producing pheromones that are harmful to developing embryos. This ensures that competition is minimised even if they cannot eat all the eggs.
The researchers placed cane toad eggs in one side of a water tank divided by a mesh. On the other side of the mesh were cane toad tadpoles. This allowed tadpole pheromones to diffuse across to the eggs while ensuring the tadpoles themselves could not touch them. After 72 hours, when the eggs had hatched, the new tadpoles were isolated and their growth and metamorphosis to toads were monitored.
Clever tricks reduce viability
After five days, the tadpoles that were exposed to pheromones were shorter, lighter and less developed than the controls that were not exposed. Even after 20 days, when metamorphosis had started, the exposed animals were significantly smaller, and 40% fewer had survived.
"The tadpoles seem to have come up with some clever tricks to reduce the viability of the next generation," said Shine. "We know that there's competition between tadpoles ... but [the discovery of] chemical cues that work during the egg stage and affect later development really is new."
In the wild, this mechanism ensures competition is reduced for older tadpoles, since there are fewer younger tadpoles, and those that remain are underdeveloped and less able to compete.
Chemical control
Cane toads are an invasive species to Australia, introduced to Queensland in the 1930s to control pests in cane fields. However, they have had devastating ecological effects, and are spreading rapidly across the most of the country. Shine thinks the pheromone that impairs tadpole development could be used to control this spread. "The really attractive thing about this [pheromone] for control purposes," he said, "is that it has such a long-term effect from such a brief exposure."
The next step is to identify the pheromone and ensure that it is safe for other wildlife before testing it. "We'd go to ponds where the toads are breeding and put in slow release forms of these chemicals," said Shine, "If there's been a really intense breeding event ... that might be the perfect time."
However, Mike Tyler at the University of Adelaide in South Australia, who discovered the first pheromones in frogs, is not convinced, and said this finding is "just another little bit of information". He added, "In terms of control, it's absolutely fanciful. The cane toad extends over about one and a half million square kilometres - how on earth are you going to control that by any sort of chemical means? It's physically and utterly impossible."
