Sneaky Toad Tadpoles Use Chemical Weapons Against Their Growing Competition

By Joseph Castro | September 2, 2011 10:34 am

spacing is important

Think you’re a survivor? You’ve got nothing on the cane toad, former native of Central and South America, now scourge of Australia. To snuff out their competition for resources, cane toad tadpoles will actually cannibalize nearby cane toad eggs. And all those eggs the tadpoles are too full to gobble up? Well, researchers recently learned that the hardy amphibians have that covered, too: cane toad tadpoles release chemicals into the water that stunt the growth of developing embryos.

Scientists already knew that cane toads communicate with pheromones and use these chemical signals to locate tasty eggs. They also wondered if the pheromones have another, more insidious, purpose. Biologists at the University of Sydney set up a simple experiment to find out. They placed cane toad eggs in 20 containers filled partially with water; in 10 of those containers, they added tadpoles and separated them from the eggs with mesh screens.

The result: five days after hatching, the amphibians that developed with drooling tadpoles next door were 24 percent shorter and weighed 41 percent less than the isolated groups. Moreover, 40 percent fewer exposed tadpoles survived beyond 20 days, when the animals begin turning into toads.

The researchers believe that older tadpoles released pheromones into the water, as evidenced by the water’s increased ammonia content, though they don’t yet know if the chemical release is continuous or sparked by the presence of nearby eggs. They also don’t know if this murderous adaptation evolved in response to food competition—if the pheromones are “designed” to do that—or if it’s a fortuitous coincidental effect.

Either way, try not to feel too bad for the little buggers. Introduced to Queensland in 1935 to control cane beetles, the poisonous amphibians have since spread rapidly across parts of Australia, wreaking havoc on the populations of other animal species. Oh, and they’ve failed miserably with their original job.

With all this in mind, the researchers believe that identifying the destructive pheromone will lead to a chemical control for the toad menace. Will it work? Absolutely not, thinks frog expert Mike Tyler—the cane toad now covers too much ground for a chemical solution. “It’s physically and utterly impossible,” he told Cosmos Magazine. Ouch.

So what can stop the cane toads? Meat ants? Cold Snaps? Droughts? Perhaps in time. But for now, it looks like the cane toad will keep on hopping across the land Down Under. Cue Destiny’s Child.

Reference: M. Crossland and R. Shine. Embryonic exposure to conspecific chemicals suppresses cane toad growth and survival. Journal of the Royal Society Biology Letters. doi: 10.1098/rsbl.2011.0794

Image courtesy of Sam Fraser-Smith / Flickr

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