In Australia, poisonous cane toads have become their own worst enemies.
Scientists have been watching cane toad tadpoles eat their younger kin in the puddles and ponds they share. The cause of cannibalism has been a mystery.
The study found that cane toad tadpoles in Australia develop an appetite when exposed to a toxin found in cane toad eggs.
Cane toads, native to South America and Central America, were introduced to Australia in 1935 by scientists who hoped they would bring down the number of cane beetles. The toads quickly became an invader of Australia's native salamanders because of their abundance of prey and lack of resistance to their poison.
As they settled into their Australian homes, something changed. In the native range of the cane toads, cannibalism had not been observed. This behavior started to be observed across Australia in recent decades, suggesting that it evolved rapidly in the Australian population.
Jayna DeVore is a researcher at the University of Sydney and is an author of the study.
A decade ago, scientists in Australia, including Michael Crossland, a research fellow at the University of Sydney who is also an author of the study, discovered that cane toad tadpoles had a fierce appetite.
Dr. Crossland conducted a series of experiments to better understand this phenomenon. Last year, he and his colleague Richard Shine showed that cane toad tadpoles are attracted to chemical compounds associated with cane toad eggs and hatchlings. The same chemical that makes cane toads poisonous and protects them from predators is found in these eggs. The researchers suspected that the chemical was triggering the tadpoles to feed on younger members of their species.
The researchers bred wild cane toads, put their tadpoles in tanks with different amounts of bufadienolide toxin in the water, and presented them with cane toad eggs as well as the eggs of Australian frog species. The tadpoles that weren't exposed to the toxin barely bothered the toad and frog eggs. The tadpoles that had been exposed to the toxin consumed both the native frog eggs and the eggs of their own kind.
The researchers gave the tadpoles eggs as they were hatching. The hatching process caused the tadpoles to exhibit the same cannibalistic hunger that they did when the bufadienolide toxin was added to their water. It's possible that the toxins within the eggs are released into the water when the hatchlings emerge.
Matthew Greenlees, a cane toad expert, said that they have known for a while that they are cannibalistic.
According to the study, cane toads in Australia most likely evolved this response to their own toxins to reduce the number of other cane toads in their habitat.
The density of cane toad tadpoles in Australia is much higher than in other countries. They are trying to eliminate future competitors.
The cane toads have been able to evolve cannibalism in a short time. It is evolution in fast motion.
Invasive species are more likely to evolve faster than native species. Scientists can watch evolution unfold over the course of decades, as opposed to centuries or millenniums.
The cane toads in Australia are evolving. They will look at how cane toad hatchlings are evolving to defend themselves against cannibalistic elders.
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