In their relentless invasion of Australia, poisonous cane toads often hop along roads, where their flattened, desiccated husks are a familiar sight during the long dry season. Nobody gave much thought to the fate of the little mummies' toxic compounds—months baking in the sun should render them harmless, researchers assumed. Not so, according to a new study, which shows that, like murderous ghouls, road kill cane toads can haunt the wilderness long after death.
Cane toads were introduced in Australia from South America in 1935 to control beetles destructive to sugar cane crops. Today, the prolific toads have invaded more than a million square kilometers of northwestern Australia, where they've caused widespread population declines among native reptiles and marsupials that try to eat them.
The notion that cane toads can even poison water has been kicking around anecdotally for years but was recently dismissed by one research team as a "myth" because the toads' principal toxic compounds, bufadienolides, aren't water soluble. Indeed, the only relevant research showed that water with live cane toads sitting in it posed no problem to chickens. The new work revives the old water-poisoning idea.
Ecologist Richard Shine of the University of Sydney in Australia, a co-author of the chicken study, has been studying how cane toads affect native fauna since 2004, when the invaders arrived at his long-term research site east of Darwin. Wondering just how harmless the thousands of sun-dried road toads they'd driven by over the years really were, Shine and postdoctoral researchers Michael Crossland and Gregory Brown collected 27 toad carcasses that had been lingering on local roadsides for at least 2 months and soaked fragments of their tissue in water. Surprisingly, the poison-producing parotid gland swelled right back up and took on a somewhat lifelike appearance, the team will report in a forthcoming issue of Biological Invasions.
When the researchers put cane toad tadpoles, native frog tadpoles, fish, and leeches in water containing scraps of cane toad tissue, they found that most of the native animals died within about a day (and sometimes much faster)—even when they couldn't touch the tissue directly. Native tadpoles and fish gave the toad tissue a wide berth. Predictably, the cane toad tadpoles were unperturbed by the poison-laced water; and in control experiments with uncontaminated freshwater, all the animals survived just fine.
Temperature-logging devices implanted near the parotid glands of toad carcasses put back out on the road for 6 days showed that the toxins withstood temperatures as high as 49˚C. The deadly bufadienolides are known to be heat stable, but because they aren't water soluble, the new research shows that other tough but as yet unidentified toxins in the cane toads' arsenal must also be involved.
Shine recalls that a friend once slapped a stamp on a paper-thin road-killed cane toad and sent it to him as a postcard. "The fact that the toxin could still be active, I think, is just extraordinary," he says. "It's like a horror movie."
The researchers chose experimental species that live or breed in puddles and ponds that form during the wet season, and they say washed-in cane toad carcasses could poison the water for native fauna. (Live toads may still be unlikely to taint water because they typically release their toxins only under duress.) The team also warns that government agencies and citizen groups that routinely kill thousands of cane toads and leave them in the wild need to rethink their disposal strategy.
Calling the extreme stability of the cane toad toxins "fascinating," Michael Tyler, a herpetologist at the University of Adelaide, North Terrace, in Australia who has been studying the invaders intermittently for over 50 years, notes that a number of other frog-skin compounds have similar durability. "Previously you thought, 'Well, okay, a dead cane toad is an ex-toad. It's no longer any problem,' " Tyler says. "What these guys have done is to demonstrate that it remains a very serious conservation problem."