Rick Shine's "Cane Toad Wars"

Rick Shine is a Professor in Biology at the University of Sydney. Besotted by snakes since childhood, he spent most of his research career prying into the private lives of serpents; but in 2005 his main study site, a floodplain in tropical Australia, was overrun by an invading army of toxic amphibians. Cane Toads, among the largest frogs in the world, had been released on the eastern shores of the continent 80 years before, and had spread westwards at ever-increasing speeds. Deadly if eaten, the toads had already killed millions of native animals. His new book, Cane Toad Wars, tells the story of the toad invasion, and of attempts by scientists, community groups, government authorities and politicians to do something about the killer frog that is rampaging through Australia’s last great wilderness.

Shine applied the “Page 99 Test” to Cane Toad Wars and reported the following:

My immediate thought on turning to page 99 was to offer an apology to that page. It faces a glorious photograph of a Freshwater Crocodile that has just seized a huge Cane Toad; pressed up against that photograph whenever the book is closed must make page 99 fear that it can’t live up to the high-impact image that precedes it. But on re-reading page 99, I think it can relax.

The text on page 99 begins with a perplexing observation. When invading Cane Toads sweep through tropical Australia, they kill vast numbers of Freshwater Crocodiles. In some rivers, more than 90% of the giant reptiles are fatally poisoned by the toad’s powerful toxins within a few weeks. Bloated corpses clog the waterway. But along adjacent rivers, the toads pour through but the crocs are unaffected. Why the difference?

A possible explanation came to me via a grizzled old bushman as we sat around a campfire beside his ramshackle house deep in the wilderness of Kakadu National Park. I and my (supposedly smart) students had tried to explain the contrasting impacts of toads on crocodiles in different rivers, and we had failed. But Dave Lindner had thought about it as well, and he had an idea. It came to him soon after Cane Toads invaded his idyllic home, when a tame monitor lizard (at six feet long, a true monster) seized a toad in Dave’s backyard, and immediately began convulsing from the poisons that were squirted into its mouth from the toad’s shoulder glands. Dave seized a bottle of water and flushed out the lizard’s mouth to remove the toxin, and the big reptile survived.

And if predators survive if they can flush away the poison, Dave thought, then maybe this is why crocs are at risk in some rivers but not others. In a small river where the crocs leave the water to forage along the sandy banks, any encounter with a Cane Toad is likely to be fatal. No convenient mouth-flushing facility in reach. But if the croc stays in the stream, and meets a toad that has come down for a drink, it may be able to wash away most of the poison as it thrashes around in the water. So maybe the crocs die in droves in a river where the local ecology encourages them to leave the water to look for food, but are unaffected if they remain aquatic.

Is Dave right? We don’t know. But an old bushman’s insights may have solved a problem that stumped my very clever research team.

Learn more about Cane Toad Wars at the University of California Press website.

--Marshal Zeringue