Natives strike back against toad
Native animals are fighting back against the infamous deadly cane toad, which has devastated large parts of northern Australia.
The toad produces a poisonous cocktail of bufotoxins and bufogenins, making it deadly to most Australian animals. However, according to new research published in Australian Mammalogy, the native water rat or rakali has been found to tear out the toad’s heart and liver with such surgical precision so as to avoid poisonous glands located behind the toad's head.
Dr Marissa Parrott from Zoos Victoria made this discovery while working on the frontline of the cane toad advance in the Kimberley in north Western Australia. She kept finding dead cane toads lying on the side of a creek bed — all with the same incisions in their bellies. Using cameras and sampling, she found the rakali were targeting the biggest toads, flipping them over, slicing open the abdomen, and removing and eating the heart and liver — but discarding the gallbladder, which contains toxic bile salts. “It's the first time we've found native Australian animals targeting the largest [toads], which are the most dangerous and toxic,” she says.
The rakali are not the only Australian animals that have learnt to dodge the poison and make a meal out of the cane toad.
Estuarine crocodiles can eat cane toads without too much ill effect, though — strangely — freshwater crocs can’t, and will die if they do. “Saltwater crocs evolved in Asia and are likely to have evolved alongside [toads],” says environmental scientist Associate Professor Jonathan Webb from the University of Technology Sydney. “Saltwater crocs evolved in Asia and are likely to have evolved alongside [toads],” he says. While the cane toad came from South America via Hawaii, its relatives with similar bufotoxins are widespread throughout Asia. Saltwater crocodiles have developed some resistance to toad toxin in their evolutionary past, yet freshwater crocodiles evolved in Australia, and are thoroughly unaccustomed to it.
Then there are a number of insects, arachnids and other invertebrates that also eat cane toads. Reptile and toad expert Professor Rick Shine from Macquarie University says, “A lot of ants, spiders, and waterbugs don't seem too affected by the poison.” Freshwater crayfish, diving beetles, dragonfly larvae and mosquitoes also feed on cane toads, though most of these are in egg and tadpole form. By contrast, huntsman spiders and native tarantulas have been observed taking down fully grown toads.
Next up, the keelback or freshwater snake, which, according to Tim Cutajar from the Australian Museum "has the evolutionary advantage of being 'pre-adapted' to life with toads. Its ancestors were some of the most recent snakes to arrive to Australia." Cane toad toxin does make keelbacks a little sick though, and they'll preference frogs over toads given a choice. Slatey-grey snakes are another native reptile partial to the toad. While red-bellied black snakes don't have that recent Asian ancestry, they have developed a mild tolerance to the poison. "Most interesting, these snakes have evolved smaller heads, which physically prevent them from eating large and therefore more poisonous toads,” he adds.
It’s been reported crows flip toads on their backs and eat their organs, though some will eat the whole toad, parotid gland and all. ”Some crows can eat them and be fine and others will eat them and die. The same seems to be the case with kookaburras,” Webb notes. Prof. Shine says, ”A lot of the birds tend to eat the tongues out of them. The kites that patrol the highways pick up the roadkill and eat them." Tawny frogmouths and bush-stone curlews have been known to partake, and Cutajar reports a “swamphen killing an adult cane toad and extracting parts out through its mouth. It fed these parts to its chicks.”
Rounding out the list are several species of melomys — a small native rodent — that actively preference them over other creatures, according to Prof. Shine. "In captivity they'll eat them for breakfast, lunch and dinner — they love them," he notes. "Most times they don't want to eat the parotid gland, but some will. We've had them eat the whole thing."
ABOVE A crow takes on a cane toad
November 1, 2019