You might not like water rats. You might have seen them bobbing around a nearby urban river system and assumed they're rats in the traditional plague-carrying sense. But Australia's native water rats are actually called rakali, and only distantly related to their European cousins. And now scientists are finding evidence that Australian rakali are learning to hunt cane toads, making them some of the first native species to do so without getting killed.
Cane toads have been ravaging Australia’s ecosystems since they were introduced from Central America in the 1930s. When threatened, cane toads excrete a lumpy white neurotoxin from orifices around their heads known as "parotoid glands," which if eaten, result in convulsions, paralysis, and death. When cane toads migrate into new areas, our native predators hunt them as they would any other frog, only to die of respiratory paralysis a short while later. But it seems native water rats have realised certain parts of cane toads are toxin-free, and are eating only those parts.
Reproductive biologist Dr Marissa Parrott was working in the Kimberley region of Western Australia where she witnessed first-hand the damage cane toads inflicted on the local wildlife. Animals such as the yellow-spotted monitor and the endangered northern quoll were facing population collapse, while large predators such as crocodiles were being found dead with comparatively tiny cane toads in their stomachs.
“[These animals have] never seen cane toads before,” Dr Parrott explained to VICE over the phone, “so they would assume it’s similar to a native frog, pick it up, eat it, and then die.”
Then, while working near a creek in Emma Gorge, she came across something surprising.
“I found a number of very large dead cane toads, and all of them were lying on their backs with almost surgical type incisions down their chest. Every day I went there, there were up to five new bodies.”
She added, “We found that in all the cane toads, the heart and liver had expertly been removed, and the gallbladder, which contains toxic bile salts, had been removed and placed outside the body.”
After setting up infrared cameras, Dr Parrott found that water rats living in the area were behind these surgically precise deaths. Having worked with water rats before at Healesville Sanctuary, she knew how intelligent and adaptable they were, and said she felt a strange sense of pride when she found out they were the predator.
“Within just two years of cane toads moving into the area, they had learned how to disable, kill, and eat something that had killed so many other predators in the region.”
While Dr Parrott couldn’t quite get a close up view of how rats were killing the toads, she believes they most likely sliced opened the toad's chests with their teeth, which are less poisonous than their backs. The rat would then remove the desired organs with their paws. The dead toads were almost exclusively large ones, believed to be targeted for the payoff of their bigger organs and the fact that it was easier for the rats to avoid the toxic gallbladders, according to Dr Parrott.
“They didn’t eat as many medium-sized toads, but when they did, it was fascinating to note that as well as the heart and liver they had also eaten one or both of the thigh muscles after stripping away the toxic skin,” she said. “We’re not sure if they just wanted a bigger payoff for their efforts in overpowering the toad, or if it was easier to subdue the toads by holding down the legs first. Interestingly, they never attacked the leg muscles on the larger toads.”
It’s possible that the rats had learned to attack these toads based on the way they kill other prey. They can use their teeth to slice through the shells of crayfish and yabbies, and can manoeuvre their prey around with their ambidextrous paws.
So now that these rats have figured out how to kill cane toads, they can finally start decimating the invasive species and let other species rebuild their ecosystems—right? Well, not quite, according to Dr Parrott.
“Unfortunately, there are billions upon billions of cane toads and not that many water rats,” she said. “The rats are in reasonably small numbers and have large territories, so they don’t become overabundant in the ways that other rodents do.”
The rats could possibly mount a defence when cane toads enter a new area. By targeting the larger toads—which are also the breeding animals—toad populations are presumably kept lower, but not for long.
“With the next flood or breeding event”, Dr Parrott says “there would be so many toads that the system would be overwhelmed. Water rats are doing their bit for the environment, but we’ll need to look at other ways to try and control the toad population.”