Scientists say native rodents in Western Australia have discovered how to kill and eat parts of the poisonous pests

This article is more than 10 months old

This article is more than 10 months old

Australian water rats have learned how to kill cane toads, eat their hearts and carve out their organs with “surgical precision”.

In only two years, highly intelligent native rakali in the Kimberly region of Western Australia discovered how to safely destroy the deadly toad – by removing its gallbladder and feasting on the heart.

The rats even targeted the biggest, most poisonous toads they could find, leaving their bodies strewn by the riverside, according to research published in Australian Mammalogy.

Cane toads were first introduced into Queensland in the 1930s and have been marching slowly west ever since, devastating native animals and driving them towards extinction. The toads first arrived in a site monitored by the researchers in WA in 2011.

But to their surprise, the scientists found the native water rat – better known as the rakali – was fighting back. The highly intelligent rodent has extremely sharp claws and teeth, and can grow to up 1kg in weight.

Dr Marissa Parrott, the paper’s co-author, said the scientists began to see dead toads appear, cut open in a “very distinctive” way.

“It was a small area of creek, three to five metres in size, and every day we were finding new dead cane toads,” she said. “Up to five every single morning.

“They were flipping them over, making a very distinctive, almost surgical precision cut down the chest. They would even remove the gallbladder outside the body, which contains toxic bile salts. They knew to remove that bit.”

“In the medium-sized toads, as well as eating the heart and liver, they would strip off the toxic skin from one or both legs and eat the non-toxic thigh muscle.

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“They have very strong sharp teeth, very dextrous little hands. They can pick up a fish or a yabby and open them up very quickly and target the areas they like.”

According to the paper, researchers observed 38 toad carcasses, floating in the river or on the creekbank, over 15 days.

“All carcasses had an incision in the chest area, measuring [on average] 10.8mm vertically and 12.2mm horizontally,” it said.

“There was no evidence of bites to the head or body of the partially consumed toads. Rather, the rats appeared to hold the toad on its back and then incise the thoracic cavity to consume organs while the toad was still alive.”

Parrott, a reproductive biologist at Zoos Victoria, said another astonishing finding was the size of the dead toads. While only 2.5% of the toads in the region were classified as large toads, the big toads made up 74% of the bodycount.

This suggested the rats were specifically targeting the biggest toads.

“Water rats are quite large themselves,” Parrott said. “They have the power to subdue a larger toad and get a bigger payload, get that larger heart and larger liver. By killing those larger toads, it may be easier to avoid the toxic organs like the gallbladder.”

This could have a positive effect for other native animals, because the largest toads are more toxic and more dangerous.

Parrott hopes other water rats around the country could develop the same technique, and help halt the march of the toad, but said other measures were needed.

“The water rats could protect small areas and could slow the progression of toads,” she said. “There have been anecdotal reports of water rats killing cane toads, across Queensland and the Northern Territory. But there are so many hundreds of millions of cane toads those areas could get swamped. It’s a major issue for our native predators.”

The researchers hypothesise that the rats either learned from scratch – by figuring out which parts of the toad made them sick – or already had previous experience from eating Australian native toxic frogs.

Either way, Parrott said, it was likely helped by the fact the rats spent a lot of time raising their children.

“The parents have quite a long period of care with their offspring. The baby rats will stay with their mother – and they can learn from their parents. It would make very good sense that their parents are teaching their children how to kill those cane toads and avoid those poisonous areas.

“And it is very possible that those children will spread to other areas and teach their children how to kill and eat those biggest toads.”

Other animals, like crows and kites, have been observed turning cane toads inside out to avoid the toxic skin and only eat non-poisonous organs, the report said.

Parrott said her focus was now on promoting water rat conservation. The rats face threats from pollution of waterways, can be caught in fishing line and discarded balloons, and hunted by stray cats, foxes and dogs.

“[The findings] show the intelligence of our native rodents,” she said. “A lot of people don’t really know we have native rodents in Australia. A story like this has really raised their profile and made people not only realise they are very clever but they are a very beautiful animal we should be protecting.”