Could water rats be the key to controlling pest rats?

The largest rodent in Australia is amphibious, fills the same niche as an otter and could be used to get rid of the introduced black rat. Ann Jones meets the researchers trying to find out more about the mysterious water rat.

There are a number of 1.5 kilogram rats swimming around Sydney Harbour. No one knows exactly how many, not even rat researchers. No one really knows too much about them either.

Despite the astounding lack of knowledge, associate professor of conservation biology Peter Banks is an upbeat man: he love rats.

‘Some people say that people look like their study animal, and I’ve got these two large incisors‚Äîmaybe that’s it!’

‘No,’ he says, ‘rats are amazing, they’re 25 per cent of the mammal diversity, they’re 25 per cent of the native mammals that we’ve got in Australia.’

‘They’re some of the worst pests of humans as well as doing amazing things in ecosystems‚Äîthey are by far the coolest animals.’

It’s not what you normally hear about rats, and Professor Banks’ takeaway message from conservation biology in some way differs from the norm as well.

‘I think too often the focus on Australian wildlife has been on the impacts introduced species, and the fact that we’ve got so many species on the endangered list and the fact we’ve got the world’s worst record of mammal extinction is in Australia,’ he says.

‘I don’t know if it’s a cultural cringe or whatever, but I think we’ve certainly got a view that Australian wildlife is fragile. I don’t think that that’s actually true. In some cases, the fauna we have now has a lot of opportunity to be robust.’

We meet on a cool morning in the affluent Sydney suburb of Mosman, where the only people awake are the street sweepers arriving for work and intrepid biologists keen on a water rat hunt.

We are on the lookout for the water rat, also known as the Rakali.

‘It’s quite distinct, it’s semi-aquatic so it spends a lot of its time in water, and it’s filling the same niche as otters,’ says Professor Banks. ‘They’re the biggest rodent we have, so they’re filling a particular predatory niche.’

The rats are central place feeders, meaning they’ll go off and collect invertebrates, molluscs or crabs, and then return to a safe place to chow down. When finished, they’ll leave behind little piles of remains and exoskeletons in middens, and seeing a refuse pile is one of the best ways to tell they’ve been foraging in your area.

Otherwise you might see their footprints on the sand or mud, which are distinguishable from other ratty markings by their large, partially-webbed back feet.

If you do glimpse one or more water rats up close they’ll look slightly oily, as if they’ve just got out of the water. They probably have, but the oily look is a bit of waterproofing on their fur.

They have a much more regal looking snout than the average rat too, and their small, low-set ears are offset by a handsome set of luscious whiskers which curve gracefully around to the front. They probably use the whiskers to sense prey in the water, according to Professor Banks.

The water rat does not have your average rat tail. It’s thicker at the base and is furry right through to the tip, not scaly. The last third of the tail is white, and this is the easiest way to make sure that you’ve got a water rat in front of you. It’s like a flag of innocence, waving around in the water.

The water rat’s scientific name is Hydromys chrysogaster, for its golden belly fluff. In my opinion, the water rat draws the race for the cutest amphibious mammal in Australia with the platypus.

But the Rakali do suffer from PR issues; it’s hard to overcome the fact you are commonly called a rat and that you do look a little bit like one of the most persistent and damaging pests known to mankind.

To get an idea of the damage that some rodents can cause, the invasive black rat is implicated in about 54 per cent of bird extinctions on islands, according to Jenna Bytheway, who works with Professor Banks.

‘They are a big pest in Australia and around the world,’ Ms Bytheway says. ‘They carry diseases which are transferable to humans and other animals. They also are big ecological pests, they’re prolific climbers.’

‘They’re big predators of birds and other Australian native animals. They also out-compete other native animals for space and they can be quite aggressive in their competition.’

Of course, that means that Professor Banks considers them amazing.

‘They’ve colonised the world‚Äîeverywhere we’ve gone, they’ve gone‚Äîso they’re fantastic at using what we don’t need and don’t want.’

‘When they’re in houses, they chew wires and they are responsible for house fires and they carry whole suite of diseases that spread to humans and to wildlife.’

In 1900, when the Bubonic Plague hit the shores of Sydney, a bounty was put on rats. We know that water rats were killed as well as the pest rats, because one appears laid out like a prize catch in a photo of deceased rodents from the time.

‘But these guys, the water rats, are the goodies,’ says Professor Banks.

‘Our research agenda is really about trying to see whether or not native species can play some role in controlling these introduced rats. Because theory tells us that wherever you’ve got an intact biodiversity, it’s going to be robust to invasive species. So if we can make and understand what makes for robust native fauna, I think we can understand how we can control these rats.’

‘So the water rat work is about trying to understand whether or not these water rats are playing any role in controlling black rats.’

‘It makes sense, because water rats being up to five times the size of a black rat, they should beat up black rats. It’s basically the idea that the big dog beats up the little dog and the water rat is definitely the big dog, and the black rats don’t really like a fight.’

The hypothesis is that where there are water rats, there should not be black rats.

In setting up hair tube traps and infra-red video cameras, the scientists had to innovate because so little is known about the ecology of the water rat that even the best attractant to use was unknown.

It turns out that water rats are most attracted to sardine oil, but even with a stinky lure, the water rats proved elusive and hard to survey for. Researchers found them in about 12 of the sampled locations around the harbour, where as black rats were almost everywhere.

Although limited, the results from these initial surveys indicate that Professor Banks’ hypothesis is supported.

‘Where we had high activity of water rats we had low activity of black rats,’ he says.

The implications of this finding could include the creation or further protection of water rat habitat in order to encourage their presence, and by implication, the absence of the pest rats.

Further research could lead to an application of water rat odour which could repel pest rats.

‘If you give this sort of native species a chance, there’s a good opportunity that they can play these pest management roles,’ says Professor Banks. ‘I mean, the fact that they can live in Sydney shows that they’re pretty adaptable and they can exploit the resources that are here.’


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