They ate the supplies belonging to Burke and Wills, and more recently chewed on the electrical wiring through-out homesteads, and have even disrupted communication networks. An invasion that presumably leaves the Channel Country in western Queensland and spreads out across central Australia, holding siege to some of the remotest and driest regions of Australia.

“…we remained there (Camp 63) until the 5th when we were driven out by the rats…” – Burke 1860

It sounds like a scene from Rommel’s army, moving across the African deserts in World War 2, but it is rare irruption of native rats. One that people rarely see or want to see again. Once every ten years or more conditions change in central Australia and flooding rains fall across the desert. The desert blooms and turns a vivid bright green – such a contrast against the red sand and deep blue sky. A mass seeding event takes place as grasses pour out their life boats in hope of more rain. These are the conditions the long-haired rat, otherwise known as the plague rat (Rattus villosissimus), take advantage of.

Long-haired rat or plague rat. Photo by Aaron Greenville

The long-haired rat feeds on seeds and vegetation, but also anything else they can get to. If camping in central Australia this rat is known to eat all the paper labels off your tins and even have a go at your swag – it doesn’t matter if you are still trying to sleep in it! Weighing in at up to 280 g they are much bigger than other desert-dwelling rodents and they are not shy in throwing their weight around either. They are aggressive,  quickly monopolising resources, but they do have their enemies; native predators,  including snakes, dingoes, and the nocturnal letter-winged kite. In addition feral cats and foxes take advantage of this abundance in prey.

A letter-winged kite comes into land. Channel Country, Queensland. Photo by Aaron Greenville

Given the quite conspicuous nature of the rat, it is surprising that we still do not know that much about its biology. Thus we set out to find out more. In our study published in the Greening of arid Australia: new insights from extreme years, a special issue of Austral Ecology, we collated historic, media and anecdotal records to work out if we could predict when this rat may irrupt. We found there was an 80% chance of a rat plague following a large flooding annual rainfall event of 750 mm. To put that in perspective, central Australia has an average annual rainfall of 150-200 mm.

But where do the rats come from in the first place? They are quite rare in the predominantly dry times in central Australia and disappear from much of the desert, persisting in small and localised refugia. The species is not physiologically well adapted to dry conditions, so the refugia are crucial in providing a reliable supply of green food and moist areas for shelter and digging burrows. Are they holding out in one or multiple refugia? Important information to know if we want to conserve, or manage, this species. We don’t want all our rats in one basket, so to speak. Plomley (1972) suggested that the rats irrupt from a single refuge in the Barkly Tablelands, to the north of our study region, and follow drainage lines into the desert. To investigate this we set up live-trapping grids twelve sites across our study region. We assumed if the rats have only one refugia, as suggested by Plomley (1972), then we would predict that they would move in a wave in one direction from the north and if they followed drainage lines, we would get greater captures at trapping grids closer to these features. We did find greater captures at trapping grids closer to drainage lines, but the direction the rats came from was from the south. This suggested that other refugia exist and drainage lines provide important corridors for dispersal.

So what else was happening at the invasion front? For mammals, it is quite common that juveniles, particularly males, are the predominate dispersers, heading out to find their place in the world. However, we found that the larger rats that were at the invasion front and regardless of sex. Presumably, larger individuals could move greater distances and exploit the new food sources. This could be an important finding for understanding dispersal of invading species, like the cane toad.

Even though the long-haired rat can be an inconvenience to some, it has some interesting biology. Perhaps, in other 10 years time we can find out some more.

References:

Burke, R. O. 1860. Robert O’Hara Burke’s dispatch, Cooper’s Creek, 13 December. MS 13071, Box 2082/1a, Item 13. Records of the Burke & Wills Expedition. Australian Manuscripts Collection, State Library of Victoria, Victoria.

Greenville, A.C., Wardle, G.M. and Dickman, C.R. (2013). Extreme rainfall events predict irruptions of rat plagues in central Australia. Austral Ecology, 38: 754–764.

Plomley N.J.B. (1972) Some notes on plagues of small mammals in Australia. J. Nat. Hist. 6, 363–84

*up-date: This article was re-published in Biology News.