If you’re asked to name an Australian mammal, the word “rat” probably isn’t the first thing that comes to mind.
But about a quarter of all Australian mammals are rodents.
Key points:
- The ancestors of native rats arrived in the region from Asia, but what happened next was a mystery
- Scientists have mapped the DNA of 150 living and extinct species to find out
- Rodents increased after arriving in Australia 5 million years ago, but many species became extinct after European arrival
These are not the black and brown rats with long tails and pointed noses associated with pestilence, which arrived with the First Fleet.
“I think native rodents can get a bit of a bad rap because a lot of them have the word ‘rat’ in their name,” said Emily Roycroft of the Australian National University.
From the mighty rakali that kills the cane toad to the smoked mouse, native rodents have come to occupy every corner of our landscape.
“There are more than 150 species in Australia and New Guinea that are found nowhere else in the world,” Dr Roycroft said.
The smoky mouse (Pseudomys fumeus) lives in high altitude mountainous areas such as the Grampians National Park. (Supplied: David Paul/Museums Australia)
But very little was known about when the first ancestors of today’s native rodents arrived in Asia and how they evolved.
Now a DNA study led by Dr Roycroft has found they first set foot on the continent around 5 million years ago.
Their family tree, published today in the journal Current Biology, is intimately tied to the geological transformation of Australia and New Guinea over time.
From Asia to Australia via New Guinea
Dr Roycroft and colleagues from Museums Victoria and the University of Melbourne mapped the DNA of more than 150 living and extinct species from Australia, New Guinea and the Melanesian Islands.
The land masses of New Guinea and Australia were once connected in a supercontinent known as Sahul. (Wikimedia commons: Maximillian Dorrbecker)
The DNA showed that the first ancestor of today’s species split from Asian species in the Philippines about 8.5 million years ago.
The lineage possibly began with a single pregnant rat that floated across the seas to New Guinea on driftwood.
“We know they came by water because Asia and New Guinea were never connected,” Dr Roycroft said.
Dr Roycroft said these animals were probably tree dwellers, similar to the group that includes rainforest species such as New Guinea tree mice.
Despite their reputation as island hoppers, the rodents stayed in New Guinea for 3.5 million years.
That changed when the buckling of Earth’s tectonic plates created the mountain ranges of New Guinea and a land bridge to Australia, forming a prehistoric continent called Sahul.
The first rodent to appear in Australia 5 million years ago was the ancestor of the Pseudomys group, which includes most of Australia’s living species today, such as the smoky mouse (Pseudomys fumeus), jumping mice of the genus Notymus and the broad-toothed rat. (Mastacomys fuscus).
The broad-toothed rat (Mastacomys fuscus) is a herbivorous rodent native to southeastern Australia. (Supplied: David Paul/Museums Australia)
As sea levels fluctuated over 4 million years, at least four or five waves of native rats moved into Australia.
The rakali or native water rat, the species name Hydromys chrysogaster refers to the golden skin on its belly, was one of the most recent settlers arriving a million years ago.
However, it was not a one-way ticket. There were also movements of Australian species back to New Guinea and the islands between three and five times.
“This ‘fauna exchange’ has been very important in the evolutionary history of species in our region and highlights the shared history of Australia and New Guinea over the past five million years,” Dr Roycroft said.
Native rats exploded across the landscape
Once here, the native rodents thrived.
“When they came to Australia, they encountered all these really unique habitats that were really different to what they would have experienced before,” Dr Roycroft said.
They quickly adapted to arid areas. The jumping mouse, for example, has the most efficient kidney of any mammal in the world for dealing with low water conditions.
A jumping mouse (Notomys) from the barren desert of Australia. (Supplied: David Paul/Museums Victoria)
And they had very different diets and lifestyles than marsupials, said Peter Banks, an ecologist at the University of Sydney who was not involved in the study.
One major thing that set them apart was their incisor teeth, which grow continuously and can grind up almost anything.
“Marsupials didn’t have anything like that, so the native rodents probably didn’t face a lot of competition for the kind of niches they were trying to explore,” Professor Banks said.
But the fortunes of native rodents changed after European settlement.
The loss of species can destabilize an ecosystem
Today, 11 of the 60 species that were endemic to Australia have been lost.
“Native rats have suffered a lot since Europeans got here, largely due to land clearing, foxes and cats,” Professor Banks said.
The first species to be lost was probably the white-footed rabbit rat (Conilurus albipes) which lived in southeastern Australia.
The white-footed rabbit rat (Conilurus albipes) was about the size of a kitten. He was last seen alive in the mid-19th century. (Wikimedia Commons: John Gould/The Mammals of Australia)
Most extinct species were terrestrial animals that lived in arid areas.
Dr Roycroft said Australian rodents have the highest extinction rate of any mammal group.
“We have this incredible diversity, but it’s being lost at a very high rate.”
He said information about the evolution and genetics of these animals was vital to their future.
“We can’t conserve and protect animals when we don’t know they exist or we don’t really know what makes them different from other species.”
The heather mouse (Pseudomys shortridgei) was thought to be extinct but was rediscovered in the Grampians in the 1960s. (Supplied: David Paul/Museums Victoria)
Native rats are integral to the environment: not only do they help maintain soil and disperse seeds, but they are a vital part of the food chain for other native predators.
“If we lose even one native species, it can throw off the balance in an ecosystem,” Dr Roycroft said.
Professor Banks said native rodents such as the rakali could help keep pest rodents at bay.
“If you have a rat already living there, occupying the same niche, it’s very difficult for others to get in,” he said.
“Taking care of the rodents we have here is a good way to prevent future invasions from being successful.”
How do you tell the difference?
Although they may be called “rats” or “mice,” native rodents are more reclusive and look different than invasive species.
Rodents in Australia
There are three groups of rodents in Australia.
The oldest native rodents arrived from New Guinea between 5 and 1 million years ago. There are about 49 different species in four main groups: Pogonomys, Hydromys, Pseudomys and Uromys.
Newer native rodents include eight species of Rattus, including bush rats (Rattus fuscipes). These arrived from Asia approximately 1 million years ago and live in coastal areas.
Pest rodents include the common brown rat (Rattus norvegicus), the black rat (Rattus rattus) and the house mouse (Mus musculus). These made it to the First Fleet.
“A lot of native rodents are nocturnal, so you’ll never see them during the day,” Dr Roycroft said.
“If there’s a rodent in your house or around your property, you’re pretty sure it’s not going to be a native rodent.”
Many native rodents are much smaller than invasive species.
The smallest of them is the delicate mouse (Pseudomys delicatulus), which weighs only 8 grams.
Invasive rats tend to have very long tails, large ears and a pointed nose, Professor Banks said.
And, if you’re brave enough to pick one up, you’ll see that they also have more nipples than native rodents.
Where an introduced rodent can have six pairs, meaning it can have 12 babies, native rodents usually have only two or three pairs.
“Native rats have adapted to lower resources in the Australian environment, so they reproduce more slowly,” Professor Banks said.
“The new ones still have the ‘live fast, die young’ strategy.”
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