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The Australian Rat Race

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Professor Peter Banks, from the University of Sydney’s School of Biological Sciences, shines a light on a mammal that many revile – the rat. A quarter of Australia’s native mammal species are some form of rodent but there are also four introduced species of rat. Australia’s native rats came when the sea level dropped, creating land bridges to Indonesia.

The first invasion happened about 3-4 million years ago which gave rise to the ancestors of most of our native rodents. Another, some 1 million years ago, introduced another 8-9 species in the Rattus genus, the same genus as the black rat. Bush rats, like the black rat, are omnivorous, eating anything from seeds, green vegetation, invertebrates and fungi to small vertebrates. They generally remain in the dense bush areas, leaving the urban spaces to introduced rats like the black rat. Bush rats usually have 4-6 young two or three times a year, depending on the available resources.

Black rats originated in India, spreading west to meet up with human civilisation there. They then spread across the world. The black rat is also known as the roof rat and the ship rat and, despite its name, the black rat is more often a tawny colour, only 10% being black. Having lived with humans for thousands of years they have learned to exploit those places where humans can’t reach, like rooves and wall cavities. The black rat has a tail that is longer than its body.

The brown rat (another introduced urban rat), also known as the sewer rat or Norway rat, has a tail as long as its stocky, thick-set body. Native rats often have quite thin tails which can be as long as their body or shorter. All of the introduced rats have resisted attempts to wipe them out so they have dealt well with the selective population pressures of living with people, something they have done for thousands of years. The good thing about an intact native rat fauna like the bush rat is that it will stop the invasion of introduced rats into the bush. In New Zealand, where there are no native mammals apart from bats, the ship rat has taken over, killing birds (the rats can climb readily) and invertebrates and reptiles because there are no predators or competitors.

Because they eat pretty much everything, black rats will exploit garbage. One of the reasons why garbage is collected regularly is to control the food supply for rats. Black rats do carry diseases like the plague, famously associated with the Black Death which had a lot of impact on humans. They also carry salmonella, leptospirosis, as well as other bacterial and viral infections with the potential to harm humans. These rats communicate with scent marks which also carry the diseases and those scent marks are left in common places where people put their things. Diseases are also left in their urine marks so controlling rats is important for disease control. Rats have shown a remarkably tenacious co-existence with humans – the human race alongside the rat race.

Summary text by Victor Barry, August 2015

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