Is it a Black Rat?

The Australian Museum is frequently asked about mammals visiting suburban gardens or houses. People may think that they have seen a baby Ring-tailed Possum or a native rat species. Unfortunately, in many cases it turns out to be a Black Rat, which is an introduced species and an age-old pest in and near human habitation.

How can I tell whether it is a Black Rat or some other small mammal?
The first thing to look at is the animal's behaviour:

Is it active during both day and night, and is it relatively fearless around humans?
Is it an agile climber, often seen in fruit trees, scaling fences and electrical wires or the roof of a house?
Have you found a nest in your roof, made of shredded materials such as paper, insulation and other debris?
Have you seen evidence of it feeding on grains and discarded foodstuffs as well as fruit and even pet food.

All of these traits combined are characteristic of the Black Rat, which is often called the Roof Rat for its nesting and climbing habits. Native rats, such as the Bush Rat, are much shyer animals, and are not found in places where human traffic is frequent - they prefer to nest in dense forest understorey, sheltering in short burrows under logs or rocks, and they line their nests with grass. In fact, native rats such as the Bush Rat, have not been recorded in the inner city for many years. Possums do nest in roofs, but are mainly active at night, and although they are agile climbers, are heavier than rats in their movements. They may eat fruit and other human leftovers, but tend to feed on native vegetation, and are not found as often as rats are scavenging indoors and/or ransacking stored foodstuffs.

The next thing to look at is its tail. Is it:

Long in relation to the body, sparsely haired and scaly and not used to grip branches when climbing?
It is a Black Rat.
Shorter than the body length?
It is a Bush Rat.
Long, with a white tip, furred on the upper surface and naked underneath, and used to grip branches or held with the end slightly curled?
It is a Ring-tailed Possum.

Lastly, the overall size, shape and colour of the animal should be looked at:

Black Rats are about 16 cm to 20 cm long, and are charcoal grey to black or light brown above, cream or white below, with a sleek smooth coat. They have big thin ears and quite a round face.
A Ring-tailed Possum of similar size would still be in its mother's pouch or on her back, and would not be fully furred. An adult possum is much larger than a rat, reaching about 30 cm to 35 cm in length. The coat colour is quite variable, the ears are short with a white patch behind, and the prehensile (gripping) tail has a white tip.
The Bush Rat, Charcoal grey to black or light brown above, cream or white below; sleek smooth coat, is grey to grey-brown or reddish above, grey or cream below and has dense soft fur. The ears are rounded.

What about other small native mammals?
One animal that is sometimes seen and mistaken for a rat is in fact a small carnivorous marsupial - the antechinus. While there are several species of antechinus in Australia, they share several traits in common, which, taken together, can set them apart from rodents such as rats and mice. These include:

Their front teeth.
- Rodents have one pair of distinctive chisel shaped incisors that have
  hard yellow enamel on the front surfaces.
- Antechinus have four rows of small sharp incisors.
Their ears. Many antechinus species have large thin crinkly ears that have a notch in the margin, although not all will have this notch.
Their tail. Antechinus have a sparsely haired tail, which is the same length as the body or shorter (65-110mm).
Their habits. Antechinus are mainly nocturnal insect eaters, which are found in forest habitats and are not found often in urban areas. They shelter in spherical nests in hollow logs or crevices, but can sometimes found nesting in furniture in bush areas or farms.