Brown Rats Biology


Common name : Brown Rat (also called Sewer, Hanover, Norway, Brown Norway, Norwegian and Wharf Rat)

Scientific name : Rattus norvegicus

Adult size when fully grown is around 250 mm long plus the tail of 250mm for both the male and female of the species.

Adult weight when fully grown is around 350 grams for the male of the species and 250 grams for the female. Some male Brown rats can weigh in at up to 900 grams but this is an extreme exception.

They have has a short fur coat that can range from brown to dark grey. It can have white or patchy coloured stomach fur.

They originated from China. It migrated to Europe and then England in 1728 as a passive traveller on timber ships from Norway, hence the Brown rat was dubbed the “Norwegian rat” and got its scientific name Rattus norvegicus. From England they spread to Australia on many ships including the first colony ships that landed in Australia back in the late 18th century. Worldwide, the Brown rat is the most wide spread and established rat. It is second only to humans among the mammals that dominate the world.

Tunnel Made By a Rat

Tunnel Made By a Rat


It has a wide ranging foraging behaviour when searching for staple foods. They consume a wide range of food types though their natural diet is mainly fruit, seeds and greenery such as leaves, stems and buds. Brown rats will also dine on small creatures for meat including insects and bugs plus smaller rodents.

Scientists and observers have discovered that they are very adaptable to their surroundings and their chosen diet can be directly influenced by their geographical location and available sources of food. For instance it has been recorded hunting and catching small fish from an American fish hatchery and in Italy the rat has adapted to diving into a river for fresh molluscs.

Brown rats on the island of Norderoog in the North Sea have been recorded hunting and eating the local small sparrows and ducks.

They are also a scavenger and will happily live in garbage and sewers eating out of rubbish bin refuse and any food scraps that humans discard. It consumes around 15 grams of food per day and drinks about 15 ml of fluids per day.

They have been identified as a serious threat to Australian natural habitats because they eat the foods that Australian natives need and they attack and eat the eggs and babies of many Australian natives. Farmers are also affected by them as they are responsible for damage to valuable agricultural crops.

Like all rodents, it is very adaptable when it comes to living in a new environment. In Australia the Brown rat can be found from coastal environments to high density urban living. They have no problem inhabiting cavities in people’s homes, office spaces and industrial premises.

Unlike the Black rat, the Brown rat is not an accomplished climber. It prefers to burrow into the ground or use ground level nooks and crannies for its refuge and homes. The Brown eat a wider variety of foods than the Black and are more resistant to weather extremes.

It prefers to nest in small dry areas that are hidden from view and possible predators. It creates a nest from shredded materials such as leaves and dry grass or paper and scraps of cloth. It scavenges for whatever is available and then tears it up to create a cozy home. So the walls and ceiling spaces of a house are ideal!


Colonies create a social hierarchy. Each rat has its own place in the pecking order of the pack based on aggression. Groups indulge in play fighting which involves physical confrontation through rolling, chasing, tumbling and wrestling each other. They will strike at each other’s necks during social play, serious confrontation and jostling for hierarchy positioning, involves more serious fighting and frequent biting of other rats rear quarters.

When foraging for food the Brown rat will try and eat from a range of foods as this allows the them to diversify their food intake to balance their diet nutritionally and exposes them to the choice of the best quality food available. It is a key part of their success in surviving in many different countries and spreading across the world on ships and hidden inside cargo on planes.


It is able to breed all year round as long as there are suitable conditions. The female Brown rat can produce up to 5 litters a year when she reaches sexual maturity, which occurs within 5 weeks of birth. The young are born after only 21 days and the female can produce up to 14 off spring at one time. However, the average young produced in a litter is seven, though this is still enough to ensure an explosive rate of growth in the population when there is food available. It has been noted that when the female Brown rat has a small litter she will usually spend more time attending to them, so they are careful parents.

The maximum life span is up to three years, but most do not make it past just 1 year of life. Estimates indicate a general figure that 95% of the Brown rats born will perish in their first year, mainly at the hands of carnivorous predators such as hawks, owls and snakes.

Since they live in large groups with a definite established hierarchy of social structure, when food is not readily available, the Brown rats that are lower down in social order are the first to die off. It has also been noted that after a significant portion of a rat colony dies off, the remaining rats will increase their reproductive rate to try to restore the previous population levels, as fast as available food will allow.


Rats can cause physical damage to a home and its contents plus spread unwanted diseases to the inhabitants. The rat can chew through walls, cupboards and other parts of a house’s envelope to gain entry. The Brown rat does carry diseases that are serious and sometimes fatal to human beings. Rodent droppings and urine when left in cupboards and home surfaces are an easy way for humans to become infected with the diseases rats carry. The risk of human salmonella food poisoning when they eat and forage through rubbish and then enter people’s homes and contaminate stored food, is significant.

The associated risk of leptospirosis spread by rats is also called Weil’s disease. Though recognized among the world’s most common diseases transmitted to people from animals, leptospirosis is nonetheless a relatively rare bacterial infection in humans. This infection is commonly transmitted to humans by allowing water that has been contaminated by animal urine to come in contact with unhealed breaks in the skin such as cuts and abrasions, human eyes, or with the mucous membranes of the nose. Outside of tropical areas, leptospirosis cases have a relatively distinct seasonality with most of them occurring in the spring and autumn. In the Brisbane area it has been noted as a minor risk to humans. Although it is not common it can have devastating effects on humans.


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