Brown rat or sewer rat (Rattus norvegicus) in New Zealand

Norway rat or brown rat

Whilst the correct common name is the Norway rat, it is more commonly called the brown rat, sewer rat, common rat or wharf rat.

The brown rat (sewer rat) is an invasive rodent.

Found worldwide, adapts well to most climatic conditions. In Australia, in contrast to most of the rest of the world, the brown rat is less common than Rattus rattus (the roof rat).

The brown rat (Norway rat) is mainly a ground dweller, so will commonly be found nesting in burrows, often located under tree roots, ground cover plantings, slab edges and flooring.

In cities they will often live and nest in the sewers, which gave rise to their name, the sewer rat.

The Norway rat is a prolific breeder and a female can have 6-10 young in a litter and 3-6  litters per year depending on availability of food. They are happy to intra-breed within families. Social groups are often formed of multiple males and multiple females. One male is dominant. Two to three females are often dominant to all other group members except the dominant male. Lifespan in the wild is 12-18 months.

Colour: Variable – Dark grey or brown to black with a lighter underside

Size range (adult):  Body 15-25 cm, Tail 10-20 cm, Weight: up to 500 g

The Norway rat is stocky with a blunt nose, small close-set ears and a long naked tail. It is a significantly bigger animal than the roof rat, but despite this, it is less common in Australia.

Brown rats (Norway rats) are omnivorous and will eat a wide range of foods, including seeds, fruit, stems, leaves, fungi, and a variety of invertebrates and vertebrates. Their diet will vary depending on what is readily available. They will eat just about anything that a human might consume. An adult will eat 15-30 g of food in a day.

  • Brown rats (Norway rats) produce 20-50 droppings per day. Norway rat droppings are 12-20 mm in length. The droppings are capsule shaped and have blunt ends. Colour varies depending on diet.
  • Because they consume and destroy the food source during feeding, they can cause devastating damage to farms and livestock.
  • They will always look for a ready food and water source along with warmth and shelter.
  • If these are readily available, then the rat will not travel any great distance, Norway rats will normally not travel further than 40 m from their harbourage.
  • They will commonly use the same pathways from harbourage to food and water sources, leaving rub marks, urine trails and droppings to indicate these pathways.

Brown rats (Norway rats) will spread disease in various ways. Their urine and faeces will transport infections and their fur is recognised as being a carrier of several bacteria. Diseases of note include Leptospirosis, Salmonella, Hantavirus and others.

With their liking to forage in garbage and in the sewers, gave rise to their name the sewer rat.

Good housekeeping is primary in reducing the likelihood of a Norway Rat infestation.

  • Keep the property clean and tidy, limiting protected runways and potential harbourages. Keep vegetation trimmed and away from the building. Cut back overhanging tree branches.
  • Close up potential entry points into a building. Keep doors closed as much as practicable. Make sure that there is no space for entry below doors.
  • Eliminate or reduce accessible food and water ie keep bins closed, do not leave pet foods out overnight and repair dripping taps or air conditioning.

Information on other rats and mice.

Treatment Notes:

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