Norway rat Rattus norvegicus

About this pest

Key Clues

Distinctive features: Largest of the rat species in New Zealand. Tail shorter than body, thick, with pale underside. Grey-brown or black fur. Small ears (won’t meet across the top of the head) that are lightly haired compared with ship rats. Has beady eyes and long whiskers. Females have 12 nipples on the belly.

Size: Average body length, without tail, c.192 mm for males and 184mm for females. Weight 170-270 g, up to 450 g. Can be much larger around towns.

Droppings: Cylindrical pellets, 13.4-19.1 mm long, blunt ended or tapering to a point.

Footprints: Four-toed forefeet and five-toed hind feet.

Kill signs: Double incisor marks in food remains, droppings around kills. Shell fragments in nests.

Vegetation damage: Seeds with neatly chewed holes to access contents, larger fruit may show distinctive parallel double incisor marks.

Eye shine: Red.

Distribution: Mainly found around waterways (coastal and freshwater), in towns and around farms.

WHY ARE NORWAY RATS PESTS?

Ecological impacts

Norway rats are omnivorous and have a very broad and varied diet. They have been recorded eating seeds, fruits, leaves, lizards, insects, molluscs, crustaceans, stored grain, animal carcasses, garbage,sewage . . .


Norway rats arrived in New Zealand in the late 1700s on sailing ships from Europe and they rapidly spread across the country. They are substantially larger than kiore (the first rat to arrive) and as a result were able to prey on species that had been able to survive the kiore invasion. Norway rats are able to climb, but they spend most of their time on the ground making them a particular threat to indigenous species that live, roost or nest on the ground. They are excellent swimmers, and some of the most dramatic impacts of Norway rats have been seen on islands where they cause significant damage to seabird populations by eating eggs, chicks and occasionally adults. Their distribution is patchy on the mainland, but they are still a significant threat to unprotected populations of ground nesting birds, especially in braided river beds and on beaches. Large invertebrates are frequently eaten and rat predation can lead to extinctions of these species.

Other impacts

Norway rats are an important agricultural pest throughout the world and cause damage by eating stored human and animal food, by soiling food, by burrowing around buildings and other structures, and by chewing timber and walls to gain access to buildings. They are also vectors for a wide range of human diseases such as leptospirosis and plague.

Read more about Norway rats.


Norway rat (vert & eggs)

Droppings

Separate

Norway rat droppings are cylindrical with blunt ends and average 16mm long (range 13.4-19.1mm) and 6mm to 8mm wide. Droppings are deposited singly in small groups along commonly used tracks and at feeding sites.

For scientific people, there is a formula that helps distinguish between ship rat and Norway rat droppings – it is correct about 95% of the time.

  • Y=100 L/W3 - where length (L) and width (W) of pellets are measured to the nearest 0.1 mm. If the average of Y from a sample of at least five pellets is less than 20 then it will most likely be Norway rats.

Can be confused with:
Norway rat droppings are larger and have a blunt end compared to the tapered ends seen in ship rat droppings. Kiore droppings are smaller than ship rat droppings and they also have pointed ends. Mouse droppings are a lot smaller than Norway rat droppings.

Tree weta droppings

Large wētā droppings can resemble Norway rat droppings in size and shape (including the blunt or rounded ends). However, wētā droppings are often ridged lengthwise, as in the example at right. The ridging can be darker, resulting in a striped appearance, which fades with time.

 

 

Not the droppings you were looking for?

Have a look at all our droppings clues.

Footprints and Tracks

Paws and feet

Rat forefeet have four toes and the hindfeet have five toes, both with non-retractable claws. The toes of the forefoot are widely spaced in a circular pattern, while the three central toes of the hindfoot are in a line with the two side toes set slightly back. Forefeet have three main pads that are visible in tracks and hindfeet have around five. Rat footprints show up clearly in tracking tunnels and fore- and hind-feet are easily distinguished. Adult Norway rat forefoot prints are about forefoot about 11.5mm long by 20.2mm wide and hindfoot prints are 15.7mm long by 23mm wide . Only the front half of the foot leaves a print. Norway rat footprints are broad in relation to their length, but this isn’t a reliable way to distinguish between rat species.

Rats have two main gaits – walking and running. If tracks are found in soft sediment. it is possible to distinguish the gait by the arrangement of the footprints. When rats are walking, the prints are evenly spaced with distinct fore- and hind-footprints visible. When they are running, all four paw prints will be close together as the animal bounds along the ground.

Can be confused with:
Rat footprints from tracking tunnels can be confused with mustelid prints. They can be distinguished by drawing a line between toes 1 and 4 (A and B in photo “How to distinguish rat from stoat prints”) on the forefoot and 1 and 5 on the hindfoot. In rats, this line will either bisect the centre pad or be slightly behind it. In mustelids, the line will be in front of the central pad. Norway rat footprints can also be confused with ship rat and kiore prints, as they can overlap in size at different ages.

Trails and Runs

Norway rats form trails linking burrows and feeding sites and these may become well worn and obvious if used frequently.

Can be confused with:
Many animals make trails, and the trail should be examined closely for footprints, hair or droppings to help identify what species is using it.

Dens and nests

Norway rats dig burrows 60-90 mm in diameter underneath rocks or tree roots, alongside buildings or in the banks of watercourses. The excavations can leave piles of soil and rubble around the burrow entrance. Food may be hoarded in burrows and discarded; inedible items may be present around the burrow entrance.

Can be confused with:
Burrows may be mistaken for rabbit burrows but rabbits generally have many burrows in one area and the entrances are larger.

Not the footprints or tracks you were looking for?

Have a look at all of our tracks.

Vegetation Damage

Bark

There are records of Norway rats stripping bark from trees .

Can be confused with:
Other species such as deer, possums, rabbits and hares may strip bark from trees.

Fruits and Flowers

Norway rats eat virtually anything, including fruit.

Can be confused with:
Possums and other rat species also eat fruit.

Not the vegetation damage you were looking for?

Browse all types of vegetation damage.

Kill Sign

Fur, feathers or scales (vertebrate)

Birds

Norway rats will kill birds by biting the back of their necks. The head is often eaten first and the rest of the body later. Flesh tends to be gnawed from bones and feathers will be left.

Lizards and frogs

Norway rats eat lizards and are known to cause lizard population declines.


Can be confused with:
Kill sign of other predators. Stoats and weasels leave distinctive canine-teeth puncture marks that would not be seen in a rat kill. Remember, too, that animal remains might have been scavenged.

Insects or snails (invertebrate)

Norway rats predate snails by chewing a hole in the shell at the apex of the whorl. Evidence of insect predation can be found at feeding stations or around nests. Norway rats don’t eat the legs or heads of larger insects, so these parts can accumulate at favoured feeding locations.

Can be confused with:
Predation by other rats leaves similar sign. Snails can also be predated by blackbirds and thrushes, so shells need to be examined carefully to determine whether they have been chewed or smashed open with a beak or on a rock.

Eggs

Nests that have been predated by Norway rats tend to have disturbed nest linings where rats have sorted through the lining looking for dropped morsels of food. Large fragments of egg shell are left in the nest and sometimes underneath. If chicks are predated, they may be eaten in the nest or removed to a feeding location nearby. Faeces are not always left at the nest.

Can be confused with:
Stoats or other rat species leave similar sign. It will be difficult to distinguish between rat species. Stoats often leave paired puncture marks in prey remains where the canines enter. Remember, too, that egg remains might have been scavenged.

Not the kill signs you were looking for?

Have a look at all our kill signs.

Other Clues

Sound

Norway rat social calls include shrill squeals and whistles and a screaming alarm call.

Can be confused with:
Other rodent species make similar sounds to Norway rats.

Body covering

Norway rats have coarse grey-brown, grey or black fur.

Can be confused with:
The fur of Norway rat is similar to that of other rat species.

Bite marks

Rats (like mice) have four incisor teeth at the front of the jaw (two top and two bottom). These long front teeth grow continuously so the animal must gnaw or chew enough to keep wearing them down. Rats have no canine teeth, a premolar midway back on each side of the bottom jaw and three molars, top and bottom, at the back on each side.

For guidance on identifying rat bite marks go to Landcare Research, 'Rodents' heading or pages 16 of their identification guide.

Eye shine

Red.

Distribution

Norway rats are patchily distributed on both the North Island and South Island. They are found on many offshore islands but have been eradicated from others.

Norway rats tend to be found around waterways and in close proximity to humans. Large populations are found in close proximity to humans in cities, towns and around farms. Isolated populations are scattered throughout the country but there is little information available on their overall distribution. They are apparently rare in mainland forests.

Can be confused with:
Norway rats could be confused with ship rats. Both species occur in towns and around farmhouses and waterways, but ship rats are much more widespread in forests and shrubland.

Not the clues you were looking for?

Have a look at all our other clues.


MORE ABOUT NORWAY RATS

Origin

Norway rats are thought to have originated in north-eastern China and then spread westwards towards Europe. They started to become common in European cities from the mid 1800s and from there were spread around the world on sailing ships. Once they arrived in New Zealand, they displaced the kiore and colonised the whole country until they in turn were displaced by the arrival of ship rats.

Description

The classic rat. Norway rats area large, and chunky, and have grey-brown fur and fat, relatively short looking, tails. . They have small ears and beady eyes. If you pull the ears across the head (caution live rats will bite!), the ears won’t touch – unlike the ears of ship rats.
Size and weight: Norway rats normally weigh 200-300g, but individuals have been caught that weigh up to 500 g. Commensal animals (those in close proximity to humans) are normally larger. Combined head-body length is around 180mm (maximum 250mm) and the tail is clearly shorter than the body.

Behaviour

Norway rats are mainly nocturnal and most active just after dusk and just before dawn. They have a strong urge to explore their surroundings. However, their wariness around new objects can make them hard to control.

Diet

The diet of Norway rats is very broad, incorporating plant and animal material, refuse, stored food, offal, and sewage. Norway rats are wary of new objects and may be initially cautious when a new food is presented. They like to feed under shelter and will often hoard food.

Habitat

Non-commensal (not living near humans) Norway rats in New Zealand tend to be found around waterways, wetlands and coastal areas.

Distribution

See distribution clue.

Other

Norway rats have several other common names – brown rat, water rat and sewer rat. The white laboratory rat is a highly inbred version of a Norway rat. Interestingly, when laboratory rats are released into the wild, they quickly revert to wild behaviour.

How to get rid of Norway rats

Contact your local DOC or Regional Council office for advice (see Next Steps). Traps and poison.


 NORWAY RAT PAGE SPONSORED BY: DOC x57


FIND MORE INFORMATION IN:

Allen R.B., Lee W.G., and Rance B.D. (1994). Regeneration in indigenous forest after eradication of Norway rats, Breaksea Island, New Zealand. New Zealand Journal of Botany, 32, 429–439.

Beveridge, A. E. (1964). Dispersal and destruction of seed in central North Island podocarp forests. Proceedings of the New Zealand Ecological Society 11, 48-55.

Innes J.G. (2005). Norway rat. In King, C.M., (Ed.), The handbook of New Zealand mammals (pp. 174-187). Auckland: Oxford University Press.

Ratz H. (1997). Identification of footprints of some small mammals. Mammalia 61(3), 431-441.