Ship Rat Rattus rattus

About this pest

KEY CLUES

Distinctive features: Beady eyes, dark tail much longer than body, grey-brown or black fur, rounded hairless ears, long whiskers. Unlike a Norway rat, a ship rat’s tail reaches its nose and the ears are big enough to cover its eyes. Females usually have 10 nipples on the belly.

Size: Maximum body length 225 mm, excluding tail. Weight 120 g-160 g, up to 225 g.

Droppings: Small cylindrical pellets on average 6.8 mm - 13.8 mm long, ends tapering to a point.

Footprints: Four-toed forefeet and five-toed hindfeet.

Kill signs: Shell fragments in predated nests, distinctive parallel double incisor marks in chewed flesh, neatly chewed holes in snail shells. Piles of insect legs near feeding areas. Food caches in trees or epiphytes.

Vegetation damage: Seeds with neatly chewed holes to access contents, larger fruit may show distinctive parallel double incisor marks about 2 mm wide, damage to growing tips of some species may occur.

Eye shine: Red.

Distribution: Throughout New Zealand in all habitats particularly forests. The most abundant rat species in New Zealand.

 

WHY ARE RATS PESTS?

Ecological impacts

Ship rats are omnivorous and have very broad diets incorporating both plant and animals. Their diet varies seasonally and depends on what food sources are available, but rats will always find something to eat. The fruit of many species are eaten which reduces food availability for native birds. Rat chewing destroy seeds, which has adverse impacts on forest regeneration. Invertebrates (insects) are a common food source, particularly weta, beetles, spiders and stick insects. Bird eggs and chicks are also eaten and, since ship rats spend most of their time in trees, they can have a major impact on forest birds. They are the most common culprit in the loss of birds eggs, chicks and sitting adult birds in non-beech forest on the New Zealand mainland, and in beech forest after large seed masts.

Predation by ship rats has been linked to the decline or local extinction of many forest bird and large invertebrate species in both the North Island and South Island and as a result some indigenous species are now restricted to predator-free islands and sanctuaries. In the early 1960s, ship rats invaded Big South Cape Island, off Stewart Island, resulting in the extinction of four endemic bird species. Ship rats are also likely to adversely affect reptiles through predation and competition for food.

Other impacts

Ship rats are commonly found around buildings and farms, where they cause significant damage to any stored food or crops they are able to access. They can also chew through wire insulation, which can results in fires. They can carry and transmit diseases to humans, including leptospirosis.

Read more about ship rats


Droppings

Separate

One sample of droppings measured 6.8mm to 13.8mm long (average 8.6mm) and 2.7mm to 5.0mm wide (average 3.6mm). The droppings are cylindrical and taper slightly at the ends. They are deposited singly as rats walk or in small groups close to 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:
Mouse, kiore, and ship rat droppings have pointed ends. Small ship rat droppings could be mistaken for those of mice or kiore, and large mouse or kiore droppings for ship rats. Norway rat dropping tend to be large and have much rounder tips.

Tree weta droppingsLarge wētā droppings can resemble small ship rat droppings but tend to be thinner, have a blunter end, and are often ridged lengthwise, as in the example at right. The ridging can be darker, resulting in a striped appearance, which fades with time.

Stick-insect droppings are even thinner and can look like a tube of stacked circular disks, especially when dry.

 

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 hindfeet have five toes, both with non-retractable claws. The toes of the forefoot are widely spaced in a circular pattern, the three central toes of the hindfeet are in a line with the two edge toes set slightly back. Forefeet have three main pads that are visible in tracks and hindfeet have around five pads. Rat footprints show up clearly in tracking tunnels and fore- and hind-feet are easily distinguished.

It difficult or impossible to determine which rat species made each print, because there is a lot of overlap in foot size between adults and juveniles of the three different species. Thus educated guesses have to be made based on where the tracks were observed (which species are likely to occur or use that habitat) and what size the animal was. Ship rat forefeet prints are be about 13.1 mm wide and 11.9 mm long. The adult hindfeet prints are about 17.7 mm wide by 16.5 mm long, with the middle three toes in front of the main pads (up to five) and the outer toes either side of the main pads .

Rats have two main gaits on the ground – walking and running. If tracks are found in soft sediment, it is possible to distinguish the gait from 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 photograph “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. Ship rat prints can also be confused with Norway rat and kiore prints as they can overlap in size at different ages.

Dens and nests

Ship rats are mainly arboreal (they live up in the trees) and do not create trails. Their dens are normally in epiphytes or tree hollows but if suitable locations are not available they will dig small burrows or create loosely woven nests out of twigs and leaves in hedgerows and young trees. Feeding platforms and larders/caches are created in sheltered areas such as under logs or in trees on old bird nests.

Can be confused with:
Possum damage to fruits and flowers. Other rat species can also have larders, but these are usually on the ground.

Not the footprints or tracks you were looking for?

Have a look at all of our tracks.

Vegetation Damage

Fruits and Flowers

Fruits and seeds can make up the majority of a ship rat’s diet. Small seeds will be completely destroyed through chewing and small fruits will be eaten whole. Ship rats will carefully chew away hard parts of larger seeds to get to the fleshy kernel within. When a ship rat eats a larger fruit, distinctive tooth marks are left by the parallel incisors.

Can be confused with:
Ship rat damage to fruit, seed, and flowers can be confused with damage caused by other rodent species and possum. However, the width of possum incisor teeth is generally wider than 5 mm, rat teeth range from about 2 mm to 5 mm, and mouse teeth are about 0.5 mm wide.

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Kill Sign

Fur, feathers or scales (vertebrate)

Birds

Ship rats will kill and eat small birds and nestlings and will scavenge larger dead birds. Typical evidence of rat predation includes flesh stripped from bones and gnawed bones, possibly with tooth marks visible. They will also harass larger birds, including those as large as kereru, until the incubating bird leaves the nest and the rats can eat the eggs or chicks.

Mammals

There is indirect evidence that the ship rat will eat other rat species and also mice. It is uncertain whether ship rats kill mammals or scavenge what they find.

Lizards and frogs

Rat-predated lizards have been recorded and lizards have been found in rat stomach contents.

Can be confused with:
Predation of mustelids (stoat or weasel), possums, cats or other rats. Mustelid kills can show puncture marks from the canine teeth but mustelids often remove and hide their prey. With bird kill, cats often eat the entire body except for large wing and tail feathers, heads and feet; possums tend to only eat the head and breast and leave the rest. Remember, too, that animal remains might have been scavenged.

Insects or snails (invertebrate)

Ship 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. Ship rats don’t eat legs or the heads of larger insects, so these parts can gather in favoured feeding locations.

Can be confused with:
Predation of invertebrates by ship rats can be confused with predation by other rat species. 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 ship 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. Rat faeces are not always left at the nest.

Can be confused with:
Predation of stoats or other rat species. It will be difficult to distinguish between rat species but stoats often leave paired canine tooth puncture marks in prey remains. 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

Smell

Areas with rodent infestations (such as sheds or food storage buildings) have a very distinctive and unpleasant musty smell.

Can be confused with:
The smell of ship rats may be confused with the smell of other rodent species, including mice. It is difficult to distinguish what species is present by smell alone.

Sound

High pitched squeaks, very difficult to hear. Can be heard rustling in undergrowth or running through buildings. In buildings ship rats may chew through woodwork and walls to create tunnels. From personal experience, this can be very loud when it is going on above your head in the middle of the night!

Can be confused with:
The sounds of all rodent species – Norway rat, kiore (Pacific rat) and mice – may be confused with each other. High pitched squeaks may sound like begging calls of nestling birds.

Body covering

Ship rat fur is brown or black on the back and brown, white or black on the belly. Ship rats have three major genetically determined colour morphs that vary in prevalence around New Zealand.

Can be confused with:
Other rat species – Norway rat and kiore (Pacific rat) – may be confused with ship rat. Stoats or weasels could also be confused with ship rats if they are moving quickly.

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.

Can be confused with:
Ship rat eye shine could be confused with possum or other rat species, but possums have much larger eyes.

Distribution

 In New Zealand, ship rats are found in all habitats except alpine tussock grasslands. They can be found in close proximity to humans and also in wild areas. Ship rats often climb and nest in trees but, apparently, few or none live there entirely.

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MORE ABOUT SHIP RATS

Origin

Ship rats originated in India and spread throughout Europe to the United Kingdom by the third century AD. They are now found around the world. Ship rats arrived in New Zealand in the late 1800s and started displacing the Norway rat. They are now the most common rat species in New Zealand. Interestingly, ship rats are now one of the rarest species in the United Kingdom after being displaced by Norway rats. As befits a species with a worldwide distribution, there are several common names including black rat, roof rat and bush rat.

Description

Ship rats have a slender body with a scaly tail. The tail is a uniform dark grey, is longer than the head-body length in adults, and is important for climbing and balancing. They have small beady eyes that are adapted to foraging at night; they have poor vision in daylight. Ship rats are highly dependent on their sense of smell and touch and have very sensitive noses and whiskers. There are three colour morphs present in New Zealand. Most North Island rats are the ‘frugivorous’ morph with a grey-brown back and a white underside. In the South Island, the commonest morph is ‘alexandrinus’, which have a grey-brown back and a slate grey belly. The final morph (‘rattus’) gave the species one of its other common names (the black rat). ‘Rattus’ morph individuals have a black back and a slate grey body and are most commonly found in buildings.

Size and weight:

Males are longer and weigh more than females. The head-body length of males is around 180mm and females are closer to 170mcm. Maximum body length, minus the tail, can be 225mm. The weight of adult ship rats can vary from 120g to 160g, with very heavy rats weighing up to 225 g. The average weight for males is around 150g and the average weight for females is around 130g.

Behaviour

Ship rats are nocturnal and remain active from dusk until dawn in most weathers. They mostly forage in trees, but also on the ground. They can swim well, and may cross water channels up to 500 m wide (compared to 2.2 km for Norway rats). They are relatively easy to reduce in numbers provided that best practice guidelines are followed, but their rapid reproduction and reinvasion mean that control effort has to be sustained year after year, and is therefore very expensive. They can have a strong aversion to new objects within their home range.

Diet

The ship rat’s diet is very broad, incorporating plant and animal material, refuse, and stored food. Ship rats can be 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

Ship rats are most abundant in diverse lowland forests, but are found in all forest types, parks, hedgerows, farmland, and in buildings. Their climbing ability means that their habitat is three dimensional incorporating everything from the ground to the treetops.

Distribution in New Zealand

See distribution clue.

Other

Ship rats have been eradicated from over 159 islands worldwide with the largest being the 1,022 ha Hermite Island in Australia.

How to get rid of ship rats

Contact your local DOC or Regional Council office for advice (see Next Steps). Trapping and poisoning can remove small infestations of ship rats in buildings. Across larger areas, poison baits must be used, either in bait stations or, for very large areas or islands, aerially distributed.


FIND MORE INFORMATION IN:

Bell, B.D. (1978). The Big South Cape rat irruption. In Dingwell, P.R., Atkinson, I.A.E., Hay, C. (Eds), The ecology and control of rodents in New Zealand nature reserves (pp. 33-40). NZ Department of Lands and Survey Information Series 4, Wellington.

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.

Gillies C., and Williams D. 2002: Using tracking tunnels to monitor rodents and other small mammals - Changes for HAMRO-20232 (AS AT 07/01/02). Unpublished Draft Standard Operating Procedure Department of Conservation, Waikato. 4 pp.

Hoare, J.M. (2006). Novel predators and naïve prey: How introduced mammals shape behaviours and populations of New Zealand lizards.  Unpublished PhD Thesis, Victoria University of Wellington, New Zealand.

Innes J.G. (2005). Ship rat. In King, C.M., (Ed.), The handbook of New Zealand mammals (pp. 187-204).Auckland, New Zealand: Oxford University Press.

Kiwi Foundation, (2014). Kiwi project - pest monitoring protocol. Retrieved from http://kiwifoundation.org.nz/project-pest-monitoring-protocol.