Ship Rat Rattus rattus

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.

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.

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.

Other Clues

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. Characteristic sign on chewcards are ragged card edges and pairs of incisor teeth marks or holes that are about 2 mm wide.

Can be confused with

Indistinguishable from Norway rat bites. Two pairs of rat bite marks side by side with a small gap between can be mistaken for a single pair of possum incisor teeth impressions with a large medial gap between the two teeth.

For further examples and guidance see the Manaaki Whenua – Landcare Research chewcard interpretation guide and other more technical identification guides.

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.

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.

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.