Description

A sleek looking medium sized rat, with smooth looking fur, as opposed to Norway rat's shaggy look.

Tail noticeably longer than body, and large thin ears (which if pulled forward will cover the eyes). Has a more pointed muzzle than a Norway rat.

Average weight 120-150g, max 225g. Maximum body length (excluding the tail) is 225mm.

Females have 10 nipples compared with 12 for Norways and 8 for kiore.

There are three different fur colour types, two variations of grey/reddish grey and one black.

An agile and excellent climber, but a poor swimmer. Can spend considerable portions of time foraging up in trees.

Colour-blind with poor vision but has excellent hearing and smell.

Gestation 21-23 days, the litter size averaging 5-6. The young are mobile after 1 month and completely independent and sexually mature after three months. Females can produce three or more litters annually, in a breeding season which can be 6-7 months or longer. Mortality rate is high, with few rats (<10%) surviving more than one year.

Extended breeding seasons and increase in population can occur as a result of heavy seedfall of protein-rich seeds (eg. hinau, pigeonwood, beech, rimu)

Home range is variable but usually between 0.5 - 4 ha (diameter of 100-200m) and densities can also vary widely but is on mainland sites usually 1-3 per hectare, though up to 6.2 per ha has been recorded. There can be substantial overlap in ranges of individuals within and between sexes.

Less carnivorous in diet than Norway rats, with heavier reliance on plant foods, particularly in autumn and winter. Animals are readily taken though, including a variety of invertebrates, eggs, chicks, birds, lizards, snails.

Ship rats are a significant predator of many small indigenous species. The establishment of ship rats on Big South Cape Island resulted in the extinction of Stewart Island wren, Stewart Island snipe, a flightless weevil and the greater short-tailed bat, with local extermination of saddlebacks, fernbird.
Populations of the giant herb punui Stilbocarpa polaris were significantly damaged.

Ship rats are a proven and significant predator of North Island kokako and are likely to pose a threat to any smaller forest-dwelling bird species such as kakariki, mohua.

They take eggs and kill chicks and adults particularly nesting females of small birds (eg. robin).

They are also known to prey upon smaller individuals of the various Powelliphanta and Paryphanta snails, and in some instances are the major predatory factor.

They are also effective predators of lizards.

Droppings are 7-14mm long and 2.5 - 5mm in diameter, proportionately broader and shorter than Norway rat droppings. This is usually a good indicator of species but cannot be wholly relied upon as a diagnostic tool.

Ship rats create and use 'husking' stations where large quantities of husks of seeds may accumulate (kiore also do this, but Norway rats do not). These are usually near the source of seeds and on the surface of the ground near overhead cover (Norway rat ‘caches’ are of intact fuits and are almost always underground).

Ship rats are often considered to leave ‘messy’ sign as predators, but smaller prey can be almost completely consumed, with little wastage or remains.

Sign (particularly ‘nest-robbing’) can sometimes be confused with possum sign but eggshells preyed upon by rats are more liklely to have jagged shell margins and many small fragments whereas possum sign normally shows crushed shells with few small fragments. Predation on chicks by rats usually does not involve any partial skinning of the carcass which is often apparent in possum predation. Rats sometimes leave obvious gnaw marks on bones remaining whereas possums usually break bones cleanly with no apparent teethmarks. Absence of feather pellets may also suggest rat predation rather than possum predation.