Ship rats (Rattus rattus) are one of the most widespread mammals in NZ “one of the most successful mammalian weed species”. Yet they are rarely seen because they are nocturnal, arboreal and shy.
There are 3 colour forms (morphs):
‘rattus’ – a black back with a slate grey belly
‘alexandrinus’ – grey-brown back with a slate grey belly
‘frugivorus’ – grey-brown back but the chin to anus is pure or lemony white
All three morphs interbreed freely.
Most North Island rats = frugivorus
Most South Island rats = alexandrinus
The rattus morph is more docile and likely to be found in buildings, they’re evenly spread in NZ.
The normal number of nipples is 10.
Skilful climbers in buildings and forests and can scale rough vertical surfaces and traverse fine wires/branches.
They do not live entirely in trees but use the forest floor as well. They rarely burrow and are unwilling swimmers (however studies on Great Barrier Island indicate they have swum up to 750m to reach new islands).
The skull and teeth are specialized for gnawing and grinding. Large incisors grow throughout life and are self-sharpening. The molars also grow continuously, and their wear is used to indicate rat age.
Ship rats collect hard seeds (e.g. miro, hinau) and eat them under cover, such as hollow logs. Hundreds of neatly eaten seed cases may be found in a cache. They feed on any available flat surface leaving pellets, seeds and insect remains e.g. weta legs and heads. Surfaces include large limbs, branch crotches, epiphyte clumps.
Ship rat faecal pellets are around 8.5mm long and 3.5mm wide.
In most NZ forest the faeces (scats) of predators will indicate ship rat presence if the scat contains rat fur, bones and teeth.
They are most abundant in mature, lowland podocarp-broadleaf forest.
Absent in pure beech forest (except after a moderate-heavy seedfall), and scarce at high altitude.
In buildings they usually live in roofs (hence alternative name the Roof Rat).
Omnivorous. Eat both plant and animal food all year round.
Main animal food = arthropods – wetas, beetles, spiders, moths, cicadas
Main fruits = Coprosma spp., karaka, rimu, hinau, miro, kawakawa
Of conservation significance other foods include: native snails, slugs, lizards, birds’ eggs and young.
Eat about 15g of dry food a night (10% of body weight).
They feed undercover and carry their food to a sheltered place rather then eat it immediately.
They are ‘messy’ predators of bird eggs and chicks. They lick up spilt yolk, disturb nest linings, eat the flesh off chick limbs but leave them attached, eat just the brain of a chick.
In contrast mustelids cleanly remove eggs or nestlings from the nest.
Dispersed evenly through available habitat.
Ranges are three-dimensional.
Males range up to 300m (mean 194m), females up to 100m.
They know there neighbours well and if one is removed, invaders take over the empty range within days. Therefore reinfestation is rapid after control programmes.
Difficult to find as they’re in epiphyte clumps, tree hollows, and may build roughly spherical, sparrow-like nests in hedges/young trees. They are substantial, stable structures, 30-100cm across, woven of twigs and leaves.
They also construct smaller feeding platforms up trees.
In a study, the rats used 3-4 daytime nests over a 5 week period. Variable amounts of time are spent in each nest.
Social Organisation and Behaviour
Nocturnal, great sense of smell, touch, hearing and taste. They emerge to feed just on dark, very alert to unusual sounds and testing the air for scents.
In one case study, radio-tracked rats were mostly arboreal (73%), but frequently recorded on the ground.
Active from dusk till dawn regardless of moon phase or weather.
They cover their home range each night in an unsystematic way, often noisy rustling through vegetation and vocalising.
Gestation period is around 20 days, up to 3 litters per year.
Litter size 3-10, averaging 5-8. They reach sexual maturity at 3-4 months old.
However a NZ study suggests that a female’s total lifetime productivity is around 16 young. Indoor ship rats have a higher productivity than outdoor populations.
Seasonal breeding causes seasonal density changes:
Low numbers in spring/early summer, peak numbers in autumn.
No long-term association between males and females, females alone raise the young.
Predators, Parasites & Diseases
Predation by feral cats is an important source of mortality. They are also predated by stoats and rarely by moreporks.
11 parasites have been found including mites, the louse and fleas (including the main flea vector of the bubonic plague is found on rats in Auckland – there were nine deaths between 1900 and 1911).
It also hosts a parasite that can cause in leptospirosis in humans.
Damage can not be prevented by using repellents – the rats show no aversion to predator
(canid, felid, mustelid) odours in the wild.
Aerial and ground poisoning and trapping can be very effective in reducing ship rat populations.
Possums and rats are often co-targeted in poisoning programmes.
Strict precautions needed to prevent the establishment of Asian-type ship rats (we have Oceanian-type).
Large-scale eradication has never been attained anywhere. Temporary population reduction in small areas is achievable, although reinvasion is rapid. The population can recover within months (although possum populations can take years).