The best-known rat species are the Black, or Ship Rat (Rattus rattus) and the Brown, or Common Rat (Rattus norvegicus). The group is generally known as the Old World rats or true rats, and originated in Asia. Rats are bigger than most Old World mice, which are their relatives, but seldom weigh over 500 grams in the wild.
The Polynesian Rat (Rattus exulans), known to the Maori as "kiore", is the third most widespread species of rat in the world behind the Brown Rat and Black Rat. It's distribution in New Zealand however is limited to Fiordland, Stewart Island and some off-shore islands.
Also known as the Asian black rat, Ship Rat, Roof Rat or House Rat, is a common long-tailed rodent of the genus Rattus (rats) and the subfamily Murinae (murine rodents). The species originated in tropical Asia and spread through the Near East in Roman times before reaching Europe by the 6th century and spreading with Europeans across the world. Today it is again largely confined to warmer areas, having been supplanted by the Norway Rat (Rattus norvegicus) in cooler regions.
Despite its name it comes in several colour forms. Compared to the Norway Rat, it is a poorer swimmer, but more agile and a better climber, tending even to flee upwards. It is usually black to light brown in colour with a lighter underside. A typical rat will be 15 to 20 cm long with a further 20 cm of tail. It is nocturnal and omnivorous, with a preference for grains. In a suitable environment it will breed throughout the year, with a female producing three to six litters of up to ten young. Females may regulate their production of offspring during times when food is scarce, throwing as few as only one litter a year. R. rattus lives for about 2-3 years. Social groups of up to sixty can be formed.
R. rattus has been known to fall victim of a number of diseases, of which bubonic plague (via the rat flea), typhus, toxoplasmosis and trichinosis are the most well known.
In New Zealand, ship rats have an internationally unusual distribution and importance, in that they are utterly pervasive through native forests, scrublands, and urban parklands. This is typical only of oceanic islands which lack native mammals, especially rodents. Around most of the world, ship rats are found only in disturbed habitats near people, mainly near the coast. Ship rats are the most frequent predator of small forest birds, seeds, invertebrates, and perhaps lizards in New Zealand forests, and are key ecosystem changers. Controlling their abundance on usefully large areas of the New Zealand mainland is a crucial current challenge for conservation managers.
Originally called the "Hanover rat" by people wishing to link problems in 18th century England with the House of Hanover, it is not known for certain why it was named Rattus norvegicus (Norwegian rat) as it did not originate from Norway. However, the English naturalist John Berkenhout, author of the 1769 book Outlines of the Natural History of Great Britain, is most likely responsible for the misnomer. Berkenhout gave the Norway rat the binomial name Rattus norvegicus believing that it had migrated to England from Norwegian ships in 1728, although no Norway rat had entered Norway at that time.
By the early to middle part of the 19th century, British academics were aware that the Norway rat was not native to Norway, hypothesizing (incorrectly) that it may have come from Ireland, Gibraltar or across the English Channel with William the Conqueror. As early as 1850, however, a more correct understanding of the rat's origins was beginning to develop. The species came from central Asia and (likely) China to England less than two hundred years ago and to have spread from thence to other countries visited by English ships.
The fur is coarse and usually brown or dark grey, the underparts are lighter grey or brown. The length can be up to 25 cm (10 in), with the tail a further 25 cm (10 in), the same length as the body. Adult body weight averages 350 g (12 oz) in males and about 250 g (9 oz) in females, but a very large individual can reach 500 g (18 oz).
Norway rats have acute hearing, are sensitive to ultrasound, and possess a very highly developed olfactory sense. Their average heart rate is 300 to 400 beats per minute, with a respiratory rate of around 100 per minute. Their vision is poor, around 20/600 for normal rats. They are dichromates who perceive colours rather like a human with red-green colorblindness, and their colour saturation may be quite faint.
The Norway Rat is usually active at night and is a good swimmer, both on the surface and underwater, but unlike the related Black rat (Rattus rattus) they are poor climbers. Norway rats dig well, and often excavate extensive burrow systems. A 2007 study found Norway rats to possess metacognition, a mental ability previously only found in humans and some primates.
The Norway rat is a true omnivore and will consume almost anything, but cereals form a substantial part of its diet.
Foraging behavior is often population-specific, and varies by environment and food source. Norway rats living near a hatchery in West Virginia catch fingerling fish. Some colonies along the banks of the Po river in Italy will dive for mollusks, a practice demonstrating social learning among members of this species. Rats on the island of Norderoog in the North Sea stalk and kill sparrows and ducks.
The Norway rat can breed throughout the year if conditions are suitable, a female producing up to five litters a year. The gestation period is only 21 days and litters can number up to fourteen, although seven is common. The maximum life span is up to three years, although most barely manage one. A yearly mortality rate of 95% is estimated, with predators and interspecies conflict as major causes.
When lactating, female rats display a 24 hour rhythm of maternal behavior, and will usually spend more time attending to smaller litters than large ones.
Norway rats live in large hierarchical groups, either in burrows or subsurface places such as sewers and cellars. When food is in short supply, the rats lower in social order are the first to die. If a large fraction of a rat population is exterminated, the remaining rats will increase their reproductive rate, and quickly restore the old population level.
Likely originating from the plains of Asia, Northern China and Mongolia, the Norway rat spread to other parts of the world sometime in the Middle Ages. The question of when Norway rats became commensal with humans remains unsettled, but as a species they have spread and established themselves along routes of human migration and now live almost everywhere humans do.
In the absence of humans, Norway rats prefer damp environments such as river banks. However, the great majority are now linked to man-made environments, such as sewage systems.
The only Norway rat-free zones in the world are the Arctic, the Antarctic, some especially isolated islands, the province of Alberta in Canada, and certain conservation areas in New Zealand.
First arriving before 1800 (perhaps on James Cook's vessels), Norway rats have posed a serious threat to many of New Zealand's native animals. Rat eradication programmes within New Zealand have led to rat-free zones on offshore islands and even on fenced "ecological islands" on the mainland. Before an eradication effort was launched in 2001, the sub-Antarctic Campbell Island had the highest population density of Norway rats in the world.
Rats are not a kiwi predator but a competitor; the invertebrate diet of rats and kiwi is almost identical. This becomes especially critical when Kiwi chicks and rats are competing for the same food supply. Unfortunately the only widespread predator that actively hunts rats is the Stoat.