The brown rat, common rat, sewer rat, Hanover rat, Norway rat, Brown Norway rat, Norwegian rat, or wharf rat (Rattus norvegicus) is one of the best known and most common rats. One of the largest muroids, it is a brown or grey rodent with a body up to 25 cm (10 in) long, and a similar tail length; the male weighs on average 350 g (12 oz) and the female 250 g (9 oz). Thought to have originated in northern China, this rodent has now spread to all continents, except Antarctica, and is the dominant rat in Europe and much of North America—making it the most successful mammal on the planet after humans. Indeed, with rare exceptions (see below) the Norway rat lives wherever humans live, particularly in urban areas.
Naming and etymology
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 the brown rat is 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 popularizing the misnomer. Berkenhout gave the brown rat the binomial name Rattus norvegicus believing that it had migrated to England from Norwegian ships in 1728, although no brown rat had entered Norway at that time.
By the early to middle part of the 19th century, British academics were aware that the brown 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 British novelist Charles Dickens acknowledged the misnomer in the 2 June 1888 edition of his weekly journal, All the Year Round, writing:
"Now there is a mystery about the native country of the best known species of rat, the common brown rat. It is frequently called, in books and otherwise, the 'Norway rat,' and it is said to have been imported into this country in a ship-load of timber from Norway. Against this hypothesis stands the fact that when the brown rat had become common in this country, it was unknown in Norway, although there was a small animal like a rat, but really a lemming, which made its home there."
Academics began to understand the origins and corrected etymology of the brown rat towards the end of the 19th century, as seen in the 1895 text Natural History by American scholar Alfred Henry Miles:
"The brown rat is the species common in England, and best known throughout the world. It is said to have travelled from Persia to England less than two hundred years ago and to have spread from thence to other countries visited by English ships."
Though the assumptions surrounding this species' origins were not yet entirely accurate, by the 20th century it was established among naturalists that the brown rat did not originate in Norway, rather that the species came from central Asia and (likely) China. Despite this, this species' common name of "Norway rat" is still in use today.
The fur is coarse and usually brown or dark grey, while 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). Rats weighing over 1 kg (2.2 lb) are exceptional, and stories of rats as big as cats are exaggerations, or misidentifications of other rodents such as the coypu and muskrat.
Brown 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. The vision of a pigmented rat is poor, around 20/600, while a non-pigmented (albino) with no melanin in its eyes has both around 20/1200 vision and a terrible scattering of light within its vision. Brown rats are dichromates who perceive colours rather like a human with red-green colorblindness, and their colour saturation may be quite faint. Their blue perception, however, also has UV perceptors, allowing them to see ultraviolet lights that some species cannot.
Biology and behavior
The Brown 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. Brown rats dig well, and often excavate extensive burrow systems. A 2007 study found brown rats to possess metacognition, a mental ability previously only found in humans and some primates.
Brown rats are capable of producing ultrasonic vocalizations. As pups, young rats use different types of ultrasonic cries to elicit and direct maternal search behavior, as well as to regulate their mother's movements in the nest. Although pups will produce ultrasounds around any other rats at 7 days old, by 14 days old they significantly reduce ultrasound production around male rats as a defensive response. Adult rats will emit ultrasonic vocalizations in response to predators or perceived danger, the frequency and duration of such cries depending on the sex and reproductive status of the rat. The female rat will also emit ultrasonic vocalizations during mating.
Rats may also emit short, high frequency, ultrasonic, socially induced vocalization during rough and tumble play, before receiving morphine, or having sex, and when tickled. The vocalization is described as a distinct "chirping," has been likened to laughter, and is interpreted as an expectation of something rewarding.. Like most rat vocalizations, the "chirping" is too high in pitch for humans to hear without special equipment.
In clinical studies, the chirping is associated with positive emotional feelings, and social bonding occurs with the tickler, resulting in the rats becoming conditioned to seek the tickling. However, as the rats age, there appears to be a decline in the tendency to chirp.
The brown rat is a true omnivore and will consume almost anything, but cereals form a substantial part of its diet.
Martin Schein, founder of the Animal Behavior Society in 1964, studied the diet of brown rats and came to the conclusion that the most-liked foods of brown rats were (in order) scrambled eggs, macaroni and cheese, and cooked corn kernels. According to Schein, the least-liked foods were raw beets, peaches, and raw celery.
Foraging behavior is often population-specific, and varies by environment and food source. Brown 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.
Reproduction and life cycle
The brown 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.
Brown 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.
It is common for rats to groom each other and sleep together. As with dogs, rats create a social hierarchy, and each rat has its own place in the pack. Rats are said to establish an order of hierarchy and so one rat will be dominant over another one. Groups of rats tend to "play fight", which can involve any combination of jumping, chasing, tumbling, and boxing. Play fighting involves rats going for each other's necks, while serious fighting involves strikes at the others' back ends.
Rats are known to burrow extensively, both in the wild and in captivity, if given access to a suitable substrate. Rats generally begin a new burrow adjacent to an object or structure, as this provides a sturdy "roof" for the section of the burrow nearest to the ground's surface. Burrows usually develop to eventually include multiple levels of tunnels, as well as a secondary entrance. Older male rats will generally not burrow, while young males and females will burrow vigorously.
Burrows provides rats with shelter and food storage as well as safe, thermoregulated nest sites. Rats use their burrows to escape from perceived threats in the surrounding environment - for example, rats will retreat to their burrows following a sudden, loud noise or while fleeing an intruder. Burrowing can therefore be described as a "pre-encounter defensive behavior," as opposed to a "post-encounter defensive behavior," such as flight, freezing, or avoidance of a threatening stimulus.
Distribution and habitat
Likely originating from the plains of Asia, Northern China and Mongolia, the brown rat spread to other parts of the world sometime in the Middle Ages. The question of when brown 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.
The brown rat may have been present in Europe as early as 1553, a conclusion drawn from an illustration and description by Swiss naturalist Conrad Gesner in his book Historiae animalium, published 1551-1558. Though Gesner's description could apply to the black rat, his mention of a large percentage of albino specimens – not uncommon among wild populations of brown rats – adds credibility to this conclusion. Reliable reports dating to the 18th century document the presence of the brown rat in England in 1730, France in 1735, Germany in 1750, and Spain in 1800, becoming widespread during the Industrial Revolution. It did not reach North America until around 1750-1755.
In the absence of humans, brown rats prefer damp environments such as river banks. However, the great majority are now linked to man-made environments, such as sewage systems.
It is often said that there are as many rats in cities as people, but this varies from area to area depending on climate, living conditions, etc. Brown rats in cities tend not to wander extensively, often staying within 20 m (66 ft) of their nest if a suitable concentrated food supply is available, but they will range more widely where food availability is lower. In New York City there is great debate over the size of the rat population with estimates from almost 100 million rats to as few as 250,000. Experts suggest New York is a particularly attractive place for rats because of its aging infrastructure, high moisture and poverty rates. In addition to sewers, rats are very comfortable living in alleyways and residential buildings, as there is usually a large and continuous food source in those areas.
In the United Kingdom some figures show that the rat population has been rising, with estimations that 81 million rats reside in the UK. Those figures would mean that there are 1.3 rats per person in the country. High rat populations in the UK are often attributed to the mild climate, which allow them higher survival rates during the winter months.
The only brown 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
Antarctica is almost completely covered by ice and has no permanent human inhabitants, making it uninhabitable by rats. The Arctic has extremely cold winters that rats cannot survive outdoors, and the human population density is extremely low making it difficult for rats to travel from one habitation to another. When the occasional rat infestation is noticed and eliminated, the rats are unable to re-infest it from an adjacent one. Isolated islands are also able to eliminate rat populations because of low human population density and geographic distance from other rat populations.
Rat Island in Alaska was infested with brown rats after a Japanese shipwreck in 1780. They had a devastating effect on the native bird life. An eradication programme was started in 2007 and the Island was declared rat free in June 2009.
Alberta, Canada is unusual in that rat infestation was eliminated by aggressive government action. Although it is a major agricultural area and has a fairly high human population density, it is far from any seaport and only a portion of its eastern boundary with Saskatchewan provides a favorable entry route for rats. They cannot survive in the boreal forest to the north, the Rocky Mountains to the west, nor the semi-arid High Plains of Montana to the south. The first brown rat did not reach Alberta until 1950, and in 1951 the province launched a rat-control program that included shooting and poisoning rats, and bulldozing, burning down, and blowing up rat-infested buildings. The effort was backed by legislation that required every person and every municipality to destroy and prevent the establishment of designated pests. If they failed, the provincial government could carry out the necessary measures and charge the costs to the landowner or municipality.
In the first year of the program, 64 tonnes (71 short tons) of arsenic trioxide were spread throughout 8,000 buildings on farms along the Saskatchewan border. In 1953 the much less toxic and more effective poison, Warfarin, was introduced. By 1960 the number of rat infestations in Alberta dropped to below 200 per year.
Currently, only zoos, universities, and research institutes are allowed to own caged rats in Alberta, and possession of an unlicensed rat (including pet rats) is punishable by a $5,000 fine or 60 days in jail. The adjacent and similarly landlocked province of Saskatchewan initiated a rat control program in 1972, and has managed to reduce the number of rats in the province substantially, although they have not been eliminated.
First arriving before 1800 (perhaps on James Cook's vessels), brown 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 brown rats in the world.
Similar to other rodents, brown rats may carry a number of pathogens  which can result in disease, including Weil's disease, rat bite fever, cryptosporidiosis, Viral hemorrhagic fever (VHF), Q fever and hantavirus pulmonary syndrome. In the United Kingdom, brown rats are an important reservoir for Coxiella burnetii, the bacteria that causes Q fever, with seroprevalence for the bacteria found to be as high as 53% in some wild populations.
This species can also serve as a reservoir for Toxoplasma gondii, the parasite that causes toxoplasmosis, though the disease usually spreads from rats to humans when domestic cats feed on infected brown rats. The parasite has a long history with the brown rat, and there are indications that the parasite has evolved to alter an infected rat's perception to cat predation, making it more susceptible to predation and increasing the likelihood of transmission.
Surveys and specimens of brown rat populations throughout the world have shown that this species is often associated with outbreaks of trichinosis, but the extent to which the brown rat is responsible in transmitting Trichinella larvae to humans and other synanthropic animals is at least somewhat debatable. Trichinella pseudospiralis, a parasite previously not considered to be a potential pathogen in humans or domestic animals, has been found to be pathogenic in humans and carried by brown rats.
Brown rats are sometimes mistakenly thought to harbor bubonic plague, a possible cause of The Black Death. However, the bacteria responsible, Yersinia pestis, incubates in only a few rodent species and is usually transmitted zoonotically by rat fleas - common rodents include ground squirrels and wood rats. In short, a brown rat may catch fleas that have plague, but cannot contract the disease itself, whereas other non-rodent species like dogs, cats, and humans can be bitten by diseased fleas or come in contact with an infected animal and then become infected themselves.
Uses in science
Selective breeding of albino brown rats rescued from being killed in a now-outlawed sport called rat-baiting has produced the albino laboratory rat. Like mice, these rats are frequently subjects of medical, psychological and other biological experiments and constitute an important model organism. This is because they grow quickly to sexual maturity and are easy to keep and to breed in captivity. When modern biologists refer to "rats", they almost always mean Rattus norvegicus.
The brown rat is kept as a pet in many parts of the world. Australia, the United Kingdom, and the United States are just a few of the countries that have formed fancy rat associations similar in nature to the American Kennel Club, establishing standards, orchestrating events, and promoting responsible pet ownership.
There are many different breeds of domesticated brown rats. These include variations in coat patterns as well as the style of the coat, such as Hairless or Rex.
Due to the lessened exposure that most people have to rats, it's far less likely that someone will have an allergy to rats than it is that they will have an allergy to cats or dogs, so they often make good pets for those with pet allergies.
As pet food
Because of their quick reproduction, rats are also used as live food for captive animals, commonly large reptiles such as snakes.
Different views exist on the topic of feeding live rats, or other species, to captive animals. Some organizations feel there is a large potential for injury to the reptiles if they are fed live animals instead of prekilled. A captive animal that does not kill the rat quickly enough will often suffer injury, e.g., from being bitten or scratched. Even feedings supervised by the owner of the captive animal can result in an injured or dead animal, as rats in particular are faster than humans and many other animals.
Other groups view the practice of feeding live rats to reptiles as cruelty to animals because the rat is not guaranteed a quick or painless death, and equate it to rat baiting or cockfighting, which are illegal in most parts of the world. These groups feel that reptiles should be conditioned to accept dead rats, as is the rule with many zoos.
Some countries, such as South Africa, as well as various municipalities worldwide, have banned the feeding of live vertebrate animals (like rats) to predators because the practice is seen as inhumane.
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