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The brown rat ( Rattus norvegicus), also known as the common rat, street rat, sewer rat, wharf rat, Hanover rat, Norway rat and Norwegian rat, is a widespread, common species of rat. One of the largest Muroidea, it is a brown or grey rodent with a body length of up to long, and a tail slightly shorter than that. It weighs between . Thought to have originated in northern China and neighbouring areas, it has now spread to all except Antarctica, and is the dominant rat in Europe and much of North America, having become naturalised across the world. With rare exceptions, the brown rat lives wherever humans live, particularly in urban areas. They are omnivorous, reproduce rapidly, and can be a vector for several human diseases.
Selective breeding of the brown rat has produced the fancy rat (rats kept as pets), as well as the laboratory rat (rats used as in biological research). Both fancy rats and laboratory rats are of the Domestication subspecies Rattus norvegicus domestica. Studies of wild rats in New York City have shown that populations living in different neighborhoods can evolve distinct genomic profiles over time, by slowly accruing different traits.
By the early to the middle part of the 19th century, British academics believed 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.Friends' Intelligencer. (1858) Volume 14. William W. Moore, publisher. pp. 398. As early as 1850, however, a new hypothesis of the rat's origins was beginning to develop.Chambers, William and Robert Chambers. (1850) Chambers's Edinburgh Journal. pp. 132. The British novelist Charles Dickens acknowledged this in his weekly journal, All the Year Round, writing:
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.Dickens, Charles. (1888) All the Year Round. New Series. Volume XLII, Number 1018. pp. 517.
Academics began to prefer this 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.Miles, Alfred Henry. (1895) Natural History. Dodd, Mead & Company. pp. 227
Though the assumptions surrounding this species' origins were not yet the same as modern ones, by the 20th century, it was believed among naturalists that the brown rat did not originate in Norway, rather the species came from central Asia and (likely) China.Cornish, Charles John. (1908) The Standard Library of Natural History. The University Society, Inc. Volume 1, Chapter 9. pp. 159
The brown rat has acute hearing, is sensitive to ultrasound, and possesses a very highly developed olfactory sense. Its average heart rate is 300 to 400 beats per minute, with a respiratory rate of around 100 per minute. The Visual acuity of a pigmented rat is poor, around 20/600, while a non-pigmented with no melanin in its eyes has both around 20/1200 vision and a terrible scattering of light within its vision. It is a dichromacy which perceives colours rather like a human with red-green colorblindness, and its colour saturation may be quite faint. The blue perception, however, also has UV receptors, allowing it to see ultraviolet lights that humans and some other species cannot.
Rats may also emit short, high frequency, ultrasonic, socially induced vocalization during rough and tumble play, before receiving morphine, or mating, and when tickled. The vocalization, 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. are often used by pet owners for this purpose.
In research 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, the tendency to chirp appears to decline.
Brown rats also produce communicative noises capable of being heard by humans. The most commonly heard in domestic rats is bruxing, or teeth-grinding, which is most usually triggered by happiness, but can also be 'self-comforting' in stressful situations, such as a visit to the vet. The noise is best described as either a quick clicking or 'burring' sound, varying from animal to animal. Vigorous bruxing can be accompanied by boggling, where the eyes of the rat rapidly bulge and retract due to movement of the lower jaw muscles behind the eye socket.
In addition, they commonly squeak along a range of tones from high, abrupt pain squeaks to soft, persistent 'singing' sounds during confrontations.
Also preyed upon by brown rats are chicks, mice and small lizards. Examination of a wild brown rat stomachs in Germany revealed 4,000 food items, most of which were plants, although studies have shown that brown rats prefer meat when given the option. In metropolitan areas, they survive mainly on discarded human food and anything else that can be eaten without negative consequences.
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.
The female is capable of becoming pregnant immediately after giving birth, and can nurse one litter while pregnant with another. She is able to produce and raise two healthy litters of normal size and weight without significantly changing her own food intake. However, when food is restricted, she can extend pregnancy by over two weeks, and give birth to litters of normal number and weight.
Dominant males have higher mating success and also provide females with more ejaculate, and females are more likely to use the sperm of dominant males for fertilization.
In mating, female rats show a clear mating preference for unknown males versus males that they have already mated with (also known as the Coolidge effect), and will often resume copulatory behavior when introduced to a novel sexual partner.
Females also prefer to mate with males who have not experienced social stress during adolescence, and can determine which males were stressed even without any observed difference in sexual performance of males experiencing stress during adolescence and not.
Rats, like most mammals, also form family groups of a mother and her young.Barnett, S. (1975). The Rat: a study in behavior (pp. 52–115). Chicago, MI: The University of Chicago Press. This applies to both groups of males and females. However, rats are territorial animals, meaning that they usually act aggressively towards or scared of strange rats. Rats will fluff up their hair, hiss, squeal, and move their tails around when defending their territory. Rats will chase each other, groom each other, sleep in group nests, wrestle with each other, have dominance squabbles, communicate, and play in various other ways with each other. Huddling is an additional important part of rat socialization. Huddling, an extreme form of herding and like chattering or "bruxing" is often used to communicate that they are feeling threatened and not to come near. The common rat has been more successful at inhabiting and building communities on 6 continents and are the only species to have occupied more land than humans.
During the wintry months, rats will huddle into piles – usually cheek-to-cheek – to control humidity and keep the air warm as a heat-conserving function. Just like elderly rats are commonly groomed and nursed by their companions, nestling rats especially depend on heat from their mother, since they cannot regulate their own temperature. Other forms of interaction include: crawling under, which is literally the act of crawling underneath one another (this is common when the rat is feeling ill and helps them breathe); walking over to find a space next to their closest friend, also explained in the name; allo-grooming, so-called to distinguish it from self-grooming; and nosing, where a rat gently pushes with its nose at another rat near the neck.
Burrows provide rats with shelter and food storage, as well as safe, thermo-regulated 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.
Today's laboratory rats exhibit physical and behavioral adaptations. Rats bred for laboratory use develop smaller testes and have smaller neocortexes. They struggle significantly more with digging and swimming than wild rats when placed under identical conditions.17 This latter difference results in an aversion in domesticated rats that allows researchers to test memory using the Morris water maze.19 Rats will recall an unseen platform's location in a pool and swim to it to avoid swimming. Other differences contribute to testing in more direct fashions; domesticated rats are able to learn faster and have lower resting levels of stress hormones.19 On the other hand, laboratory rats' increased agonistic behavior is less beneficial and not intentionally selected for.19 A mixture of artificial selection and random variation through genetic drift is likely responsible for these differences.
Unique adaptations have been observed in Albinism strains of laboratory rats. Albinos have smaller Visual cortex and are less active during the day than their pigmented counterparts. Both of these adaptations are believed to be connected to their diminished visual acuity.
Differences between laboratory rats and wild populations have led to increasing concerns over the representativeness of psychological studies using inbred strains. Some researchers point to the effects of selective breeding on fear response and brain size as warping the results' applicability to human fear mechanisms.
Particular attention has been given to the adaptations found in the New York City population. New York rats have longer noses and shorter molar rows than the Chinese population; these are hypothesized to be adaptations to a colder climate and a diet including human food, respectively. Genetic markers also show differences in regions associated with the metabolism and diet of New York rats. However, metabolic differences have also been connected to migrating populations prior to relocation in urban settlements. Immune response changes from this period are suspected to have enabled the eventual domestication of brown rats in Europe.
Population bottlenecks are a significant source of adaptation. Such a bottleneck is theorized to have occurred 20,000 years ago in the ancestral Chinese population, and similar reductions due to founder effects have been observed as invasive populations spread to new areas. A notable recent instance of bottleneck-induced adaptation is the rise of rodenticide resistance among wild brown rats. Resistance to warfarin was discovered in urban populations in the mid-20th century, prompting the synthesis of new forms of rodenticide. Some populations in the United Kingdom have also been found to resist the second-generation rodenticides developed. Behavioral adaptations have also made effective rodenticide more difficult to provide; fear of new stimuli in wild populations has been linked to the widespread presence of rodenticide. This fear is markedly reduced in domesticated populations.
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.Freye, H.A., and Thenius, E. (1968) Die Nagetiere. Grzimeks Tierleben. (B. Grzimek, ed.) Volume 11. Kindler, Zurich. pp. 204–211. 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.Suckow et al. (2006) The Laboratory Rat, 2nd ed. Academic Press. pp. 74. . Reliable reports dating to the 18th century document the presence of the brown rat in Ireland in 1722, England in 1730, France in 1735, Germany in 1750, and Spain in 1800, becoming widespread during the Industrial Revolution. (1999). 9780856611308, Academic Press. ISBN 9780856611308 It did not reach North America until around 1750–1755.Nowak, Robert M. (1999) Walker's Mammals of the World. JHU Press. pp. 1521. .
As it spread from Asia, the brown rat generally displaced the black rat in areas where humans lived. In addition to being larger and more aggressive, the change from wooden structures and thatched roofs to bricked and tiled buildings favored the burrowing brown rats over the arboreal black rats. In addition, brown rats eat a wider variety of foods, and are more resistant to weather extremes.Rowland, Ta . "Ancient Origins of Pet Rats" , Santa Barbara Independent, 4 December 2009.
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 of their nest if a suitable concentrated food supply is available, but they will range more widely where food availability is lower. It is difficult to determine the extent of their home range because they do not utilize a whole area but rather use regular runways to get from one location to another. There is great debate over the size of the population of rats in New York City, with estimates from almost 100 million rats to as few as 250,000. Experts suggest that New York is a particularly attractive place for rats because of its aging infrastructure and high poverty rates. In 2023, the city appointed Kathleen Corradi as the first Rat Czar, a position created to address the city's rat population. The position focuses on instituting policies measures to curb the population such as garbage regulation and additional rat trapping. 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. With the increase in global temperature and glacier retreat, it is estimated that brown rat populations will see an increase.
In tropical and desert regions, brown rat occurrence tends to be limited to human-modified habitats. Contiguous rat-free areas in the world include the continent of Antarctica, the Arctic, some isolated islands, the Canada province of Alberta, and certain conservation areas in New Zealand.Perrow, Martin and A. J. Davy. (2002) Handbook of Ecological Restoration. Cambridge University Press. pp. 362–363. . Most of Australia apart from the eastern and south-eastern coastal areas does not have reports of substantial rat occurrences.
Antarctica is 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, although they have arrived in many coastal areas by ship. When the occasional rat infestation is found 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 the geographic distance from other rat populations.
Today the brown rat is found on seven of the eighteen Faroese islands, and is common in and around human habitations as well as in the wild. Although the brown rat is now common on all of the largest Faroese islands, only sparse information on the population is available in the literature. An investigation for infection with the spirochaete Leptospira interrogans did not find any infected animals, suggesting that Leptospira prevalence rates on the Faroe Islands may be among the lowest recorded worldwide.
The only Rattus species that is capable of surviving the climate of Alberta is the brown rat, which can only survive in the prairie region of the province, and even then must overwinter in buildings. Although it is a major agricultural area, Alberta is far from any seaport and only a portion of its eastern boundary with Saskatchewan provides a favorable entry route for rats. Brown rats cannot survive in the wild boreal forest to the north, the Rocky Mountains to the west, nor can they safely cross the semiarid 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, poisoning, and poison gas rats, and bulldozing or burning down some 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 rat control program, of arsenic trioxide were spread throughout 8,000 buildings on farms along the Saskatchewan border. However, in 1953 the much safer and more effective rodenticide warfarin was introduced to replace arsenic. Warfarin is an anticoagulant that was approved as a drug for human use in 1954 and is much safer to use near humans and other large animals than arsenic. By 1960, the number of rat infestations in Alberta had dropped to below 200 per year. In 2002, the province finally recorded its first year with zero rat infestations, and from 2002 to 2007 there were only two infestations found. After an infestation of rats in the Medicine Hat landfill was found in 2012, the province's rat-free status was questioned, but provincial government rat control specialists brought in excavating machinery, dug out, shot, and poisoned 147 rats in the landfill, and no live rats were found thereafter. In 2013, the number of rat infestations in Alberta dropped to zero again. Alberta defines an infestation as two or more rats found at the same location, since a single rat cannot reproduce. About a dozen single rats enter Alberta in an average year and are killed by provincial rat control specialists before they can reproduce.
Only zoos, universities, and research institutes are allowed to keep caged rats in Alberta, and possession of unlicensed rats, including by anyone else is punishable by a penalty of up to C$5,000 or up to 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. The Saskatchewan rat control program has considerably reduced the number of rats trying to enter Alberta.
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 this species is often associated with outbreaks of trichinosis,Samuel et al. (2001) Parasitic Diseases of Wild Mammals. Blackwell Publishing. pp. 380–393. . 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.
They can also be responsible for transmitting Angiostrongylus larvae to humans by eating raw or undercooked snails, slugs, molluscs, crustaceans, water and/or vegetables contaminated with them.
Brown rats are sometimes mistakenly thought to be a major reservoir of bubonic plague, a possible cause of the Black Death. However, the bacterium responsible, Yersinia pestis, is commonly endemic in only a few rodent species and is usually transmitted zoonosis by —common carrier rodents today include Spermophilus and . However, brown rats may suffer from plague, as can many nonrodent species, including dogs, cats, and humans. During investigations of the plague epidemic in San Francisco in 1907, >1% of collected rats were infected with Y. pestis. The original carrier for the plague-infected fleas thought to be responsible for the Black Death was the black rat, and it has been hypothesized that the displacement of black rats by brown rats led to the decline of bubonic plague.John M. Last. "Black Death", Encyclopedia of Public Health, eNotes website. Retrieved 31 December 2010. *Ethne Barnes. Diseases and Human Evolution, University of New Mexico Press, 2007, , p. 247. This theory has, however, been deprecated, as the dates of these displacements do not match the increases and decreases in plague outbreaks.
During the COVID-19 pandemic, one study of New York City sewer rats showed that 17 percent of the city's brown rat population had become infected with SARS-CoV-2.
The many different types of domesticated brown rats include variations in coat patterns, as well as the style of the coat, such as Hairless or Rex, and more recently developed variations in body size and structure, including dwarf and tailless fancy rats.
Rattus norvegicus genome and use as model animal
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