The brown rat, common rat, street 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 by at least this particular definition the most "successful" mammal on the planet after humans.^[2] Indeed, with rare exceptions, the brown rat lives wherever humans live, particularly in urban areas.

Selective breeding of Rattus norvegicus has produced the laboratory rat, an important model organism in biological research, as well as pet rats.

Naming and etymology [edit]

Originally called the "Hanover rat" by people wishing to link problems in 18th century England with the House of Hanover,^[3] 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 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.^[4] As early as 1850, however,a more correct understanding of the rat's origins was beginning to develop.^[5] The British novelist Charles Dickens acknowledged the misnomer in 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."^[6]

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. it is believed that the Brown Rat is so called due to the resemblance its fur holds to rat faeces or alternatively, sloppy rat diarrhoea."^[7]

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 the species came from central Asia and (likely) China.^[8] Despite this, this species' common name of "Norway rat" is still in use today.

Description [edit]

Comparison of the physique of a black rat (Rattus rattus) with a brown rat (Rattus norvegicus)

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 550 g (19 oz) in males and about 350 g (12 oz) in females, but a very large individual can reach 900 g (32 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 nonpigmented (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 which 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 receptors, allowing them to see ultraviolet lights that some species cannot.^[9]

Biology and behavior [edit]

Brown rat skull

The brown rat is nocturnal and is a good swimmer, both on the surface and underwater, but unlike the related black rat (Rattus rattus), it is a poor climber. 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,^[10] but further analysis suggested they may have been following simple operant conditioning principles.^[11]

Communication [edit]

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,^[12] as well as to regulate their mother's movements in the nest.^[13] 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.^[14] Adult rats will emit ultrasonic vocalizations in response to predators or perceived danger;^[15] the frequency and duration of such cries depends on the sex and reproductive status of the rat.^[16][17] The female rat will also emit ultrasonic vocalizations during mating.^[18]

Chirping [edit]

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.^[19] Like most rat vocalizations, the chirping is too high in pitch for humans to hear without special equipment. Bat detectors are often used by pet owners for this purpose.

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, the tendency to chirp appears to decline.^[20]

Rat chirp also can be used for mosquito control.^{[citation needed]}

Other ultrasonic vocalisations, including a lower-frequency 'boom' or 'whoom' noise can be produced by bucks in a calm state, when grooming or settling down to sleep.^{[citation needed]}

Audible communication [edit]

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.

In addition, they commonly squeak along a range of tones from high, abrupt pain squeaks to soft, persistent 'singing' sounds during confrontations.

Diet [edit]

The brown rat is a true omnivore and will consume almost anything, but cereals form a substantial part of its diet.

Brown rat eating sunflower seeds

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 include scrambled eggs, macaroni and cheese, and cooked corn kernels. According to Schein, the least-liked foods were raw beets, peaches, and raw celery.^[21]

Foraging behavior is often population-specific, and varies by environment and food source.^[2] Brown rats living near a hatchery in West Virginia catch fingerling fish.^[22] Some colonies along the banks of the Po River in Italy will dive for mollusks,^[23][24] a practice demonstrating social learning among members of this species.^[25] Rats on the island of Norderoog in the North Sea stalk and kill sparrows and ducks.^[26]

Reproduction and life cycle [edit]

The brown rat can breed throughout the year if conditions are suitable, with a female producing up to five litters a year. The gestation period is only 21 days, and litters can number up to 14, although seven is common. They reach sexual maturity in about five weeks. 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.^[27]

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.^{[citation needed]}

Social behavior [edit]

Rats commonly groom each other and sleep together.^[28] 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, so one rat will be dominant over another one.^[29] 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.^[30] If living space become limited, rats may turn to aggressive behavior, which may result in the death of some animals, reducing the burden over the living space.

Rats like most mammals also form family groups, a mother and her young.^[31] This applies to both groups of males and females. However, rats are territorial animals, meaning that they usually act aggressively or scared of strange rats. Rats will fluff up their hair, hiss, squeal, and move their tails around when defending their territory.^[32] 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.^[32] Huddling is an additional important part of rat socialization. Huddling is often supposed to have a heat-conserving function. Nestling rats especially depend on heat from their mother, since they cannot regulate their own temperature. Huddling is an extreme form of herding. Other forms of interaction include, crawling under, which is literally the act of crawling underneath one another,walking over, also explained in the name, then there is allo-grooming, so-called to distinguish it from self-grooming. And lastly there is another type of contact called nosing, where a rat gently pushes with its nose at another rat near the neck.^[31]

Burrowing [edit]

Rats are known to burrow extensively, both in the wild and in captivity, if given access to a suitable substrate.^[33] 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.^[34] Burrows usually develop to eventually include multiple levels of tunnels, as well as a secondary entrance.^[33] Older male rats will generally not burrow, while young males and females will burrow vigorously.^[33][35]

Burrows provide rats with shelter and food storage, as well as safe, thermoregulated nest sites.^[33] 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.^[36] Burrowing can therefore be described as a "pre-encounter defensive behavior", as opposed to a "postencounter defensive behavior", such as flight, freezing, or avoidance of a threatening stimulus.

Distribution and habitat [edit]

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.^[37][38][39] 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 are. .^[40]

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.^[41] 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.^[42] 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,^[42] becoming widespread during the Industrial Revolution.^[43] It did not reach North America until around 1750–1755.^[41][44]

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.^[45]

In the absence of humans, brown rats prefer damp environments, such as river banks.^[43] However, the great majority are now linked to man-made environments, such as sewage systems.

Brown rat in a flower box in the East Village of New York City

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.^[46] Experts suggest New York is a particularly attractive place for rats because of its aging infrastructure, high moisture and poverty rates.^[46] 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.^[47]

In the United Kingdom, some figures show the rat population has been rising, with estimations that 81 million rats reside in the UK.^[48] Those figures would mean 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, such as Iceland, the province of Alberta in Canada,^[49] and certain conservation areas in New Zealand.^[50][51]

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 reinfest 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.

Alaska [edit]

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 program was started in 2007 and the island was declared rat free in June 2009.

Alberta, Canada [edit]

Alberta, Canada, is unusual in that rat infestation was eliminated by aggressive government action. Although it is a major agricultural area, 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 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 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.^[52]

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.^[53]

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.^[54]

New Zealand [edit]

First arriving before 1800 (perhaps on James Cook's vessels),^[55] 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.^[56]

Diseases [edit]

Similar to other rodents, brown rats may carry a number of pathogens,^[57] which can result in disease, including Weil's disease, rat bite fever, cryptosporidiosis, viral hemorrhagic fever, Q fever and hantavirus pulmonary syndrome. In the United Kingdom, brown rats are an important reservoir for Coxiella burnetii, the bacterium that causes Q fever, with seroprevalence for the bacteria found to be as high as 53% in some wild populations.^[58]

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.^[59] 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.^[60]

Surveys and specimens of brown rat populations throughout the world have shown this species is often associated with outbreaks of trichinosis,^[61][62] 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.^[63] 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.^[64]

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 zoonotically by rat fleas—common carrier rodents today include ground squirrels and wood rats. However, brown rats may suffer from plague, as can many nonrodent species, including dogs, cats, and humans.^[65] 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.^[66] This theory has, however, been deprecated, as the dates of these displacements do not match the increases and decreases in plague outbreaks.^[67]

Control [edit]

Some of the common methods used to control the number of Brown rats include:

Trapping [edit]

Using traditional break-back traps, glue traps, live cage traps and other humane traps

Poisoning [edit]

There are many types of poison available for the purpose of controlling the Brown rat. The use of poison is controlled in most of the countries in the world so it is important to check state legislations.

Proofing [edit]

Prevention is the best cure. There are ways to stop the infestation occurring at the first place, such as blocking access points, better waste management, better sewage design.

In captivity [edit]

Uses in science [edit]

Selective breeding of albino brown rats rescued from being killed in a now-outlawed sport called rat baiting has produced the albino laboratory rat.^[68] 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.

As pets [edit]

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.

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..

References [edit]

The fancy rat is a domesticated brown rat (Rattus norvegicus), which is the most common type of pet rat.^[1] The name fancy rat derives from the idea of animal fancy or the phrase, "to fancy" (to like, or appreciate).^[2]

Fancy rats have their origins as the targets for blood sport in 18th and 19th century Europe. Specially bred as pets since then, fancy rats now come in a wide variety of colours and coat types and there exists several rat fancy groups worldwide. Fancy rats are commonly sold as pets in stores and by breeders. In fiction, pet brown rats tend to be depicted as tamed rather than domesticated, akin to when a character befriends a wolf. As tamed pets, they have played roles that vary from evil, to ambiguous, to lovable.^[3]

Domesticated rats are physiologically and psychologically different from their wild relatives, and—when acquired from reliable sources—they pose no more of a health risk than other common pets.^[4] For example, domesticated brown rats are not considered a plague threat^[5], while exposure to wild rat populations could introduce diseases like Salmonella into the home.^[6] While fancy rats are subject to different health risks than their wild counterparts, they are consequently less likely to succumb to other illnesses prevalent in the wild.

History

Jack Black made his living not only from catching rats, but also from turning around and selling them for use in baiting.

The origin of the modern fancy rat begins with the rat-catchers of the 18th and 19th centuries who trapped rats throughout Europe.^[1] These rat-catchers would then either kill the rats, or—more likely—sell the rats to be the victims in bloodsport.^[7] Rat-baiting was a popular sport until the beginning of the 20th century, it involved filling a pit with several rats and then placing bets on how long it would take a terrier to kill them all. It is believed that both rat-catchers and sportsmen began to keep certain, odd-coloured rats during the height of the sport—eventually breeding them, and then selling them as pets.^[1][8] The two men thought to have formed the basis of rat fancy are Jack Black, rat-catcher to Queen Victoria, and Jimmy Shaw, manager of one of the largest sporting public houses in London. These two men are responsible for beginning many of the colour varieties present today.^[1][9]

The bloody sport of rat-baiting is an unlikely antecedent for the practice of keeping rats as pets.

Rat fancy as a formal, organized hobby began when a woman named Mary Douglas asked for permission to bring her pet rats to an exhibition of the National Mouse Club at the Aylesbury Town Show in England on October 24, 1901. Her black and white hooded rat won "Best in Show," and ignited interest in the area. After Douglas's death in 1921, rat fancy soon began to fall back out of fashion. The original hobby formally lasted from 1912 to 1929 or 1931, as part of the National Mouse and Rat Club, at which point Rat was dropped from the name returning it to the original National Mouse Club. The hobby was revived in 1976 with the formation of the English National Fancy Rat Society (NFRS).^[1][9] Pet rats are now commonly available in stores and from breeders, and there exist several rat fancy groups worldwide.

Differences from wild rats

While domesticated rats are not so far removed from their wild counterparts as to justify a distinct subspecies (compare Canis lupus familiaris and Capra aegagrus hircus), there are several significant differences that set them apart, the most apparent of which is colouring. While random colour mutations may occur in the wild, these are rare—most wild R. norvegicus are a dark brown colour, but fancy rats may be anything from white to blue.^[10]

Behaviourally, pet rats are tamer than those in the wild.^[11] They are more comfortable around humans, have decreased reactions to light and sound, are less wary of new foods, and can better tolerate overcrowding, though some rats prefer to be alone. They are shown to mate earlier, more readily, and for a longer period of time over their lifespan.^[12] Also, domesticated rats exhibit different behaviours when fighting with each other. While wild rats almost always flee a lost battle, caged rats spend protracted amounts of time in a belly-up or boxing position.^[13] These behavioural traits are thought to be products of environment as opposed to genetics. However, it is also accepted that there are certain underlying biological reasons for why some members of a wild species are more receptive to domestication than others, and that these differences are then passed down to offspring.^[12][14]

At a physiological level, while still the same species, domesticated rats have different average statistics than wild rats. The chief difference is lifespan. Because domesticated rats are protected from predators and have ready access to food, water, shelter, and medical care, their average lifespan is around 3-5 years, in contrast to wild R. norvegicus, which average a lifespan of less than one year.^[15] However, wild rats generally have larger brains, hearts, livers, kidneys, and adrenal glands than laboratory rats.^[12] The fancy rat and wild rat also both face a multitude of differing health concerns—the former is at risk of developing a pneumococcal infection from exposure to humans, while the latter may harbour tapeworms after coming in contact with carriers like cockroaches and fleas.^[16]

Varieties

The American Blue is one of several standardized coat colours.

As in other pet species, a variety of colours, coat types, and other features that do not appear in the wild have either been developed, or have appeared spontaneously. Any individual rat may be defined one or more ways by its colour, coat, marking, and non-standard body type. This allows for very specific classifications such as a ruby-eyed cinnamon rex berkshire dumbo.^[2]

Colouring

While some pet rats retain the "agouti" colouring of the wild brown rat (three tones on the same hair), others may be black based colours (a single colour on each hair). Agouti based colours include agouti, cinnamon, and fawn. Black based colours include black, beige, and chocolate.^[17] Additionally, eye-colour is considered a subset of colouring, and coat-colour definitions often include standards for the eyes as many genes which control eye colour will also affect the coat colour. The American Fancy Rat and Mouse Association (AFRMA), a United States-based club, lists black, pink, ruby, and odd-eyed (two different types) as possible eye-colours depending on the variety of rat shown.^[18] Ruby refers to eyes which normally appear black, but are shown to be red under bright light. Colour names can vary for more vague varieties, like lilac and fawn,^[17] while the interpretations of standards can fluctuate between and even within different countries or clubs.^[18][19][20]

Markings

Himalayan rats have a unique color and marking variation.

Further dividing the varieties of fancy rats are the many different markings. Pet rats can appear in any combination of colour and marking. The markings are typically in reference to the patterns and ratios of coloured hair versus white hair. Two extremes would be a self (completely solid, non-white colour) and a Himalayan (completely white with a gradual blend of colouring toward its nose and feet—called "points").^[21]

Markings have a strict standard for showing. For example, in the case of hooded rats, the stripe or "saddle" should be a single, unbroken line that runs down the spine and possibly partly down the tail.^[20] However, many rats are not bred to the standard, such as those found in pet shops, and will have "mismarkings".

Commonly recognized standards include:

• Berkshire – coloured top, white belly
• Hooded – colour runs from full head down spine
• Capped – colour on full head only
• Variegated – a blaze, or short white strip on the forehead, with a fully coloured head and splotches or flecks of colour run down the back only
• Irish or English Irish – In England the Irish is standardized by the NFRS as an equilateral triangle of white with a side that begins at the chest, or between the front legs, and where the point ends mid length.^[20] In the United States and elsewhere, clubs like the AFRMA distinguish this marking as the English Irish and allow for another standard Irish in which the rat may have white of an even or symmetrical nature anywhere along its underside.^[18][22]

Other marking varieties include Dalmatian-like spotting, blazes, masks, Siamese (typically a gradient of colour along the body, darkest at the base of the tail), and "downunders" (an Australian variety which has a solid colour stripe on the belly or a colour marking that corresponds to those on top).^[23]

Body types

A male "Dumbo" rat, a variety with ears placed lower on the head. Named for resemblance to Dumbo the elephant.

Two of the most prominent (and thus standardized) physical changes applied to rats through selective breeding are the development of the Manx and Dumbo. The Dumbo, whose origins are in the United States, is characterized by having large, low, round ears on the sides of its head, while the Manx rat shares both its name and mutation with tailless Manx cats.^[18]

Coat types

There is a relatively small variety of coats in relation to the number of colours and markings, and not all are internationally standardized. The most common type is the Normal or "Standard," which is allowed variance in coarseness between the sexes; males have a coarse, thick, rough coat, while females' coats are softer and finer.^[18][20] Other standardized coats include: Rex, in which all the hairs are curly, even the whiskers; Velveteen, a softer variation on the Rex; Satin or Silky coats, which are extra soft and fine, with a sheen; Harley, characterized by long straight hairs.^[18][20][24] Remaining coat types are not defined by the hair itself, but rather by the lack of it.

Hairless rats

Even though this rat is hairless, the markings on its skin indicate that it is a hooded rat.

Hairless rats are a coat variety characterized by varying levels of hair loss. Hairless rats, bred from Rexes, have varying areas of bare skin, or very short fur on their bodies. Because the Rex coat is a dominant trait, it only needs one Rex-parent to affect the rat's appearance. However, when two copies of the trait appear, by breeding two Rexes together, the coat is affected differently—causing hairlessness, and earning the colloquial name, "Double-rex". Hairless varieties are produced by different combinations of the various genes that cause Rex coats. One subset of semi-hairless rats, patchwork rats, constantly lose hair and regrow it in different "patches" several times throughout their lifetimes.^[18][20]

Ethics of selective breeding

There is controversy among rat fanciers in regards to selective breeding.^[25][26] On one hand, breeding rats to conform to a specific standard or to develop a new one is a large part of what the hobby was founded on. On the other hand, the process results in many rats who do not conform and are then either given away, sold as food, or killed—referred to as culling.^[27] Additionally, there are concerns as to whether or not breeding hairless and tailless rats is ethical. The tail is vital for rats' balance and for adjusting body temperature. Tailless rats have greater risk of heat exhaustion, poor bowel and bladder control, falling from heights, and can be at risk for life-threatening deformities in the pelvic region like hind leg paralysis and megacolon.^[28] Similarly, hairless rats are less protected from scratches and the cold without their coat, and have weakened immune systems. Groups such as the NFRS prohibit the showing of these varieties at their events and forbid advertisement through affiliated services.^[29]

Accessibility

Because R. norvegicus and related species are seen as pests, their intentional import into foreign countries is often regulated. For example, the importation of foreign rodents is prohibited in Australia,^[30] and so various coat types, colours, and varieties have been bred separately from foreign lines, or are just not obtainable within that country. In other areas, like the Canadian province of Alberta, the ownership of brown rats outside of schools, laboratories, and zoos is illegal.^[31]

Health

Fancy rats of both sexes commonly develop mammary tumours as they age. These are usually benign, but multiple tumours can persist even after removal.^[32]

Human-raised R. norvegicus are more prone to specific health risks and diseases than their wild counterparts, but they are also far less likely to succumb to certain illnesses that are prevalent in the wild. The major considerations for susceptibility include exposure, living conditions, and diet.

Rats that live their entire lives indoors usually are able to avoid disease-causing bacteria such as Salmonella and Pseudomonas aeruginosa, the latter is absent in treated water. They may also more easily avoid vectors like cockroaches, beetles, and fleas who are essential for the spread of endemic typhus, and intestinal parasites like the Rat tapeworm.^[33][34] Additionally, pet or laboratory rats enjoy the natural benefits of having a consistent and well-balanced diet, along with access to medical care.

While living indoors decreases the risk of contracting certain diseases, living in close quarters with other rats, being unable to always seek proper protection from environmental factors (e.g. temperature, humidity, drafts), being fed an unhealthy diet, and the stresses naturally associated with living in an unnatural habitat can all adversely affect a rat's health to make them more prone to specific conditions.^[33][35][36] Specifically, Tyzzer's disease, protozoic infections (e.g. Giardia muris), and pseudotuberculosis are usually seen in stressed or young rats.^[34][37] Additionally, pet rats are exposed to Streptococcus pneumoniae, a zoonotic disease caught from humans, not the same bacteria associated with strep throat. Another bacteria associated with humans, Pneumocystis carinii, is actually found in almost all domesticated animals. However it is asymptomatic unless the rat's immune system is compromised by illness, then it can develop into pneumonia.^[37]

Several diseases, like Rat Coronavirus Infection (RCI), Sendai virus, and Murine Respiratory Mycoplasmosis (MRM, Mycoplasma pulmonis), are prevalent simply because their highly contagious natures work in tandem with the way rats are kept in laboratories, pet stores, and by breeders.^[34][37] It should be noted, however, that MRM is far less likely to occur in laboratory rats than in those kept as pets.^[37]

Pet rats can also develop pituitary tumors if they are given high-calorie diets,^[32] and ringtail if they are placed in areas with low humidity, high temperatures, or drafts. Staphylococcus spp. are a mostly benign group of bacteria that commonly reside on the top of the skin, but cuts and scratches from social and hierarchal fighting can open up the pathways for them to cause ulcerative dermatitis.^[33]

Risks to owners

Keeping rats as pets can come with the stigma that rats transmit dangerous diseases to their owners. One fear is that all rats carry plague, when in fact R. norvegicus is not among the list of species considered a threat.^[5] In 2004, an outbreak of salmonella in the United States was connected to people who owned pet rats,^[38] however it has been determined that a pet rat's initial exposure to salmonella, along with many other zoonotic rat-diseases, typically indicates exposure to wild rodent populations, either from an infestation in the owner's home, or from the pet's contaminated food, water, or bedding. Over all domestic pet rats pose very little or no threat to their owners.^[6]

Fiction

Samantha Martin, a professional animal trainer for films, commercials, and music videos, has claimed that rats are one of the easiest animals to train due to their adaptability, intelligence, and focus.^[39] Rather than portraying pet brown rats as thoroughly domesticated, they are often cast as a wild brown rat that a character tames. An example of this is in the series of movies based on Ratman's Notebooks: 1971's Willard, the 1972 sequel Ben, and 2003's Willard. In these movies, the protagonist befriends the rats found in his home and builds up a close relationship, only to have it end in tragedy. While these movies generally emphasize the popular perception of malevolence^[40]—they kill people, cats, and ransack grocery stores—other wild rats who become pets are portrayed in more neutral to positive ways; the television show, House, shortly featured "Steve McQueen", the pet rat of the titular character,^[41] and the 2007 film, Ratatouille, is about a rat described by Roger Ebert as "earnest... lovable, determined, [and] gifted" who lives with a Parisian garbage boy.^[3] Tame rats used in religious ceremonies play a role in Lois McMaster Bujold's novel The Curse of Chalion. Splinter, the master and adoptive father of the Teenage Mutant Ninja Turtles, was once the pet rat of ninja Hamato Yoshi and learned his martial arts skills by imitating his master.

See also

• American Fancy Rat and Mouse Association, US
• Experimental evolution
• List of fictional mice and rats
• National Fancy Rat Society, UK
• Rat agility
• Rat genome database
• Working rat

References