THE Common Rat  Rattus norvegicus


Large size: pointed muzzle: Long scaly tail
Head/body length: about 280mm, tail 80 to 100 of body
Weight: average approx 500grammes

The fur is coarse and usually brown or dark grey, while the underparts are lighter grey or brown. The Brown Rat is a rather large true murid and can weigh twice as much as a Black Rat and many times more than a House Mouse. The length is commonly in the range of 20 to 25 cm (8 to 10 in), with the tail a further 18 to 25 cm (7 to 10 in), thus being roughly the same length as the body. Adult body weight averages 350 g (12 oz) in males and about 250 g (9 oz) in females. Exceptionally large individuals can reportedly reach 900 to 1,000 g (32 to 35 oz) but are not expected outside of domestic specimens. Stories of rats attaining sizes as big as cats are exaggerations, or misidentifications of other rodents, such as the coypu and muskrat. In fact it is common for breeding wild Brown Rats to weigh (sometimes considerably) less than 300 g (11 oz).

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 which perceive colors 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.

General Ecology:

Common rats are mainly nocturnal and rarely seen during the day. The have a nest usually below ground but nests can be amongst sacks or straw bales. Common rates are opportunist feeders and prefer starch rich or protein rich foods but they will eat almost anything that is left lying around Common Rates live in very dense populations which compose of mostly young individuals. The females in these dense populations are pregnant throughout the year with litter sizes of between 6 and 11 babies. Common Rats are widespread in all Parts of the United Kingdom where they  spread from eastern USSR into the British Isles during the middle of the18th century where they mostly replaced the Black Rat (Rattus rattus) Mortality is high among young rats who are fair game to most predatory animals. Larger rats may deter weasels and owls due to their aggressive behaviour. The maximum life expectancy of a Common Rat is about 1 year though very few survive so long. Common rats in terms of success are in a league of there own due to the fact that they have developed the ability to live in such close association with man.

Reproduction and life cycle:
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.
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.

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 depends 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 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. 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. Rat chirp also can be used for mosquito control. Other ultrasonic vocalizations, including a lower-frequency 'boom' or 'whoom' noise can be produced by bucks in a calm state, when grooming or settling down to sleep.

Audible communication:
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.

Social behavior:
Rats commonly 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, 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. 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. 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. 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 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.

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 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.

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 are,including being widespread throughout Thailand.

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 Ireland in 1722, 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.

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.[48]

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. 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 New York is a particularly attractive place for rats because of its aging infrastructure, high moisture, and high 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 the rat population has been rising, with estimations that 81 million rats reside in the UK.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, 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.


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, 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.
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,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 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 non rodent species, including dogs, cats, and humans. 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. This theory has, however, been deprecated, as the dates of these displacements do not match the increases and decreases in plague outbreaks.

In captivity:

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.

As pets:
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.


Common Rats are offered no protected by any  laws and are listed on the Pest Act of 1949 which requires notification of local authorities of substantial numbers And due to the contamination of foodstuffs have become a major problem throughout the world. They are also a vector of some human diseases particularly leptospirosis among rice workers in Spain and Italy. 

On the credit side has served man for decades as a laboratorial animal


Further Information: