Brown Rat

The brown rat, common rat, Norway rat, Norwegian rat or wharf rat (Rattus norvegicus) is one of the best-known and common rats, and also one of the largest. It is not known for certain why it is named Rattus norvegicus (Norwegian rat) as it did not originate in Norway, but John Berkenhout, the author of the 1769 book "Outlines of the Natural History of Great Britain", is most likely responsible for the misnomer. Berkenhout gave the brown rat the binomial name Rattus norvegicus believing that the rat had migrated to England from Norwegian ships in 1728, although no brown rat had entered Norway at that time, instead coming from Denmark. 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. It 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.

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Description

The fur is coarse and usually brown or dark grey, the underparts are lighter grey or brown. The length can be up to 25 cm (10 in.), with the tail a further 25 cm (the same as the body length). Adult body weight averages 350 g in males and about 250 g in females, but a very large individual can reach 500 g. Rats weighing over a kilogram 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 and are sensitive to ultrasound, and also possess a very highly developed olfactory sense. Their average heart rate is 300 to 400 beats per minute, with a respiratory rate of around 100 per minute. Their vision is poor, around 20/600 for normal rats. They are dichromates who perceive colours rather like a human with red-green colorblindness, and their colour saturation may be quite faint.^[2]

Food, habitat and behavior

The brown rat is a true omnivore and will consume almost anything, but with cereals forming a substantial part of the diet. Martin Schein, founder of the Animal Behavior Society in 1964, studied the diet of brown rats and came to the conclusion in his paper "A Preliminary Analysis of Garbage as Food for the Norway Rat" that the most-liked foods of brown rats were (in order) scrambled eggs, macaroni and cheese, and cooked corn kernels. Their least-liked foods were raw beets, peaches, and raw celery. They are usually active at night and are good swimmers, both on the surface and underwater, but (unlike the related Black rat Rattus rattus) are poor climbers. They dig well, and often excavate extensive burrow systems. A 2007 study found rats to possess metacognition, a mental ability previously only found in humans and some primates.^[3]

Breeding

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 interspecific conflict as major causes. 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.

Rats live almost everywhere people live. It is often said that there are as many rats in cities as people, but this varies from area to area depending on climate, etc. It is probable that New York City (with a severe winter climate), for instance, has only 250,000 rats, not eight million. However, the UK official National Rodent Survey^{[citation needed]} found a 2003 UK population of 60 million brown rats, about equal to the UK human population; winters in Britain are much warmer, making rat survival higher. Brown rats in cities tend not to wander extensively, often staying within 20 meters (65 ft) of their nest if a suitable concentrated food supply is available, but they will range more widely where food availability is lower.

Rat-free zones

The only rat-free zones in the world are the Arctic, the Antarctic, some especially isolated islands, the province of Alberta in Canada^[4] , and certain conservation areas in New Zealand^{[citation needed]}.

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.

Alberta is unusual in that rat infestation was eliminated by aggressive government action.^[5] 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 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 of arsenic trioxide was spread in 8,000 buildings (8 kg/building) on 2,700 farms along the Saskatchewan border. In 1953 the much less toxic and more effective poison Warfarin was introduced, and since then the control program has consumed between 5 and 13 tonnes of Warfarin annually. By 1960 the number of rat infestations in Alberta had dropped below 200 per year and has remained low ever since.^[6] Currently, only zoos, universities, and research institutes are allowed to own caged rats, 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.^[7]

Rats pose 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 rats in the world.^[8]

Diseases

Brown rats carry some diseases, including Weil's disease, cryptosporidiosis, Viral hemorrhagic fever (VHF), Q fever and hantavirus pulmonary syndrome. They can also act as reservoirs of bubonic plague.

Brown rats in science

In 1895, Clark University in Worcester, Massachusetts (United States) established a population of domestic white brown rats to study the effects of diet and for other physiological studies. Over the years, rats have been used in many experimental studies, which have added to our understanding of genetics, diseases, the effects of drugs, and other topics that have provided a great benefit for the health and wellbeing of humankind. Laboratory rats have also proved valuable in psychological studies of learning and other mental processes (Barnett 2002).

Domestic rats differ from wild rats in many ways. They are calmer and less likely to bite; they can tolerate greater crowding; they breed earlier and produce more offspring; and their brains, livers, kidneys, adrenal glands, and hearts are smaller (Barnett 2002).

Selective breeding of the brown rat 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.

Scientists have bred many strains or "lines" of rats specifically for experimentation. Most are derived from the albino Wistar rat, which is still widely used. Other popular strains are the Sprague Dawley, Fischer 344^[9] and Holtzman albino strains, and the Long-Evans, and (in the UK) Lister black hooded rats. Inbred strains are also available but are not as commonly used as inbred mice. Generally rat lines are not transgenic because the easy techniques of genetic transformation that work in mice do not work for rats. This has disadvantaged many investigators, who regard many aspects of behavior and physiology in rats as more relevant to humans and easier to observe than in mice and who wish to trace their observations to underlying genes. As a result, many have been forced to study questions in mice that might be better pursued in rats. In October 2003, however, researchers succeeded in cloning two laboratory rats by the problematic technique of nuclear transfer. So rats may begin to see more use as genetic research subjects. Much of the genome of Rattus norvegicus has been sequenced.^[10]

Wistar rat

Wistar rats are an outbred strain of albino rats belonging to the species Rattus norvegicus. This strain was developed at the Wistar Institute for use in biological and medical research, and is notably the first rat strain developed to serve as a model organism at a time when laboratories primarily used Mus musculus, or the common House mouse. Most laboratory rat strains are descended from a colony of rats established at the Wistar Institute in 1906 by American physiologist Henry Donaldson, scientific administrator Milton J. Greenman, and genetic researcher/embryologist Helen Dean King. This is currently one of the most popular rat strains used for laboratory research. It is characterized by its wide head, long ears, and having a tail length that is always less than its body length. The Sprague Dawley and Long-Evans rat strains were developed from Wistar rats.

Sprague Dawley rat

Sprague Dawley rats are an outbred strain of albino laboratory rats belonging to the species Rattus norvegicus. They were originally developed from the Wistar rat strain. They are used widely for experimental purposes because of their calmness and ease of handling. The adult body weight is 250-300g (female), and 450-520g (male). The typical life span is 2.5 - 3.5 years.^[11] Sprague Dawley is the registered trademark of Harlan Industries Inc.^[11]

Brown rats as pets

Main article: Fancy rat

The brown rat, along with the black rat to a lesser degree, is kept as a pet in many parts of the world. Australia, England, 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.

Brown rats as pet food

Because of their quick reproduction, rats are also used as live food for captive animals, commonly large reptiles such as snakes. Care must be taken to select rats from breeders who provide a good quality diet and captive living conditions.

There is a large risk to captive animals if they are fed live rats instead of dead rats. A captive animal that is not hungry when it is given a live rat as food may itself become food for the rat. An animal that does not kill the rat quickly enough will often suffer injury, e.g., from being bitten by the rat. Even feedings supervised by the owner of the captive animal can result in an injured or dead animal, as rats are faster than humans and many other animals.^[12]

Objections to the practice of feeding live rats to reptiles include viewing it as cruelty to animals because the rat is not guaranteed a quick or painless death, and equating it to rat baiting or cockfighting, which are illegal in most parts of the world. Reptiles can be conditioned to accept dead rats with coaching, as is the rule with most accredited 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.^[13]

References

Rattus norvegicus genome & use as model animal

• Nature: Rat Genome
• Rat Genome Database
• Bonnie Tocher Clause, The Wistar Institute Archives: Rats (Not Mice) and History, Mendel Newsletter, Feb. 1998; also see Bonnie Tocher Clause, "The Wistar Rat as a Right Choice: Establishing Mammalian Standards and the Ideal of a Standardized Mammal," Journal of the History of Biology, vol. 26, no. 2 (Summer 1993), pp. 329-349.

Details

• detailed reproductive data by cavyrescue.co.uk, a UK charity for small animals
• Waarneming.nl Pictures, sightings and distribution maps of brown rats in the Netherlands.
• The Laboratory Rat: A Natural History Downloadable short film (total time 27 min) following a colony of Wistar and hooded rats which were released into a fenced lot to see if lab rats still retain enough instincts to survive in the wild.