The Brown Rat or Norway Rat (Rattus norvegicus) is one of the most well-known and common rats, and also one of the largest. Thought to have originated in northern China, this rodent has now spread to all continents and is the dominant rat in Europe and much of North America. It is a common pest wherever humans live, particularly in urban areas and degraded environments. It is also an important model organism in biological research, as well as a common pet. It has been given a number of different names, including Norwegian Rat, Wharf Rat and Common Rat.
The Brown Rat is a true omnivore and will consume almost anything, but with grain forming a substantial part of the diet. They typically eat up to a third of their bodyweight a day. Brown Rats are known to catch small fish, other rodents, and also eat nestling birds and eggs. 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.
The fur is coarse and usually brown or dark grey, the underparts are lighter grey or brown. The length can be up to 40 cm, although 25 cm is more common, with the tail a further 15 cm or so (less than half the body length). Adult body weight averages 320 g in males and about 200 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 mis-identifications of other rodents such as the Coypu and Muskrat. Brown Rats have acute hearing and are sensitive to ultrasound, and also a very highly developed olfactory sense. Their average heart rate is 300-400 bpm, with a respiratory rate of around 100 per minute. Their vision is poor and they are unable to detect colour and are blind to long-wave light.
The Brown Rat can breed throughout the year if conditions are suitable, a female producing up to eight litters a year. The gestation period is only 21-23 days and litters can number up to fourteen, although seven is common. The lifespan is up to three years, although most barely manage one—a mortality rate of 95% is estimated: predators, intraspecific conflict, and cannibalism are 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 wherever 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 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.
The only way to truly combat the rat problem is reduce the food supply, i.e., garbage left out on the street.
Brown Rats carry a number of diseases, including Weil's disease, cryptosporidiosis, haemorrhagic fever (often HFRS), Q fever and hantavirus pulmonary syndrome. They are one of the most significant mammal pests. Unlike the Black Rat, Brown Rats rarely if ever carry bubonic plague.
The Brown Rat was given the binomial name Rattus norvegicus in 1769, at which time it was still thought to have originated in Norway.
Brown rats in science
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. Generally, these lines are not transgenic, however, 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, but 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.
Brown rats as pets
Brown Rats make excellent pets and are the focus of attention at shows of fancy rats. Brown Rats are clean, social and intelligent pets.
- Nature: Rat Genome (http://www.nature.com/nature/focus/ratgenome/)
- Rat Genome Database (http://rgd.mcw.edu/sequences/rgp_info.shtml)
Rattus norvegicus is also the title of an album by the Stranglers.