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Life Cycle & Description

Description of Brown Rat
(Rattus norvegicus)


The Norway rat is thought to have originated in south eastern Siberia and northern China and has only been recorded in Britain since early in the 18th century. It is thought to have been introduced in shipping from Russia or Norway, and did not originate in Norway, despite one of its common names.

It is a rat of the developed urban environment and is found worldwide, largely in developed towns and cities where refuse collects and drains and sewers have been introduced. It continues to increase its range.

It is now by far the more abundant of the two rat species in the United Kingdom and is widely distributed in both urban and rural areas. It is largely a terrestrial burrowing species, with relatively poor climbing skills compared with other species, but it can still, and does, climb when the need and opportunity arises. It lives largely outdoors but will find its way indoors when the environment allows, often via drainage defects and gaps in walls, and it is able to climb rough surfaces or "chimney" up behind pipes to gain access to buildings. If activity is found indoors it is often an extension of external activity. It is the species often associated with sewer systems, hence its alternative name, the sewer rat.

Norway rats live communally and often construct burrow systems. A typical burrow complex may have multiple points of entry and exit and numerous interconnecting tunnels and chambers. Food storing or "caching" behaviour is common.

The Norway rat lives in colonies with a strongly hierarchical social system in which high ranking individuals (male and female) enjoy privileged access to food and other resources. This allows them to forage less often and for shorter periods each night, and allows high-ranked females to breed at a younger age and with greater success. Behavioural differences related to social rank become especially pronounced within high population densities when the higher ranking individuals need to, and are able to, exert their influence on lower ranking individuals.

Average weight - 250 - 500g (Max 700 grams)

Combined head & body length - 200 - 270 mm

Tail length - 150 - 210 mm

Sexually mature - 12 - 16 weeks

Gestation period - 21 - 24 days

Average litter size - 6 - 12

Weaning period - 3 - 4 weeks

Average lifespan - 1 - 3 years

The Brown rat is considered one of the most prominent pests in the UK. In addition, it is highly dangerous to humans, due to carrying Weil's disease, plague and numerous other diseases. Hence why this animal is dispatched and controlled.

Good hygiene, proofing, preventative measures, pest management and monitoring.

Distribution and Habitat

The brown rat is an incredibly adaptable mammal and can be found almost everywhere in the UK, in any habitat, all it needs is shelter, food and a water source. Brown rats are omnivorous, eating pretty much anything, from fruit and seeds to human food waste, insects, birds' eggs or even small mammals. They are particularly common around towns and cities. Brown rats live in loose colonies and dig their own burrows. They are famously good breeders; a female brown rat can breed from around 3 months old, and has an average of five litters a year, each of up to 12 young.


The brown rat is nocturnal and is a good swimmer, both on the surface and underwater, and has been observed climbing slim round metal poles several feet in order to reach garden bird feeders. 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 other primates, but further analysis suggested they may have been following simple operant conditioning principles.

Brown rats are capable of producing ultrasonic vocalizations. As pups, young rats use different types of ultrasonic cries to elicit and direct maternal search behaviour, as well as to regulate their mother's movements in the nest. Although pups produce ultrasounds around any other rats at the age of 7 days, 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 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 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.

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 12, although nine is common. They reach sexual maturity in about five weeks. Under ideal conditions (for the rat), this means that the population of females could increase by a factor of three and a half (half a litter of 7) in 8 weeks (5 weeks for sexual maturity and 3 weeks of gestation), corresponding to a population growing by a factor of 10 in just 15 weeks. As a result, the population can grow from 2 to 15,000 in a year. The maximum life span is up to three years, although most barely manage two. 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 behaviour, 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.

Females are capable of becoming pregnant immediately after giving birth, and can nurse one litter while pregnant with another. Females are able to produce and raise two healthy litters of normal size and weight without significantly changing their own food intake. However, when food is restricted, females can extend pregnancy by over two weeks, and give birth to litters of normal number and weight.

Males can ejaculate multiple times in a row, and this increases the likelihood of pregnancy as well as decreases the number of still borns. Multiple ejaculation also means that males can mate with multiple females, and they exhibit more ejaculatory series when there are several oestrous females present. Males also copulate at shorter intervals than females. In group mating, females often switch partners.

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 behaviour 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 commonly groom each other and sleep together. 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 becomes limited, rats may turn to aggressive behaviour, which may result in the death of some animals, reducing the burden over the living space.

Rats, like most mammals, also form family groups of 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 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, often has a heat-conserving function; 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; walking over, 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.

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 behaviour", as opposed to a "post-encounter defensive behaviour", such as flight, freezing, or avoidance of a threatening stimulus.

Brown rats pose a huge threat to both domestic and commercial properties so it is advised to act quickly if you have, or suspect you have a problem with rats. Contact us today on 01282 777549 or email us at Our comprehensive site surveys at commercial premises are conducted free of charge and are carried out discreetly at a time both suitable and convenient in accordance with your working hours.

We will inspect all locations of the site in both internal and external locations, providing a full photographic digital report of our findings and survey results followed by our recommendations and next course of action to take.

Here at Atlas Environmental Services Ltd we are specialists when it comes to dealing with rats, our team of expert technicians have the skills and abilities to permanently eradicate and exclude these unwanted pest species from your property in Burnley or Lancashire and to prevent any further reoccurrence of ingress.

Here are some signs to look out for regarding the ingress of rats:

  • Sightings

  • Smell

  • Droppings

  • Urine and Urination Pillars

  • Hairs

  • Runs

  • Footprints

  • Smears

  • Harbourage, Burrows and Nests

  • Damage to Goods and Structures


Commercial sites normally always require regular servicing and inspections, in which these are conducted from our trained team of specialist technicians at 6 weekly interval visits to coincide with the breeding cycle  of rodents, and to ensure your premises remain free from pest related issues and in good order.

Proofing work and preventative installations are always carried out from ourselves where required - This is always highlighted to our clients and actioned whilst on site!

We are Burnley and Lancashire's leading pest control and pest management service provider - We currently look after approximately 200 commercial sites across the North West region and visit many domestic properties on a daily and regular basis!

Our policy still remains the same - " Your Site - Remains Our Priority! "


If your having issues with Rats or any other pest related problem within your commercial or domestic property then please feel free to contact us on 01282 777549 or email us at for an immediate response.

We are the leading service provider for rodent control and pest management services in Burnley and Lancashire!

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