# Brown Rat — Facts, Threats & Conservation

*Rattus norvegicus*

> A brown rat (Rattus norvegicus) is a large, adaptable rodent that lives alongside humans worldwide. Native to Asia, it has spread to every continent except Antarctica. Highly intelligent and social, it is the wild ancestor of both pet "fancy rats" and laboratory rats, and is listed as Least Concern.

**IUCN status:** Least Concern (IUCN)  ·  **WARN range:** Worldwide (all continents except Antarctica), Europe, North America, Asia (likely origin in northern China and Mongolia), Australia

## Quick facts
| Fact | Value |
| --- | --- |
| Common names | Brown rat, common rat, Norway rat, sewer rat |
| Scientific name | Rattus norvegicus |
| Likely origin | Northern China and Mongolia |
| Distribution | Worldwide; every continent except Antarctica |
| Head & body length | About 15-28 cm, plus a shorter tail |
| Weight | Typically 140-500 g |
| Diet | Opportunistic omnivore; cereals, fruit, seeds, insects, refuse |
| Lifespan | Up to ~3 years; most under 1 year in the wild |
| Litter size | Up to 14 pups (about 7 typical); ~21-day gestation |
| IUCN status | Least Concern (not CITES-listed) |

## Scientific classification
- **Kingdom:** Animalia
- **Phylum:** Chordata
- **Class:** Mammalia
- **Order:** Rodentia
- **Family:** Muridae
- **Genus:** Rattus
- **Species:** Rattus norvegicus

## Conservation status
- **Status:** Least Concern (IUCN Red List). The brown rat has an extremely large, stable and widespread global population and benefits directly from living alongside humans. It is not listed under CITES. In many parts of the world it is an introduced, invasive species that can threaten native wildlife, especially ground-nesting birds on islands.
- **Population:** Not quantified, but extremely abundant; one of the most numerous mammals on Earth
- **Trend:** Stable
- **Assessed:** Assessed by the IUCN as Least Concern (most recent assessment)
- **CITES:** Not listed in any CITES Appendix
- The conservation concern around brown rats is usually their impact as an invasive species on vulnerable island wildlife, rather than any threat to the rat itself.

## Key facts: Brown Rat
- The brown rat is among the most successful mammals alive, living wherever humans do across all continents except Antarctica.
- It most likely originated in northern China and Mongolia, despite the misleading name 'Norway rat'.
- Brown rats are highly intelligent, social animals that live in hierarchical groups and groom one another.
- They are the wild ancestor of both pet fancy rats and laboratory rats, central to modern medical research.
- The IUCN lists the brown rat as Least Concern, with a large and stable global population.
- As a non-native species in many regions, brown rats can threaten ground-nesting birds and other wildlife on islands.

## What does a brown rat look like?
The brown rat is a robust, heavy-bodied rodent with coarse grey-brown fur, a paler grey or off-white underside, small rounded ears and a blunt nose. Adults typically measure 15-28 cm in head and body length, with a scaly, sparsely haired tail of 10.5-24 cm that is shorter than the head and body combined, a useful feature for telling it apart from the black rat. Most weigh between 140 and 500 g, though exceptional individuals have reached over 800 g. Like all rodents, it has a single pair of continuously growing incisors in each jaw, kept in check by constant gnawing; in total it has 16 teeth, with a wide gap (the diastema) separating the incisors from the grinding molars. Its eyesight is relatively poor, but it has an excellent sense of smell, sensitive whiskers and acute hearing that extends well into the ultrasonic range. These senses, together with its agility, climbing ability and willingness to swim, help the brown rat exploit an enormous variety of habitats.

## Where do brown rats live and what do they eat?
Brown rats are commensal animals, meaning they live in close association with people. They are found in cities, farms, warehouses, sewers, riverbanks and rubbish tips, and have colonised every continent except Antarctica by stowing away on ships and in cargo. Despite the name 'Norway rat', genetic and historical evidence points to an origin in northern China and Mongolia, from where the species spread along human trade routes, reaching Europe in the 1700s. They are strong burrowers, digging extensive systems of tunnels and chambers near reliable food and water. The brown rat is an opportunistic omnivore that will eat almost anything, including grain, fruit, seeds, insects, eggs, refuse and carrion, though cereals form a large part of its natural diet. This dietary flexibility, combined with a willingness to try new foods cautiously, is a key reason for its remarkable global success. Living so close to people, brown rats can damage stored food and structures and can carry diseases, which is why much of their interaction with humans has historically been as a pest species.

## How intelligent and social are rats?
Brown rats are strikingly intelligent and social, which is part of why they have become such important laboratory animals. In the wild they live in large, hierarchical groups built around family units, communicating through scent, body posture and a range of squeaks and ultrasonic calls. They establish dominance hierarchies, groom one another, and recognise individual group members. Studies have shown rats are capable of problem-solving, learning complex tasks, navigating mazes and even appearing to display empathy-like behaviour towards companions in distress. Young rats engage in play, and researchers have recorded ultrasonic 'chirps' associated with positive experiences such as tickling. This intelligence and sociability is exactly what makes the domesticated brown rat, the 'fancy rat', such a rewarding companion animal: pet rats are typically gentle, curious and quick to bond with the people who care for them. The same traits underpin the laboratory rat's central role in biology and medicine, where Rattus norvegicus has contributed to countless advances in our understanding of behaviour, physiology and disease.

## Are brown rats endangered or a conservation concern?
The brown rat is in no danger of extinction. The IUCN lists it as Least Concern, reflecting an enormous, stable and widespread population that benefits directly from human activity. It is not listed under CITES, as it faces no trade-related threat. If anything, the conservation story runs the other way: in many parts of the world the brown rat is an introduced, invasive species, and on islands in particular it can devastate populations of ground-nesting seabirds, reptiles and native insects that evolved without such predators. Conservation programmes on vulnerable islands sometimes focus on removing introduced rats to allow native wildlife to recover. For the animal itself, the brown rat is a model of adaptability rather than fragility, a reminder that 'conservation status' is about a species' place in an ecosystem as much as its raw numbers. Understanding the difference between a thriving commensal species and the threatened wildlife it can affect is an important part of reading any animal's story honestly.

## Brown rat vs black rat
| Feature | Brown rat (Rattus norvegicus) | Black rat (Rattus rattus) |
| --- | --- | --- |
| Build | Larger, heavier, stocky | Smaller, slimmer, lighter |
| Nose | Blunt | Pointed |
| Ears | Small, close to head | Large, prominent |
| Tail vs body | Shorter than head and body | Longer than head and body |
| Preferred level | Ground, burrows, sewers | Roofs, upper storeys, climbing |
| Typical setting | Cities, farms, waterways worldwide | Warmer regions; more arboreal |

## Fancy Rat Varieties Guide
From the Standard and big-eared Dumbo to the white-bodied Hooded and the near-bald Hairless — explore the main fancy rat varieties, classified by coat, ear, tail and markings, with honest welfare notes, common health issues and care for these clever, sociable companions.

Full variety library (4 guides): https://worldanimalrescuenetwork.org/wildlife-guides/rat#breeds

- **Standard:** The classic pet rat — short, smooth, glossy coat and upright ears. — https://worldanimalrescuenetwork.org/wildlife-guides/rat/standard
- **Dumbo:** Large, low-set ears give the dumbo rat its endearing cartoon-like look. — https://worldanimalrescuenetwork.org/wildlife-guides/rat/dumbo
- **Hairless:** A near-bald variety that needs extra warmth, soft bedding and careful welfare. — https://worldanimalrescuenetwork.org/wildlife-guides/rat/hairless
- **Hooded:** A coloured head and shoulders with a stripe down the spine — a classic rat marking. — https://worldanimalrescuenetwork.org/wildlife-guides/rat/hooded

## What WARN does
WARN does not run field projects specifically for brown rats, which are abundant and listed as Least Concern, and in many regions are an introduced species rather than threatened wildlife. This guide is part of WARN's free educational work, helping people understand animals accurately and kindly. The wider threats woven through the rat's story, especially habitat change and the pressure invasive species place on vulnerable native animals, are exactly the kinds of challenges that affect the wildlife WARN does work to protect.

If you have found this guide useful, supporting WARN's free educational work helps us keep telling honest animal stories and caring for the vulnerable wildlife behind them.

## Frequently asked questions: Brown Rat
### Why is the brown rat also called the Norway rat?
The name 'Norway rat' is misleading. The species did not originate in Norway; an early naturalist wrongly assumed it had arrived in Britain on Norwegian ships. Genetic and historical evidence instead points to an origin in northern China and Mongolia, from where the brown rat spread along human trade routes to reach Europe in the 1700s and, eventually, almost the entire world.

### What is the difference between a brown rat and a black rat?
Both are common 'true rats', but they differ in build and habits. The brown rat (Rattus norvegicus) is larger and heavier with a blunt nose, small ears and a tail shorter than its body; it prefers ground level, burrows and sewers. The black rat (Rattus rattus) is slimmer with a pointed nose, larger ears and a tail longer than its body, and is a more agile climber that favours roofs and upper storeys.

### Are pet rats and laboratory rats the same species as wild brown rats?
Yes. Pet 'fancy rats' and laboratory rats are both domesticated forms of the wild brown rat, Rattus norvegicus. Through selective breeding they have become tamer, calmer and more varied in colour and coat than their wild ancestors, but they remain the same species. Their intelligence and sociability make fancy rats popular companion animals and laboratory rats invaluable to medical and behavioural research.

### Are brown rats dangerous to humans?
Healthy wild brown rats generally avoid people and are not aggressive, but living so close to humans they can spread certain diseases through their droppings, urine or parasites, and can contaminate stored food. Historically rats have been associated with serious illnesses. Pet fancy rats, by contrast, are bred to be gentle and pose very little risk when kept responsibly and handled with normal hygiene.

### How fast do brown rats reproduce?
Very quickly, which is central to their success. Females can breed from around five weeks old, gestation lasts only about 21 days, and a single litter can contain up to 14 pups, with around seven being typical. A female can produce several litters a year in good conditions. High reproductive output offsets a very high natural mortality rate, as most wild brown rats live barely a year despite a potential lifespan of up to three years.

### Is the brown rat endangered?
No. The brown rat is listed as Least Concern by the IUCN, with a vast, stable population spread across every continent except Antarctica. It is not protected under CITES. In many regions the conservation concern is the opposite: as an introduced, invasive species, the brown rat can threaten vulnerable native wildlife, especially ground-nesting birds on islands that evolved without ground predators.

## Sources
- [Brown rat — Wikipedia](https://en.wikipedia.org/wiki/Brown_rat)
- [IUCN Red List — Rattus norvegicus](https://www.iucnredlist.org/species/19353/197467531)
- [Rat — Encyclopaedia Britannica](https://www.britannica.com/animal/rat)
- [Rattus norvegicus — Animal Diversity Web (University of Michigan)](https://animaldiversity.org/accounts/Rattus_norvegicus/)
- [Norway rat — Smithsonian's National Zoo](https://nationalzoo.si.edu/animals/norway-rat)

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Full guide: https://worldanimalrescuenetwork.org/wildlife-guides/rat