FaunaHub

Rat (genus Rattus)

MammalsRodentsCommensal species

Brown rat (Rattus norvegicus) foraging on bare ground, showing a blunt muzzle, small furred ears and coarse grey-brown fur.

A brown rat foraging in a public park in Stevenage, England. The blunt muzzle and small, thickly furred ears separate a rat from a mouse at close range.

Image: AnemoneProjectors ( talk ), CC BY-SA 2.0, via Wikimedia Commons.

Overview

Used strictly, “rat” means a member of the genus Rattus, a group of medium-sized Old World rodents in the family Muridae that contains several dozen recognised species. Two of them account for almost everything written about rats in English. The brown rat, Rattus norvegicus, is the animal usually meant by the bare word and is also the ancestor of the fancy rat and of the laboratory rat. The black rat, Rattus rattus, is the slimmer, longer-tailed, more arboreal species that travelled the world's sea routes long before its relative did. This profile is written at group level and takes those two as its reference points.

The remaining species of Rattus are far less familiar. Most are native to South and Southeast Asia, New Guinea and the islands of the Indo-Pacific, and many are restricted to a single island, mountain range or forest type. They are ordinary wild rodents in ordinary wild habitats, and almost none of the popular associations attached to the word describe them at all. Any claim about “rats” that is really a claim about commensal brown rats in cities should not be carried across to them.

Three contexts run through this page and are kept deliberately apart. Wild rats live independently of people, as most species in the genus do. Commensal rats are populations of the brown and black rat living in and around human buildings, farms, stores, sewers and ships, exploiting what human activity makes available without being bred or managed by anyone. Domestic rats — the fancy rat kept as a companion animal and the laboratory rat used in research — are populations of Rattus norvegicus shaped by many generations of selection under human care. What is true of one of these is very often untrue of the others.

Habitat & Range

The genus as a whole is a genus of terrestrial and scansorial forest and grassland rodents. Wild species of Rattus occupy tropical and subtropical forest, montane forest, bamboo, scrub, grassland, marsh edges and cultivated land across Asia, New Guinea, Australia — including temperate south-eastern Australia and Tasmania, where the bush rat and the swamp rat are widespread native species — and the Indo-Pacific islands, and different species partition those habitats by elevation, structure and moisture. Several are described as strongly associated with intact forest and are seldom found near people.

The brown rat is best understood as a ground-dwelling generalist that has attached itself to human landscapes. It is a capable digger and typically occupies burrow systems in banks, embankments, waste ground, hedgerows and beneath structures, and it is strongly associated with water and with damp, low-lying situations; it swims readily. The black rat is the more three-dimensional animal of the two, more inclined to climb and to nest above ground — in roof spaces, palm crowns, tree cavities and dense vegetation. Where the two overlap, that vertical difference is often the clearest way they divide up the same site.

Both species also live independently of buildings in parts of their range, in field margins, riverbanks, coastal scrub, plantations and island habitats. Whether a given population is commensal, seasonally commensal or fully free-living varies with region, climate and season, and populations can shift between those patterns rather than belonging permanently to one.

Diet

Rats are omnivorous generalists, and that flexibility is central to what the genus is. Wild species are usually described as taking seeds, fruit, nuts, shoots, roots, fungi and invertebrates, with the balance shifting through the year as different foods become available. The brown rat is the broadest feeder of the two familiar species and is reported taking grain and seed, fruit, invertebrates, small vertebrates, eggs, carrion, fish and refuse. The black rat is generally described as more strongly frugivorous and granivorous, with a marked reliance on fruit, seed and nuts where it lives arboreally.

Brown rats are noted for how they approach unfamiliar food. Individuals commonly sample a small amount of something new and then leave it, returning later if nothing follows — a cautious pattern often discussed under the heading of neophobia, and one reason the species is a difficult subject for simple assumptions about feeding. Young rats also appear to be influenced by what other members of their group have recently eaten, which is generally interpreted as social transmission of food preference.

Continuously growing incisors mean that gnawing is a routine part of a rat's life and is not confined to food. This section describes feeding ecology as it is observed in the wild and in commensal settings. It is not a feeding guide, and any question about what an animal under human care should be given belongs with a qualified veterinarian rather than with a natural-history page.

Behavior

Rats are typically most active at night and around dusk, though commensal animals can shift their activity in response to disturbance, lighting and the rhythms of the places they live in. They move along established routes and follow walls, edges and pipes rather than crossing open ground, and both familiar species are strong climbers, with the black rat the more confident of the two. Brown rats swim and dive well; black rats generally do so less readily.

Social organisation is complex and varies with species and circumstance. Brown rats are commonly described as living in colonies built around related females, with burrow systems shared and defended, dominance relationships among males, and considerable tolerance between familiar animals alongside hostility toward strangers. Scent does much of the work: urine marks, glandular secretions and individual odour carry information about identity, group membership, status and reproductive condition. Rats also produce a wide range of vocalisations, many of them ultrasonic and inaudible to people, in social, exploratory and distress contexts.

The brown rat is one of the most heavily studied mammals in behavioural science, and it is worth being careful about what that literature shows. Rats learn quickly, navigate familiar space well, and are frequently reported engaging in social behaviours such as grooming, huddling and rough-and-tumble play. Much of that work was done on domesticated laboratory animals in controlled conditions, and how far each finding extends to wild rats is a live question rather than a settled one. FaunaHub does not rank animals by intelligence, and none of this should be read as such a ranking.

Human Interaction & Conservation

Few wild mammals have travelled further with people. The black rat appears to have spread west out of Asia along ancient trade and shipping routes, becoming established around the Mediterranean and then across much of Europe; the brown rat followed far later, expanding widely through Europe and onward by sea from roughly the eighteenth century. Both now reach nearly everywhere people have gone. That relationship is commensal rather than domestic: these are wild animals exploiting human environments, not animals bred, tamed or managed by anyone.

The practical consequences of that association are long documented — loss and contamination of stored food, gnawing damage to materials and fittings, and a role in the ecology of a number of pathogens, including a historically important part in the transmission cycles of plague. Guidance on rodents in buildings, and on any health question associated with them, is the province of public-health authorities and qualified professionals; none is offered here.

A separate relationship is domestic and scientific. Wild-caught brown rats taken into captivity in nineteenth-century Britain — largely from the stock assembled for rat-baiting, a blood sport of the period — produced the first tame colour varieties, and those animals became both the fancy rat kept as a companion and exhibition animal and the foundation of the laboratory rat. From around the turn of the twentieth century standardised laboratory lines, many of them albino, were developed from that domestic material, and the brown rat went on to become one of the principal mammalian model organisms in biomedical research. Any question about the health, handling, housing or welfare of a rat kept under human care should go to a qualified veterinarian with small-mammal experience and to recognised species-specific welfare organisations.

Scientific Classification

Rats in the strict sense belong to the kingdom Animalia, phylum Chordata, class Mammalia, order Rodentia, family Muridae, subfamily Murinae, genus Rattus. Muridae is the largest family of mammals, and Murinae — the Old World rats and mice — is its largest subfamily, which is why so many superficially similar rodents across Africa, Eurasia and Australasia sit close to Rattus without belonging to it.

The genus contains several dozen accepted species, and the exact number changes as populations are reassessed with genetic evidence; species boundaries within some Southeast Asian and New Guinean groups are still being revised, so different references may give different totals. The two species treated as reference points here are the brown rat, Rattus norvegicus, and the black rat, Rattus rattus. The house mouse, Mus musculus, sits in the same subfamily but a different genus, which is why rats and mice look related without being interchangeable.

For any named species, current classification, synonymy and range are best taken from an authoritative taxonomic database or the IUCN Red List assessment for that species rather than generalised from a group-level page such as this one.

Identification

The single most useful identification skill here is telling a rat from a mouse, and it is done on proportion, not absolute size. A half-grown rat is roughly the size of an adult mouse, and that is where nearly every real-world misidentification happens. Judging by length or weight alone will therefore fail exactly when it matters.

Four proportional features separate them reliably. A rat has ears that look small relative to the head, while a mouse's ears look conspicuously large for its head. A rat has a heavier, blunter muzzle; a mouse's face tapers to a fine point. A rat has a thicker, coarser, more obviously scaly tail, whereas a mouse's tail is slender and finer. And a rat has noticeably larger hind feet in proportion to its body. Taken together, these give a young rat a chunky, heavy-footed, blunt-faced look that an adult mouse does not have, however similar the two may be in overall length.

Separating the two familiar rats from each other also rests on proportion. The black rat is the slimmer, finer-boned animal, with larger and thinner ears, a more pointed face, and a tail that is usually longer than its head and body combined. The brown rat is heavier and more thickset, with smaller ears set closer to the head and a tail usually shorter than its head and body. Coat colour is an unreliable guide in both directions, and the English names are actively misleading on this point.

Appearance

Rats have the general murine build: a compact head, a pointed but substantial muzzle, prominent dark eyes, long vibrissae, short limbs, and a long, sparsely haired, ring-scaled tail. The fur is typically a grizzled mixture of dark and lighter hairs on the upperparts, with paler and often greyish or buff underparts, and the pelage is usually coarser than a mouse's.

Adult brown rats are commonly reported at somewhere around 20–27 cm in head and body length, with a tail a little shorter than that, and body weights most often given in the region of about 200–500 g. Black rats are typically lighter and slimmer, frequently reported at roughly 16–22 cm in head and body with a longer tail, and weights commonly in the region of about 75–230 g. All of these figures shift substantially with sex, age, season, food supply, population and whether the animals are free-living or held under human care, and exceptionally large individuals attract attention precisely because they are unusual. Treat any single quoted number as an approximation.

Coat colour varies more than the names suggest. Brown rats range from grizzled grey-brown through warm agouti to dark, and black rats occur both in a dark grey-black form and in browner forms with paler underparts, so a dark rat is not necessarily a black rat. Domestic fancy rats depart further still, with a wide range of selected colours, markings and coat types — including the albino form long associated with laboratory lines — none of which occurs commonly in wild populations.

Distribution

The natural range of the genus is Asian and Indo-Pacific. Most species of Rattus are native to South and Southeast Asia, southern China, New Guinea, Australia and the archipelagos between them, and a good number are island endemics with very small ranges. The Australian native species are not confined to the tropical north: Rattus fuscipes, the bush rat, runs down the eastern seaboard into Victoria and reappears in south-western Western Australia, and Rattus lutreolus, the swamp rat, reaches Tasmania. The brown rat is generally traced to north-eastern Asia, and the black rat to the Indian subcontinent and adjacent Southeast Asia, though the details of both origins continue to be refined.

What happened next is a story about ships rather than about biology. The black rat moved west with maritime and overland trade over a long period, becoming widespread around the Mediterranean and through much of Europe, and subsequently reached the Americas, Africa, Australia and a very large number of islands with European seafaring. The brown rat expanded much later, spreading through Europe and then worldwide by sea, and in many temperate regions it has displaced the black rat from buildings while the black rat has persisted in warmer climates, ports and roof spaces.

Between them the two species now occur on every continent except Antarctica, in association with people, and on thousands of islands that previously had no ground-dwelling mammals at all. That distribution is not a natural range: it is a map of human transport, settlement and storage laid over the world. By contrast, the great majority of Rattus species have never left their native ranges, and current range maps for any of them should be taken from the IUCN Red List entry for that species.

Adaptations

Rats are built for close-quarters, low-light living. Smell is the dominant sense, used for individual and colony recognition, for marking routes, for assessing reproductive condition and for finding food. Touch is almost as important: long whiskers are swept rhythmically across surfaces, allowing an animal to follow a wall or tunnel confidently in complete darkness. Hearing extends well above the human range, which is why so much rat communication is inaudible to us. Vision is generally described as suited to detecting movement in dim light rather than to fine detail or rich colour, and the eyes sit high and to the side for a wide field of view.

Physically, the genus combines a compact, compressible body with strong claws, a long balancing tail and considerable climbing ability. Rats can pass through openings that look far too small for them, but the mechanism is usually misdescribed: a rat cannot collapse its skeleton, and like other rodents it retains collar bones. The limiting dimension is the width of the skull, and a gap the head fits through is one the rest of the body can generally be worked through as well. Ever-growing incisors carry hard enamel on the front face with softer dentine behind, so gnawing wears the two at different rates and maintains a chisel edge, letting a rat open hard seeds and much tougher material besides. A large caecum supports the digestion of plant material, and the digestive anatomy of rats is such that they are unable to vomit — a well-documented peculiarity of the group that is often linked to their cautious sampling of unfamiliar food.

Behavioural flexibility may be the most important adaptation of all in the two commensal species: a broad diet, tolerance of disturbed habitats, rapid reproduction, and a wary, exploratory approach to change together explain far more about their success alongside people than any single physical trait. As with all functional interpretations, these explanations reflect current understanding and may be refined.

Life Cycle & Reproduction

Reproduction in the brown and black rat is rapid where conditions allow. Gestation is short — commonly reported at around three weeks — and litters typically contain several young, often somewhere in the range of about six to twelve, with smaller and larger litters both recorded. Females can produce several litters in a year, and in warm, sheltered, food-rich sites breeding may continue through much of the year, while in colder or drier regions it is tightly seasonal. Reported figures vary between studies, sites and populations.

Young rats are altricial. They are born blind, deaf and effectively hairless in a nest, develop fur over the following days, open their eyes within a couple of weeks or so, and are generally weaned at around three to four weeks. Sexual maturity typically arrives within a few months and can come sooner where food is abundant. That combination of short gestation, large litters, early maturity and repeated breeding is why rat populations can build and crash quickly when conditions change.

In the less familiar wild species of the genus, reproductive patterns are far more variable and in several cases poorly documented, with smaller litters and more strongly seasonal breeding reported for some forest species. This section describes reproductive ecology as it is observed. It is not breeding guidance, and nothing here should be read as instruction for animals under human care.

Lifespan

In the wild and in commensal settings: rat lifespans are short. Free-living brown and black rats are generally reported to average well under a year, with predation by owls, birds of prey, foxes, mustelids, snakes, monitor lizards and domestic cats, together with cold, food shortage, competition and disease, removing the great majority before a second breeding season. A minority of individuals live appreciably longer. These figures are estimates drawn from trapping and mark-recapture work and differ between studies, seasons and sites.

Under human care: rats held in protected conditions, including fancy rats and laboratory animals, are commonly described as living somewhere in the region of about two to three years, with individuals occasionally exceeding that. These figures come from environments without predators, with reliable food and with veterinary oversight, and they vary with line, sex and the conditions in which the animals are kept.

The two sets of numbers are kept apart on purpose. Captive longevity describes what a rat can live to in the absence of the pressures that actually end wild lives, and reading it back onto free-living animals — or the reverse — produces a misleading picture of both.

Conservation Context

The brown rat and the black rat are both extremely widespread and have been assessed in recent years as being of least concern. Conservation categories are reassessed as new information arrives and can move in either direction, so nothing on this page should be treated as a permanent status; for the current category, criteria, range and assessment date for Rattus norvegicus, Rattus rattus or any other species, consult the IUCN Red List entry for that species directly.

The security of those two says nothing about the genus as a whole. A substantial number of Rattus species are narrow-range island or montane endemics, and several have been assessed in threatened categories or are considered extinct, chiefly through habitat loss and the arrival of introduced predators and competitors on small islands. The commensal success of two species has, in effect, obscured the vulnerability of many of their relatives.

The genus also sits on the other side of the conservation ledger. Introduced brown and black rats are among the most consequential invasive mammals on islands worldwide, associated with declines and extinctions of seabirds, land birds, reptiles, invertebrates and plants that evolved without ground predators, and island rat-eradication projects are undertaken as conservation work by national agencies and conservation organisations. Abundance and conservation concern are not opposites, and an animal can be both a species of least concern in its own right and a serious threat somewhere it does not belong.

What the Name Covers

In its strict sense, “rat” means a species of Rattus. In everyday use it is a description of size and shape applied to a great many rodents that are not in the genus at all: woodrats and pack rats of the Americas, kangaroo rats, bamboo rats, cane rats, giant pouched rats, and the muskrat, which is a large aquatic relative of the voles. The distinction is not merely pedantic: the rats trained for landmine and tuberculosis detection, often reported simply as “detection rats”, are African giant pouched rats of the genus Cricetomys, not Rattus at all. The naked mole-rat is another clear case — it is not Rattus, is not closely related to it, and belongs to a quite different branch of the rodent family tree; FaunaHub covers that species on its own page. A number of unrelated animals outside Rodentia have also picked up “rat” in their common names.

Regional naming adds more confusion. Rattus norvegicus is called the brown rat, common rat, sewer rat, street rat and Norway rat, the last a historical misnomer since the species did not originate in Norway. Rattus rattus is called the black rat, ship rat, roof rat and house rat, and, as noted above, it is frequently not black. Because both names promise a colour the animal may not have, colour is the worst available basis for identifying either.

Two other confusions are worth naming. The first is rat and mouse: they are different genera in the same subfamily, separated in the field by proportion rather than size, and a juvenile rat passed off as an adult mouse is the standard error — see the Identification section above. The second is wild and domestic: the fancy rat and the laboratory rat are the same species as the brown rat but descend from wild-caught animals taken into captivity in the nineteenth century, largely from rat-baiting stock, and later from standardised laboratory lines. Generations of selection have changed coat, colour, size, temperament, reproductive pattern and physiology, and a statement about one of these populations often does not describe the other. Whether rats may be kept, bred, sold or used in research varies between countries and smaller jurisdictions and is not stated here; check current local law.

Compare With Related Animals

Side-by-side comparisons covering identification, taxonomy, habitat and behaviour.

More photos of the rat

Brown rat (Rattus norvegicus) swimming across open water with only its head and back above the surface.

A brown rat swimming at Santragachi Jheel, Howrah, India. Brown rats swim and dive readily, which is one reason they colonise sewers and waterways so successfully.

Image: Biswarup Ganguly, CC BY 3.0, via Wikimedia Commons.

Frequently Asked Questions — Rat

How do you tell a rat from a mouse?
Read the parts against each other rather than measuring the whole animal. On a rat the ears look small beside a heavy, blunt muzzle, the tail is thick with visible scale rings, and the hind feet are broad. On a mouse the face tapers to a fine point, the ears look outsized for the head, and the tail is slender. Because the two overlap in weight while a rat is still growing, size on its own settles nothing, and fur colour is unreliable in both.
Is a mole-rat a kind of rat?
No. Despite the name, African mole-rats — including the naked mole-rat, Heterocephalus glaber — are not members of the genus Rattus and are not closely related to it. They belong to a quite different branch of the rodent family tree, live underground in Africa, and differ from true rats in body plan, biology and social organisation. FaunaHub covers them separately. The word rat inside a common name is a shape description, not a statement of ancestry.
Are pet fancy rats the same animal as wild rats?
They are the same species as the brown rat, Rattus norvegicus, but not the same animal in any practical sense. Fancy rats descend from wild-caught brown rats taken into captivity in the nineteenth century, largely from rat-baiting stock, and later from laboratory lines bred for standardised genetics. Many generations of selection have changed coat, colour, size, temperament and physiology, so findings about one population should not be read across to the other.
How long do rats live?
The two settings should never be merged. Free-living brown and black rats are generally reported to live well under a year on average, with predation, weather, food shortage, competition and disease removing most animals before a second breeding season; a minority survive appreciably longer. Rats kept under human care, protected from predators and with reliable food, are commonly described as living somewhere around two to three years, varying with line, sex and conditions.
Why is the black rat sometimes not black?
Coat colour is a poor guide to species in this genus. Black rats, Rattus rattus, occur in dark grey-black and in browner, paler-bellied forms, while brown rats, Rattus norvegicus, range from grizzled grey-brown to dark. Reliable separation rests on proportion instead: the black rat is the slimmer animal, with larger, thinner ears and a tail usually longer than its head and body, whereas the brown rat is heavier with a shorter tail.

Sources and further reading

Authoritative wildlife references used for general educational context. Conservation status should always be verified against current IUCN Red List data. External links open in a new tab.