Brown Rat (Rattus norvegicus): Facts and Behavior

What does a brown rat look like?

A brown rat has coarse fur that is grayish-brown to dark brown on the back and paler gray or buff on the belly. Some individuals show scattered black or white hairs, and all-black or pale animals occasionally turn up in the wild. The body is heavy and thickset, with a blunt muzzle, small eyes and relatively small, finely furred ears that do not reach the eyes when folded forward.

The scaly, almost bare tail is thick at the base and always shorter than the head and body together. This is one of the easiest ways to tell it from its close relative, the black rat (Rattus rattus), which is slimmer, has large thin ears, a pointed snout and a tail longer than its body.

Like all rodents, the brown rat has a pair of upper and lower incisors that grow throughout life. They are kept short and sharp by constant gnawing. The front surface of each incisor has hard, orange-tinted enamel, while the back is softer dentine, so the teeth wear into chisel-like edges.

How big is a brown rat?

The brown rat is one of the largest members of the mouse family, Muridae. Typical measurements are:

Males are usually larger than females. Many breeding wild rats weigh less than 300 g (11 oz), and only exceptional animals, mostly well-fed domestic ones, approach 800 g (28 oz). A brown rat can weigh twice as much as a black rat and many times more than a house mouse, but tales of rats as big as cats are exaggerations or mistaken sightings of other animals.

Senses and intelligence

Brown rats live in a world of smell, touch and sound. Their sense of smell is excellent, and they use it to find food, recognize members of their colony and read each other’s breeding condition. Long whiskers are swept back and forth several times a second, letting a rat feel its way along walls and through total darkness. Rats are sensitive to ultrasound and can hear frequencies far above the limit of human hearing.

Eyesight, by contrast, is weak. Wild rats have blurry vision and see colors roughly as a person with red-green color blindness does, although their eyes are also sensitive to ultraviolet light.

Rats learn quickly and remember routes through complex burrows and sewers. They are cautious of unfamiliar objects and foods, a trait called neophobia, and young rats learn which foods are safe by smelling food odors on the breath of other colony members. Psychologists have studied rat learning for more than a century; see how smart are rats? for what experiments have revealed.

Habitat and range

Brown rats originally lived in the plains, grasslands and brushy river valleys of northern China, Mongolia and neighboring areas. Over the last few centuries they spread with human trade and shipping, reaching Europe by the early 1700s and North America around the middle of the 18th century. The full story is in where did brown rats come from?

Today the species is found on every continent except Antarctica, from ports and big cities to farms, rubbish dumps and remote islands. Away from people, brown rats prefer damp places such as riverbanks, ditches, salt marshes and coasts. In towns they live in sewers, cellars, drains, under sheds and along railway embankments. In hot tropical and desert regions they are mostly limited to human settlements, where the black rat is often the more common species.

There are still a few large rat-free areas. These include Antarctica, much of the Arctic, many remote islands and the Canadian province of Alberta, which has kept brown rats from establishing since the 1950s through a strict provincial program.

What do brown rats eat?

The brown rat is a true omnivore that will eat almost anything, but grains and seeds make up a large part of its natural diet. It also takes fruit, roots, insects, snails, earthworms, eggs and small animals such as mice, chicks and small lizards. In towns it lives largely on discarded human food.

Local colonies develop their own feeding traditions. Rats near a fish hatchery in West Virginia caught young fish, and rats along the Po River in Italy were recorded diving for mollusks, a habit apparently passed on by social learning. On the German North Sea island of Norderoog, brown rats were reported attacking seabirds. Rats often carry food back to a safe place to eat it, and they store surplus food in their burrows.

Behavior and social life

Brown rats are mainly nocturnal and are good diggers, climbers and swimmers. Where the ground allows, they dig extensive burrows with several entrances, nest chambers and food stores. Each rat follows regular runways between its nest, food and water, and in cities many rats stay within about 20 m (66 ft) of their nest when food is plentiful.

Rats are highly social. A colony is built around related females and their young, and members groom each other, sleep together in huddles and play-fight when young. Dominance hierarchies decide who gets the best food and mates, and larger males usually rank highest. Rats are territorial toward strangers: they fluff up their fur, hiss, squeal and chase outsiders away.

Much rat communication is ultrasonic and therefore silent to people. Pups call to their mothers, adults give lower-pitched alarm calls around 22 kHz, and playful or rewarded rats make short “chirps” around 50 kHz. Researchers have compared these chirps to laughter; see do rats laugh? for the evidence.

Reproduction and life cycle

Brown rats can breed throughout the year when food and shelter allow, with a peak in the warmer months. Females are fertile again within about a day of giving birth, so a mother can nurse one litter while pregnant with the next.

• Gestation: about 21–24 days
• Litter size: usually 6–12 pups, occasionally up to about 14 or more
• Litters per year: up to about 5–7 under good conditions
• Birth weight: about 5–6 g (0.2 oz); pups are born naked and blind
• Eyes open: about 2 weeks; weaning at 3–4 weeks
• Sexual maturity: from about 2–3 months in the wild, earlier in well-fed laboratory rats

Females in the same nest often nurse each other’s young. With such fast breeding, a small group of rats can grow into a large colony within a single year, and populations quickly bounce back after heavy losses.

How long do brown rats live?

Most wild brown rats live less than a year. Studies of wild populations have estimated that around 90–95% of rats die each year, from predators, disease, cold, fights with other rats and human control. In captivity, safe from these risks, brown rats usually live about 2–3 years, and the documented maximum is close to 4 years. Read more in how long do rats live?

Predators and threats

Brown rats are important prey for many predators. Barn owls, tawny owls, great horned owls, hawks, foxes, weasels, stoats, mink, snakes and domestic cats all eat them, and herons and gulls take young rats. Rats in cities often avoid areas used by cats, and predators help to keep populations in check in the countryside.

Wild rats have also evolved quickly in response to people. Some populations, for example in parts of the United Kingdom and Europe, have become genetically resistant to certain poisons used against them, and city rats in different neighborhoods of New York have been shown to differ genetically.

Conservation status

The brown rat is listed as Least Concern on the IUCN Red List. It is abundant and widespread and faces no threat as a species. Outside its native range, however, it is one of the world’s most damaging invasive animals. On islands without native ground predators, introduced rats eat the eggs and chicks of seabirds, along with lizards, insects and land snails.

Eradicating rats from islands has become a major conservation tool. Hawadax Island in Alaska’s Aleutians, formerly called Rat Island, was declared rat-free in 2009, and its shorebirds and seabirds have since recovered. Large rat-removal projects have also been completed on islands such as South Georgia and Campbell Island.

Myths and facts

Myth: There is a rat for every person in a city. This popular claim is not based on real counts. Estimates for New York City, for example, have ranged from a few hundred thousand to several million, and most experts consider “one rat per person” an urban myth.

Myth: Brown rats spread the Black Death. The great plague pandemic of 1347–1351 struck Europe centuries before brown rats became common there. Historians have linked medieval plague to the black rat and its fleas, among other possible routes, although the exact roles are still debated.

Fact: Rats cannot vomit. Like other rats and mice, brown rats are unable to throw up, which may help explain why they are so cautious about trying new foods.

Brown rats and people

People have lived with brown rats for centuries, mostly unwillingly. Rats damage stored food and gnaw through wood, plastic and cables, and they can carry diseases such as leptospirosis (Weil’s disease) and rat-bite fever. Anyone who becomes ill after contact with rats or their urine should seek medical help promptly.

The brown rat is also one of the most important animals in science. Albino brown rats were bred for research from the 19th century, and in 1906 the Wistar Institute in Philadelphia began breeding a standardized strain, the Wistar rat, that is still used today. When biologists say “rat”, they almost always mean this species, whose genome was sequenced in 2004. Pet or fancy rats, shown by hobby breeders since around 1900, are domesticated brown rats too.

Rats feature widely in culture, from the rat that opens the Chinese zodiac to children’s books such as The Tale of Samuel Whiskers. Their reputation is mixed, but their intelligence and sociability have earned them a place among the smartest animals in many rankings.