Brown rat

Name and taxonomy

The English naturalist John Berkenhout named the species Mus norvegicus in 1769. He believed that the rat had arrived in England on Norwegian timber ships in 1728. The rat never came from Norway, and the species was once even listed as extinct there. The name stuck all the same, and Norway rat is still common in North America. In 18th-century England some people called it the Hanover rat, to blame the ruling house for the country's troubles. Charles Dickens noted in his journal that the rat was unknown in Norway when it was already common in England.

The type locality is Great Britain, which says more about where it was described than where it began. It sits in the genus Rattus, within Muridae, which is thought to be the largest family of mammals. Within the genus it forms the norvegicus species group with the Himalayan field rat and the Turkestan rat. The domestic laboratory and pet strains are placed in the subspecies Rattus norvegicus domestica. Subspecies among wild populations remain unsettled and need genetic work.

Evolution and relatives

The brown rat's line split from the black rat's about 2.9 million years ago. Mitochondrial genomes place its origin as a separate species somewhere between 0.5 and 2.9 million years ago. Its closest relative is the Himalayan field rat, which parted from it about 700 thousand years ago. The two species met again later and exchanged genes, and some of the brown rat's smell receptors came from that mixing. The Turkestan rat, the other sister species, lives near the brown rat's ancestral range in Nepal.

The species keeps evolving in the cities it has colonised. Rats in New York carry longer snouts and shorter molar rows than Chinese rats, perhaps to suit cold and human food. Roads and clean districts split city rats into separate populations, sometimes a single block wide. Resistance to the poison warfarin appeared in the mid-20th century, and some British rats now resist newer poisons too. Laboratory rats have smaller testes and brains than wild ones and are far worse at digging and swimming.

Appearance

The brown rat is a robust, heavy-bodied rat with a broad, blunt snout. The coat is long, coarse and shaggy, dirty greyish brown to dark brown on the back. The colour lightens along the sides to a grey or white belly, without a sharp line between the two. The ears are short and hairy, and they do not cover the eyes when folded forward. The tail is thick, almost bare, and darker above than below. Albino and black rats turn up quite often in wild colonies, and pet strains come in many colours and coat types.

Head and body length runs from 15 to 28 cm, and the tail adds 10.5 to 24 cm. The tail is always shorter than the body, which helps separate this species from the black rat. Adults weigh 140 to 500 grams, and wild breeding rats often weigh under 300. A few well-fed city rats pass 800 grams, but the tales of rats the size of cats are false. Males are usually larger than females. A brown rat may be double the weight of a black rat and several times heavier than a house mouse.

Habitat and range

The native range is thought to cover northern China, Mongolia, southeast Siberia and perhaps Japan. There the rat lived in moist forests, brush and fields. It reached Europe by the 16th century at the latest, and a Swiss naturalist may have drawn one in 1553. Reliable records place it in Ireland in 1722, England in 1730, France in 1735 and Germany in 1750. It crossed to North America in about 1750 to 1755 as a stowaway on ships. Today it lives on every continent except Antarctica, though it is less common in Africa.

In cool climates the rat lives outdoors as well as in buildings, and it favours damp ground near water. In the tropics and in deserts it is limited to places that people have built or changed, such as ports and sewers. City rats nest in sewers, wall cavities and burrows, and they rarely stray more than 20 metres from the nest when food is close. As it spread, the brown rat pushed out the black rat, because brick and tile buildings suit a burrower better than a climber. A few large areas remain rat-free: Antarctica, the high Arctic, some islands and the prairie province of Alberta.

Diet and feeding

Brown rats eat almost everything, and grain makes up a large share of the diet. One wild stomach examined in Germany held 4,000 food items, most of them plant matter. Given a choice, rats prefer meat. In cities they live mainly on thrown-away human food. Tests of pet rats found scrambled egg, raw carrot and cooked sweetcorn among the favourites, and raw beetroot and celery among the rejects.

Feeding habits differ from colony to colony and pass from rat to rat by learning. Rats near a fish hatchery in West Virginia catch young fish, and colonies on the Po River in Italy dive for molluscs. On the North Sea island of Norderoog they stalk and kill sparrows and ducks. They also take chicks, eggs, mice, small lizards and shorebirds. Wild rats are wary of new food and objects, and this caution makes poison harder to use. Small crumbs are eaten on the spot, while larger pieces are carried to cover.

Behaviour and social life

The brown rat is nocturnal, and most activity happens before midnight. It swims well on the surface and under water, which suits life in a sewer. It climbs when it must and has been seen shinning up thin metal poles to reach bird feeders. Rats dig long burrow systems with several levels, a second entrance, food stores and nest chambers. A burrow usually starts beside a wall or an object, which forms a firm roof for the top section. Young rats and females dig hard, while old males seldom dig at all.

Rats live in large colonies with a hierarchy, and bigger rats usually rank higher. They groom each other, sleep in piles and play-fight by chasing, tumbling and boxing. Play aims at the neck, while a serious fight aims at the rump. Rats defend their colony against strangers with fluffed fur, hisses and squeals. When food runs short, the lowest-ranked rats die first. If people kill a large share of a colony, the survivors breed faster and the numbers soon recover.

In winter rats huddle cheek to cheek to keep warm. They learn routes through sewers and burrows and follow the same runways every night. One study in 2007 claimed that rats can judge their own knowledge, though later analysis suggested simpler learning. Their quick learning made them the standard animal of the psychology laboratory.

Communication and senses

Smell is the rat's best sense, and it uses scent to find food and to tell colony members apart. Hearing is sharp and extends into ultrasound. Eyesight is poor, around 20/600 in a pigmented rat, and worse in an albino. The rat sees colour much as a red-green colour-blind person does, but it can also see ultraviolet light. Whiskers and paws let it feel its way through total darkness, and it senses faint vibrations in the ground.

Pups call in ultrasound to bring their mother back to the nest. Adults give ultrasonic alarm calls when a predator appears, and a female calls during mating. Young rats chirp in ultrasound when they play or are tickled, and researchers compare the sound to laughter. Pet owners use bat detectors to hear it. Within human hearing, rats squeak in pain, sing softly in a stand-off, and grind their teeth when content. Hard tooth-grinding makes the eyes bulge and retract, a sign called boggling.

Reproduction and life cycle

The mating system is polygynandrous, and both sexes take several partners. A female is in oestrus for about six hours, and in that time she may mate as many as 500 times. Dominant males mate more often, and females tend to use their sperm. A female prefers a new male to one she has already mated with. She can mate again about 18 hours after giving birth, so she can nurse one litter while carrying the next.

Rats breed all year where conditions allow, with peaks in the warmer months. Gestation lasts 21 to 24 days. A litter can hold up to 14 pups, and seven is common. A female can produce up to five litters a year, and by one estimate as many as 60 young. Newborns weigh about 6 grams, and their eyes open at 14 to 17 days. Weaning comes at three to four weeks, and the young can breed at about five weeks in the best conditions.

Several females often share a nest and nurse each other's pups. If a mother dies, another female takes over her litter. Males take no part in raising the young. Wild rats seldom live beyond a year, and about 95 percent die each year. The wild maximum is about 3 years, and a captive rat can reach 4.

Predators and threats

Cats, dogs, foxes, owls, hawks, snakes and many other carnivores eat brown rats. People kill far more rats than any predator does, with poison, traps, gas and dogs. The rat's own kind is a threat too, since fights over space and food kill many. The parasite Toxoplasma gondii can change an infected rat's fear of cats, which makes it easier prey.

Warfarin and later anticoagulant poisons are the main weapon against rats. Rats that survive poison can store the residue in the liver and pass it on to owls and hawks that eat them. Rats also carry lice, fleas, ticks and worms. None of this threatens the species, which the IUCN lists as Least Concern.

Population and conservation

Nobody has counted the world's brown rats, and city figures are guesses. Estimates for New York City range from about 250,000 to almost 100 million. In 2023 New York appointed its first official to lead rat control, with a focus on rubbish rules and trapping. One British estimate puts 81 million rats in the United Kingdom, or 1.3 for every person, helped by mild winters. The saying that a city holds one rat per person is an urban myth for most cities.

The conservation effort runs the other way: people try to remove rats, not save them. Alberta started a rat-control programme in 1951 and has stayed almost rat-free ever since. In 2012 the province dug out and killed 147 rats in a landfill at Medicine Hat, and it recorded zero infestations again in 2013. Keeping a pet rat in Alberta can cost a fine of up to 5,000 Canadian dollars or 60 days in jail. Hawadax Island in Alaska was declared rat-free in 2009, and its shorebirds have since recovered. New Zealand has cleared rats from offshore islands and fenced reserves to protect ground-nesting birds.

Relationship with humans

The brown rat is one of the most damaging animal pests in the world. It eats and fouls stored grain, gnaws buildings and wiring, and kills poultry and game birds. One estimate puts its damage in the United States at almost 1 billion dollars a year. It can carry Weil's disease, rat-bite fever, Q fever, hantavirus and other infections. In one survey of New York sewer rats, 17 percent had caught the COVID-19 virus. It is often blamed for the Black Death, but the black rat and its fleas carried that plague.

On the other side of the ledger stands the laboratory rat. Bred from white rats rescued from the sport of rat baiting, it became a model organism for medicine, genetics and psychology. When a biologist says rat, this species is almost always meant. Fancy rats are popular pets, with breed clubs in Britain, the United States and Australia. Trained working rats, mostly domestic brown rats, do tasks for people. Wild rats bite when cornered, and there are reports of rats attacking human infants.

In culture and myth

In the Chinese zodiac the Rat comes first among the twelve animals. In the Great Race story it crossed the river riding the ox, then leapt ahead at the bank to win. It also failed to wake the cat, which explains why cats hunt rats and have no year of their own. People born in a Rat year, such as 1996, 2008 or 2020, are said to be clever, creative and quick-tempered. In Hindu tradition the rat carries the god Ganesha, and rat statues stand in his temples. At the Karni Mata temple in Rajasthan about 20,000 rats are fed as holy animals, and a white one is a sign of luck.

In Europe the rat has a bad name. To rat on a friend means to betray him, and an informer is called a rat. The town of Autun in France held a formal trial of its rats in 1522 for destroying the crops. The Pied Piper of Hamelin is a German tale set in the late 13th century. A piper charms the rats away with music, and after the town withholds his fee he leads the children away too. On the Isle of Man people avoid saying the word rat at all. Modern stories swing from villains such as Professor Ratigan to the cook Remy in Ratatouille.