Brown rat

• 1. Introduction

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  The brown rat, Rattus norvegicus, goes by more names than almost any other creature on Earth: common rat, street rat, sewer rat, wharf rat, Hanover rat, Norway rat, Norwegian rat. Yet nearly every one of those names is wrong, or at least misleading. A rodent that did not come from Norway, was never especially confined to sewers, and is far from common in the sense of ordinary. It is, in fact, one of the most successful mammals ever to share a planet with humans.

  This is a story about an animal that spread to every continent except Antarctica, evolved distinct genomic profiles block by block in New York City, and learned to resist the very poisons designed to kill it. It weighs as little as 140 grams. It has outlasted empires. How did a rodent from the plains of northern China become the dominant rat on Earth? And what does its relationship with us reveal about the strange, entangled nature of urban life?

• 2. A Name Built on Mistakes

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  John Berkenhout, an English naturalist, handed the brown rat one of science's great misnomers when he published his 1769 book Outlines of the Natural History of Great Britain. Berkenhout assigned the species the binomial name Mus norvegicus, convinced it had arrived in England from Norwegian ships in 1728. The name stuck even as evidence against it piled up.

  The label "Hanover rat" had actually appeared even earlier, coined by people in 18th-century England who wanted to attach the nuisance to the House of Hanover. By the early to the middle of the 19th century, British academics were already walking back the Norway theory, proposing instead that the rat might have come from Ireland, Gibraltar, or arrived with William the Conqueror across the English Channel. Charles Dickens engaged the debate directly in his weekly journal, All the Year Round, pointing out a rather obvious flaw: when the brown rat had become common in England, it was essentially unknown in Norway.

  An 1895 natural history text by American scholar Alfred Henry Miles moved the origin story closer to the truth, suggesting the rat had travelled from Persia to England less than two hundred years prior. By the 20th century, naturalists had settled on Central Asia and, most likely, China as the genuine homeland. Today the species is understood to have possibly originated from the plains of northern China and Mongolia, spreading outward along routes of human migration.

• 3. Body, Senses, and Speed

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  At its largest, a wild brown rat's body can reach 28 cm in length, with a tail slightly shorter than that. It can weigh up to 500 g, roughly twice as much as a black rat and many times the weight of a house mouse. Stories of rats reaching the size of cats are exaggerations, or misidentifications of larger rodents.

  The senses that make the brown rat so effective are not the ones humans tend to prize. Its vision is poor, around 20/600 in pigmented individuals, and even worse in albino variants at roughly 20/1200. It perceives color somewhat like a person with red-green colorblindness, and the saturation of what it does see may be quite faint. But its blue-light receptors also detect ultraviolet, giving it access to a spectrum of the world that humans cannot see. Its hearing extends into ultrasound, and its sense of smell is highly developed.

  The heart of a brown rat beats at 300 to 400 times per minute, with a respiratory rate of around 100 per minute. That metabolic urgency extends to reproduction: a female can produce up to five litters a year, each potentially numbering up to 14 pups, after a gestation of just 21 days. Pups reach sexual maturity in about five weeks. Under ideal conditions, a population can grow from two individuals to around 15,000 in a single year.

• 4. What They Eat and How They Learn

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  Brown rats near a hatchery in West Virginia have been observed catching fingerling fish. Colonies along the banks of the Po River in Italy dive for mollusks, a practice understood to represent social learning within the species. Rats on the island of Norderoog in the North Sea stalk and kill sparrows and ducks.

  The brown rat is a true omnivore, and cereals make up a substantial part of its diet. Its preferred foods include scrambled eggs, raw carrots, and cooked corn kernels. Its least-liked include raw beets, peaches, and raw celery. In metropolitan areas it survives mainly on discarded human food, and examination of wild brown rat stomachs in Germany turned up 4,000 distinct food items in a single study, most of which were plants, though the rats preferred meat when given a choice.

  A 2007 study found brown rats to possess metacognition, a mental ability previously identified only in humans and certain other primates. Subsequent analysis suggested the rats may instead have been following operant conditioning principles rather than genuine self-reflection, but the debate itself speaks to the complexity of the animal. Social knowledge clearly passes between individuals: the Po River diving behavior would be difficult to explain otherwise.

• 5. The Language of Chirps and Bruxing

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  At seven days old, rat pups produce ultrasonic cries to direct their mother toward them and to regulate her movements in the nest. By 14 days, they have already learned to reduce those calls around male rats as a defensive response.

  Adult rats emit high-frequency vocalizations when they detect predators or sense danger, and the pitch and duration of those calls varies with the sex and reproductive status of the individual. In more relaxed circumstances, rats produce a short, high-frequency sound during rough-and-tumble play, before receiving morphine, or when tickled by a researcher. The sound has been described as a distinct chirping and likened to laughter. It is associated with positive emotional states and social bonding, and rats conditioned to seek tickling from a human handler will actively pursue that interaction. As rats age, the tendency to chirp appears to decline.

  Bruxing, or teeth-grinding, is a sound audible to humans and most commonly triggered by contentment, though it can also serve as self-comforting in stressful situations like a veterinary visit. Vigorous bruxing is sometimes accompanied by boggling, in which the eyes rapidly bulge and retract due to the movement of jaw muscles behind the eye socket. Bat detectors are often used by pet owners to pick up the ultrasonic range of rat vocalizations.

• 6. Conquest and Resistance

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  Reliable historical records place the brown rat in Ireland in 1722, England in 1730, France in 1735, Germany in 1750, and Spain by 1800. It did not reach North America until around 1750-1755. In the early 20th century, as recently as 1925 to 1927-50% of ships entering the port of New York were rat-infested.

  As the species moved across Europe, it systematically displaced the black rat. The brown rat had advantages beyond size and aggression: the shift from wooden structures and thatched roofs to brick and tile buildings favored burrowers over the arboreal black rat. Brown rats also tolerate a broader range of foods and weather extremes.

  By the mid-20th century, wild populations in urban areas had begun developing resistance to warfarin, the anticoagulant rodenticide that had replaced arsenic as the control tool of choice. Warfarin was approved as a drug for human use in 1954, but some rat populations were already evolving around it. Resistance to second-generation rodenticides has also appeared in some United Kingdom populations. A behavioral shift compounded the problem: widespread rodenticide use produced a strong fear of novel stimuli in wild rats, making effective bait placement far harder. Some rats without the genetic adaptations for resistance still accumulate rodenticide anticoagulants in their livers, sometimes above known toxic thresholds, a pattern associated with secondary poisoning of predators, particularly raptors.

• 7. Alberta's War

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  The first brown rat did not reach Alberta until 1950. A year later, the province launched a rat-control program that included shooting, poisoning, gassing, and the bulldozing or burning of rat-infested buildings. In the first year alone, 64 tonnes of arsenic trioxide were spread across 8,000 buildings along the Saskatchewan border.

  Legislation required every person and every municipality to destroy rat populations and prevent new ones. If they failed, the provincial government could carry out the measures itself and bill the landowner. By 1960, infestations had dropped to below 200 per year. In 2002, Alberta recorded its first year with zero infestations. From 2002 to 2007, only two infestations were found.

  Alberta is the largest rat-free, human-populated area in the world. Its geography helps: the boreal forest to the north and the Rocky Mountains to the west are impassable, while the semiarid High Plains of Montana to the south present a crossing the rats cannot safely make. The only viable entry point is a stretch of the eastern border with Saskatchewan, and even there, Alberta defines an infestation as two or more rats in the same location, since a single rat cannot reproduce. About a dozen single rats enter the province in an average year and are killed before they can find a mate. Possession of an unlicensed rat, including a pet fancy rat, carries a penalty of up to C$5,000 or up to 60 days in jail.

• 8. Evolution in City Blocks

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  New York City rats have longer noses and shorter molar rows than the Chinese population. Researchers hypothesize the longer nose is an adaptation to a colder climate, while the shorter molar rows may reflect a diet built around processed human food. Genetic markers also show differences in regions associated with metabolism and diet.

  Urban geography structures these differences directly. Roadways and districts with low levels of garbage act as barriers, separating rat populations and restricting gene flow between them. Brown rats in cities tend not to wander more than 20 m from their nest when a concentrated food source is available. That limited range means populations on different city blocks can evolve distinct genomic profiles over time, as has been documented in New York, Vancouver, and Salvador.

  In 2023, New York City appointed Kathleen Corradi as its first Rat Czar, a position created specifically to address the city's rat population through garbage regulation and additional trapping. Estimates of how many rats actually live in the city range wildly, from almost 100 million to as few as 250,000. Experts point to the city's aging infrastructure and high poverty rates as factors that make it particularly hospitable. A 2024 study using Vancouver data found that most municipal complaints about rats were linked to garbage, framing the animal less as a pest problem and more as a signal of deeper community conditions.