The Norway rat, and the city beneath the city.
The Norway rat did not come from Norway. The eighteenth-century naturalist who named it had received specimens from a Norwegian port and assumed wrongly. The species in fact originated in the steppe grasslands of northern China and Mongolia, where it lived as a burrowing rodent on the margins of rivers and human settlements. From there it followed the rise of agriculture westward through Asia, into Europe by the early Middle Ages, and across the Atlantic on cargo ships during the seventeenth and eighteenth centuries. It arrived in North American port cities along with the people who paid for the ships. It has been here ever since.
Montreal received its first Norway rats in the late seventeenth century, almost certainly through the port. The species has been part of the city's underground ecology continuously for more than three hundred years. Whatever the rat population in the city looks like at any given moment, it is best understood not as an infestation in the temporary sense but as an established native fauna that we periodically attempt to manage downward.
The animal
An adult Norway rat measures roughly twenty to twenty-five centimetres from nose to base of tail, with a tail almost as long again. The fur is brown or greyish brown on the back, pale grey on the belly. The body is heavy and thick, the tail shorter and thicker than the body itself, the muzzle blunter than that of a mouse. Its weight when mature is in the range of two hundred to five hundred grams — roughly the weight of a coffee mug.
This animal is much larger than a mouse and physically very different. Where the mouse climbs and explores aerially, the rat burrows and works horizontally. Where the mouse is wary but generally curious, the rat is wary and persistently neophobic — that is, deeply suspicious of new objects in its environment. A new bait station placed in a rat's habitual route may be avoided for weeks. The species has not survived alongside us by being foolish.
Where they live in Montreal
The Norway rat is primarily a ground-level and below-ground animal. In Montreal it lives in:
The space underneath sheds, garages, and decks. Loose soil at the base of foundations. The voids left where backfill settled away from poured concrete. The narrow channels along the inside of sewer access pits. The slack space inside abandoned drain tile. Empty lots with overgrown vegetation. Restaurant alleys with reliable garbage and concealed access.
The rats dig. A rat burrow is typically a metre or so deep, with at least two entrances. The main entrance is the obvious one. A second, smaller "bolt hole" is hidden some distance away, used to escape when the main entrance is threatened. Inside the burrow, the rats maintain chambers for nesting, food caching, and waste. A productive burrow may shelter a dozen or more individuals.
Above ground, rats use travel routes along walls, fences, and curbs. They are guided by their sense of touch — the whiskers and guard hairs read the texture of surfaces — and they prefer to keep at least one body in contact with a solid edge. This is why rat droppings, when they appear inside a structure, are usually along the base of a wall and not in the middle of a room.
What they eat
A Norway rat is omnivorous. In a city, this means anything organic. Garbage, dropped food on the street, restaurant grease bins, dog waste, pet food set out on porches, fruit fallen from trees, seed spilled at bird feeders, compost that has not been properly contained. The animal needs roughly twenty to thirty grams of food per day and a steady source of water. The water requirement is part of what determines where rats can sustain a colony — a building without ongoing water access is a much less attractive long-term habitat than one with a leaky basement pipe or an exterior puddle that does not dry out.
This is also why a long, dry summer in Montreal sometimes corresponds to rats appearing in unusual places — they are searching wider for water.
The reproductive engine
A female Norway rat reaches sexual maturity at three to four months. Gestation is about twenty-two days. A typical litter contains seven to ten pups, though larger litters of fourteen or more are recorded. Females can become pregnant within twenty-four hours of giving birth. A single healthy female can produce four to six litters per year. In a stable colony with adequate food and water, the population doubles roughly every two months.
This is the math that makes a rat infestation different in character from a mouse infestation. With mice, the population grows quickly but at a small absolute scale. With rats, the population grows quickly at a scale that produces meaningful structural damage, ongoing visible activity, and substantial public-health concern.
What they carry
Norway rats are documented vectors of a number of pathogens of public-health significance. Leptospirosis, transmitted through contact with urine or contaminated water. Rat-bite fever, transmitted through bites. Salmonellosis. Hantavirus, though more commonly associated with deer mice. Historically, the species also carried the fleas that transmitted bubonic plague during European outbreaks, though Norway rats themselves do not transmit plague directly.
The risk of an individual urban resident contracting any of these is low under normal circumstances. The risk rises in situations involving direct contact with rat-contaminated water or surfaces, particularly during cleanup of infested spaces. Public health guidance consistently recommends professional removal and decontamination rather than attempting to clean active rat-infested areas without protective equipment.
Why they are difficult to eliminate
Treatment of an individual building can be effective. Sealing entry points, removing food and water access, deploying tamper-resistant bait stations, addressing the burrow systems immediately outside the structure — these methods, applied together and maintained over time, produce real and lasting reductions. What they cannot do is alter the fact that the city as a whole sustains a large Norway rat population in its underground infrastructure, and that population perpetually presses outward in search of new resources. Any individual building exists in equilibrium with that pressure. When the pressure rises — because of nearby construction, a sanitation disruption, a closure of a previously available food source — the rats appear at the next available building.
This is why responsible rodent management is treated as ongoing rather than one-time. The goal is not to certify a building rat-free in perpetuity. The goal is to maintain a structural and behavioural environment in which rats find no purchase, and to monitor for changes in conditions that would invite them back.
The historical scale
Estimating an urban rat population is famously difficult. Studies in other large North American cities have arrived at estimates ranging from one rat per several human residents to ratios approaching one-to-one, depending on methodology. No reliable figure is available specifically for Montreal in 2026 that we can cite. What is reliably known is that the species is present continuously across the city's older neighbourhoods, that population pressure varies with construction activity and sanitation conditions, and that the species has been a part of the city for as long as the city has existed in its modern form.
Related reading: why the Sud-Ouest neighbourhoods have so many rats, why Verdun rodent populations spike, and the related but smaller house mouse.
The honest takeaway
If you find a rat in your building, you are not the cause of it. The species has been waiting at the perimeter for a long time. What matters now is the timing of the response. The first individual found inside a building is the right moment to act. Waiting until there are several individuals — or until burrows are visible in the yard — turns a manageable situation into a more involved one. The Norway rat has been doing this work for centuries. It is patient. The response should be prompt.
Seeing rats outside? Hearing larger movements in walls?
Rats need a different protocol than mice. We do both, but they are not the same job.