The house mouse, considered as a tenant.
The house mouse weighs about as much as a 2-dollar coin. From nose to base of tail, the body is around seven to ten centimetres. The tail adds another seven or eight. The fur is light brown or grey, the belly a paler cream. The eyes are black and proportionally large. None of these facts on their own are surprising. What is surprising is how completely this small animal has adapted to live alongside us.
The house mouse is, after humans, the most widely distributed mammal on the planet. It lives on every continent except Antarctica. It crossed the oceans in our ships. It crossed the prairies in our grain wagons. It has been a part of every human settlement, large or small, for at least ten thousand years. By the time you find one in your kitchen, it has been doing this kind of work for a very long time.
The size of the door
The single most useful number to know about a mouse is the size of the gap it can squeeze through. A house mouse can pass through an opening approximately six millimetres wide — roughly the diameter of a standard pencil. The mouse does this by collapsing its skull and rib cage slightly and threading itself through. It is not magic. It is just very efficient anatomy.
What this means in practice is that almost every gap in a building's exterior is a potential entry point. The space where a gas line enters the foundation. The clearance around a dryer vent. The gap under a poorly hung garage door. The hole drilled for a cable that was later abandoned. Anywhere you can see a sliver of light from the outside, a mouse can enter.
What they actually do all night
Mice are crepuscular — most active at dusk and dawn — with secondary activity peaks through the night. During the day, they sleep in nests built from shredded paper, fabric, insulation, dryer lint, and any other soft material they have stolen. The nest is usually within a few metres of a food source. The food source does not need to be obvious. A bag of pet food. Crumbs behind a stove. A spilled bag of birdseed in a garage. A neglected pantry shelf.
A house mouse needs to eat several times a day but in tiny portions. A typical individual consumes about three grams of food daily. This is why a single mouse can live for weeks in a building before anyone notices. The total damage to the food supply is minimal. The droppings are small. The mouse itself is rarely seen.
What does get noticed is the byproducts. Mice produce somewhere in the range of fifty to seventy-five droppings per day, each about the size of a grain of rice, dark brown to black, slightly pointed at the ends. They are deposited along travel routes and inside the nest. A small infestation produces remarkable quantities of evidence within a week.
The reproductive math
A female mouse reaches sexual maturity at six weeks. Gestation is about twenty days. A litter typically contains five to eight pups. After giving birth, the female can be pregnant again within twenty-four hours. Under good conditions — meaning a warm building with reliable food — a single female can produce five to ten litters per year.
Run those numbers and the implication is clear. A single pregnant female mouse who finds her way into a triplex in October can, by April, have produced descendants in the dozens. This is why even one mouse in a building is not a "small problem." It is an early-stage version of a larger problem.
How they find their way around
The mouse has poor distance vision but excellent senses of smell, hearing, and touch. The long whiskers around its face are highly sensitive — they map the texture and width of every surface it passes. Once a mouse has explored a route, it tends to use the same route repeatedly. The route runs along walls, against furniture edges, and through the same gaps. This is part of why mice deposit so many droppings in repeating patterns: they are using the same paths.
Mice are also strong climbers. They scale rough vertical surfaces — brick, stucco, wood siding, rough plaster — without difficulty. They use the inside of a wall cavity as a vertical highway. A mouse that enters the basement of a triplex can be in the attic by morning.
What makes a Montreal building so suitable
The factors that draw mice into a building are the same factors that define most older Quebec residential housing. Insulation is variable. Wood-frame construction leaves voids and runs that mice navigate easily. Pipe penetrations are often loosely sealed or not sealed at all. Garage and rear doors are sized to swell and shrink with humidity, opening and closing the gap at the threshold seasonally. The neighbouring building, if it is attached, has all the same characteristics, and the wall between the two is rarely tight enough to stop a small mammal.
This is why mouse populations move between buildings on the same block. It is why a renovation that disturbs one building can drive its mouse population into the two next door. And it is why one apartment with mice often signals a broader building or neighbourhood pattern.
What they actually carry
Mice are documented vectors of several pathogens of public health concern, including hantavirus (transmitted through aerosolized contact with droppings or urine), salmonella, and leptospirosis. The risk of any individual person contracting one of these is low. The risk goes up sharply when droppings are disturbed without proper precautions — sweeping a basement floor that has not been cleaned in years, for example, can aerosolize particles that should have been wet-cleaned with disinfectant first.
The standard health-authority guidance is to wet-clean, never dry-sweep, areas with mouse droppings, and to ventilate the space well during cleaning. This applies whether the infestation is current or historical.
Why they are not easy to catch
People expect a mouse to behave like a cartoon mouse — bold, curious, drawn to cheese. The real animal is wary. It investigates new objects in its environment cautiously and incrementally. A snap trap placed in a high-traffic area is often ignored for several days because it is a new object. The mouse will pass near it, sniff it, and continue on its established route. Only after the trap has been there for a while, blending into the environment, will the mouse approach it directly.
This is why effective trapping uses many traps placed early and along established routes. It is also why one or two traps in a kitchen, hopefully placed and rarely moved, are usually ignored.
Related reading: why NDG triplexes have so many mice, why basement nests are so common in the suburbs, and whether one mouse means there are more.
The honest takeaway
The house mouse is not a problem you can prevent by being clean, in the everyday sense of clean. The species is too well-adapted to human structures and too good at exploiting overlooked entry points. What you can control is whether the gaps it could use are sealed, whether the food it might find is contained, and whether the population in your area is being monitored. The rest is biology doing what it has done since the first granaries were built.
Hearing scratching in the walls?
An inspection finds the entry points, not just the droppings. That is what stops them.