Mice and rats do not cohabitate in a shared, cooperative living arrangement. Both species are common commensal rodents, meaning they live in close association with humans, but their relationship is fundamentally one of separation driven by size and aggression. These distinct biological entities occupy parallel, non-overlapping territories. This separation is a survival mechanism for the smaller species, ensuring competition for resources does not escalate into lethal conflict.
Establishing the Difference: Taxonomy and Size
The house mouse (Mus musculus) and the common rat species, such as the Norway rat (Rattus norvegicus) and the roof rat (Rattus rattus), belong to the same family, Muridae, but they are members of entirely different genera. This taxonomic separation indicates they are not closely related enough to interbreed and have been evolving along separate lines. The most obvious difference between the genera is a stark contrast in physical size and weight, which dictates the nature of their interaction.
An adult house mouse typically weighs between 12 and 30 grams. In contrast, an adult Norway rat can easily weigh between 250 and 500 grams, making the rat up to 20 times heavier. This significant difference in mass establishes an immediate dominance hierarchy that prevents cohabitation.
The Primary Barrier: Predation and Direct Aggression
The primary reason mice and rats do not share space is that rats are predators of mice. This behavior, known as muricide, is a well-documented predatory action where rats actively hunt, kill, and consume mice, viewing them as a viable food source. Studies show that a high percentage of wild rats, sometimes over 70%, will exhibit this mouse-killing behavior.
The act of muricide is swift and stereotyped, often concluding with a single, fatal bite aimed at the mouse’s head, neck, or upper back. This behavior is not solely driven by hunger; laboratory experiments have shown that even well-fed rats may kill mice, suggesting hormonal or neurological components are involved. Following a successful kill, a rat often consumes parts of the mouse, particularly the brain, liver, and viscera, confirming the predatory nature of the interaction.
The mere presence and odor of a rat are enough to cause immense stress in mice, driving them out of an area to avoid being killed. For a mouse, the physical presence of a rat represents an immediate and existential threat.
Ecological Niche Partitioning
When mice and rats are found in the same structure, their coexistence is maintained not by cooperation, but by a complex strategy of spatial and resource separation called niche partitioning. They utilize different microhabitats within the same building to minimize direct conflict.
Mice, being smaller and more agile, tend to prefer higher, drier, and more secluded spaces like wall voids, cabinets, attics, and upper storage areas. Rats, due to their larger size, prefer to occupy the lower levels, basements, sewers, and deep ground burrows. This spatial segregation minimizes the chances of a lethal encounter, as the mouse can access small, inaccessible areas where the rat cannot follow.
Resource Needs
This physical separation is further reinforced by differences in their resource needs. While both species are omnivorous, their dietary requirements differ significantly. Rats require a larger volume of food and a consistent source of water, often leading them to exploit bulk food sources and plumbing systems. Mice can survive on smaller, scattered crumbs and extract most of the water they need from dry food, allowing them to thrive in more isolated locations.