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Recognizing Mice and Rats in Ontario – Appearance, Behaviour, and Dangers

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Mouse & Rat Library

House Mouse 

Appearance: Small (6–10 cm body length), dusty gray with a lighter underside. Long, hairless tail nearly the same length as the body.

Habitat: Nests in walls, attics, basements, storage areas, and near food sources. Found in homes, cottages, barns, and commercial buildings year-round.

Behaviour: Nocturnal, fast breeders, and excellent climbers. Enters through tiny gaps as small as 6 mm.

Risk to Humans: Contaminates food and surfaces with urine and droppings. Can spread disease and trigger allergies.

Interesting Fact: A single pair can produce dozens of offspring annually, leading to fast-growing infestations.

The House Mouse (Mus musculus) is the most common mouse species found in Ontario homes and businesses.

Deer Mouse - "Field Mouse"

Appearance: Small (7–10 cm body), brown back with a white underside and a distinctly bi-colored tail.

Habitat: More common in rural areas, cottages, farms, and wooded regions. Nests in sheds, cabins, garages, and stored materials.

Behaviour: Active year-round. Less likely to infest homes long-term but often enters buildings in fall or winter to escape the cold.

Risk to Humans: Can carry hantavirus, which is transmitted through droppings, urine, or nesting materials.

Interesting Fact: Unlike house mice, deer mice have excellent jumping ability and are more commonly found in unoccupied buildings or seasonal properties.

White-Footed Mouse

Appearance: Similar to the deer mouse—small and brown with white feet, white underbelly, and a slightly shorter tail.

Habitat: Found in wooded or brushy areas, including around cottages and lakefront properties.

Behaviour: Solitary and nocturnal. Enters homes in fall for shelter. Nests in walls, basements, and sheds.

Risk to Humans: Potential vector for Lyme disease (via ticks). Droppings may also carry hantavirus.

Interesting Fact: UMore common in southern Ontario than deer mice, especially near conservation land and forested properties.

Norway Rat – "Brown Rat"

Appearance: Large and stocky (20–25 cm body length plus tail), brown or gray with a blunt nose, small ears, and a thick tail shorter than its body.

Habitat: Nests in basements, sewers, crawl spaces, and burrows near foundations or garbage. Common around urban and rural properties.

Behaviour: Excellent diggers and swimmers. Forages mostly at night and follows regular routes.

Risk to Humans: Can spread diseases, contaminate food, and cause structural damage by gnawing.

Interesting Fact: Despite the name, Norway rats likely originated in Asia and are now widespread in Ontario’s cities and farms.

Roof Rat – "Black Rat"

Appearance: Sleek body, 35–45 cm including tail. Black or dark brown fur, pointed nose, large ears, and long thin tail.

Habitat: Rare in Ontario but may be found in port cities or transported indoors accidentally. Prefers high places—attics, roofs, rafters.

Behaviour: Excellent climbers. More cautious than Norway rats. Nests above ground in walls, ceilings, or sheds.

Risk to Humans: Like other rats, can carry disease and contaminate food. Presence indoors requires immediate pest control.

Interesting Fact: Sometimes called “ship rats” due to their history of traveling in cargo. Rare but increasingly reported in southern Ontario.

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