House Mice

Identification | Biology | Damage ID | Management | Resources

Figure 1. House mouse (Mus musculus). Photo by US Department of Health and Human Services-National Institutes of Health. 

Learning Objectives 

  1. Understand how the physical abilities of mice apply to their control. 
  2. Describe how to inspect for mice and how to identify their scat.  
  3. Explain the techniques used to manage damage by mice and causes of failure.  

Identification 

Native to central Asia, house mice (Mus musculus) arrived in North America with settlers from Europe. Mice often live in close association with humans and therefore are called “commensal” (table-sharing) rodents. Mice are more common in residences and commercial structures than rats. They are considered among the most troublesome and economically important rodents in the US. 

Legal Status 

House mice are not protected by laws and are considered pests. They can be controlled using pesticides that are registered for the control of mice, or they may be trapped.  

Physical Description 

House mice are small, slender rodents with a slightly pointed nose; small, black, somewhat protruding eyes; large, sparsely haired ears; and a nearly hairless tail with obvious scale rings (Figure 1). House mice are gray-brown with a gray or buff-colored belly. Similar species include deer, white-footed, and jumping mice, which have white bellies that contrast sharply with the brown dorsal fur. Harvest mice have grooves on their upper incisors. For more details on identification of species, consult a field guide to mammals.  

Adult house mice have a body length (nose to base of tail) of 3 to 4 inches and a tail length of 2 to 4 inches. They typically weigh 0.4 to 0.9 ounces.  

Species Range 

House mice exist across the continental US, Hawaii, and Alaska (Figure 2).   

Figure 2. Distribution of the house mouse in the US.  Image by Stephen M. Vantassel. 

Voice and Sounds 

Mice emit a high-pitched squeak. Their movements, especially on drop ceilings, can be loud. 

Tracks and Signs 

Tracks, including footprints and marks from tails, may be seen on dusty surfaces or in mud (Figures 3 and 4). A tracking patch made of flour rolled smooth with a cylindrical object can be placed overnight in pathways to determine if rodents are present. 

Figure 4. Tracks of house mice in flour. Photo by the University of Nebraska-Lincoln (UNL). 
Figure 4. Tracks of house mice. Image by Dee Ebbeka. 

Droppings may be found along run-ways, in feeding areas, and near shelter. Droppings of house mice average ¼ inch long, whereas those of cockroaches usually are 1/to ¼ inch long and under a magnifying glass, show distinct longitudinal ridges and squared-off ends. In comparison, droppings of bats contain fragments of insects and are easily crushed.  

Urine, both wet and dry, will fluoresce under ultraviolet light as do many other items, such as cleaning products. Look for tiny droplets, ranging in size from a couple of millimeters to microscopic, in places where mice are likely travel or feed.  

Marks from gnawing may be visible in corners and materials in walls and on doors, ledges, stored materials, and other surfaces wherever mice are present. Mice gnaw marks are 1/32 inch while rat gnaw marks are 1/16 inch in diameter. Fresh accumulations of wood shavings, insulation, and other material that has been gnawed indicate activity. Size of entry holes, often 1½ inches in diameter or less, can be used to distinguish gnawing by rats from gnawing by mice. Mice keep their paired incisor teeth, which grow continuously, worn down by gnawing on hard surfaces and by working the teeth against each other.  

Smudge and rub marks may occur on beams, rafters, pipes, walls, and other parts of structures. They are the result of oil and dirt rubbing off mouse fur along frequently traveled routes (Figure 5). They may be less apparent than those left by rats.  

Sounds of gnawing, climbing in walls, running across the upper surface of ceilings, and squeaks are common where mice are present. 

Visual sightings may be possible during daylight. Mice can be seen after dark with the aid of a flashlight, spotlight, or infra-red light.  

Nests may be found in garages, closets, attics, basements, and outbuildings where mice are present. Nests consist of fine, shredded, fibrous materials.  

Odors may indicate the presence of mice. A characteristic musky odor is a positive indication that mice are present, and this odor can be used to differentiate their presence from that of rats.  

Figure 5. Black smudge line created by house mice climbing the concrete to enter the hole in the metal.  
Photo by Robert M. Timm.