Commensal rodents are species that live in close association with humans, depending on our buildings, food, and water for survival. The term “commensal” literally means “sharing the table,” and it applies to three species in particular: the house mouse (Mus musculus), the Norway rat (Rattus norvegicus), and the roof rat (Rattus rattus). These three have followed human civilization across the globe and are now found on every continent except Antarctica, thriving wherever people store food, generate waste, and build structures.
The Three Commensal Species
Each of the three commensal rodents has distinct physical traits and behavioral tendencies that determine where and how it lives alongside people.
The house mouse is the smallest, with adults weighing less than one ounce. It has a slender body, a pointed muzzle, and a nearly hairless tail that runs about the same length as its head and body combined. Coloring is typically grayish-brown with a slightly lighter belly, though house mice can range from black to white. Their small size allows them to squeeze through gaps as narrow as a pencil’s width, making them the most difficult of the three to exclude from buildings.
The Norway rat is the largest and most widespread. Adults average about one pound, with a stocky body, blunt nose, and brownish or reddish-gray fur. Its tail is shorter than its head and body combined, which helps distinguish it from the roof rat. Norway rats are ground dwellers. They dig burrows along foundations, under debris piles, and in basements or sewers.
The roof rat is more slender than the Norway rat, with larger eyes and ears, a pointed muzzle, and a tail that extends beyond the length of its body. Its fur tends to be darker, often blackish with a lighter belly. True to its name, the roof rat prefers elevated spaces. It nests in attics, trees, vine-covered fences, and upper floors of buildings. In warehouses where both species coexist, roof rats typically occupy the upper levels while Norway rats stay on the ground floor or basement.
How They Behave Around People
Commensal rodents are primarily nocturnal, beginning their search for food shortly after sunset. All three species are omnivorous, but their preferences differ. Roof rats favor fruit and nuts, with foraging habits that resemble those of tree squirrels. Norway rats and house mice are less selective and will eat nearly anything available. All three species require water daily, though roof rats can sometimes get enough moisture from high-water-content foods like fruit.
Roof rats are remarkably agile climbers, able to scale walls, travel along power lines, and leap between branches. They will travel 100 to 300 feet from their nesting site to reach food and often carry large food items to a hiding place before eating. Many rats cache or hoard solid food for later consumption.
Both rat species display neophobia, a wariness of new objects in their environment. Roof rats are especially cautious and may avoid a newly placed trap or bait station for several days. House mice, by contrast, are curious and tend to investigate new objects quickly, which makes them easier to trap but harder to control with bait stations since they nibble small amounts from many sources rather than feeding heavily at one location.
Reproductive Capacity
One reason commensal rodents are so difficult to control is their reproductive speed. Norway rats reach sexual maturity at two to three months of age, and gestation lasts only about 22 days. A single litter can range from 2 to 22 pups, with 10 being typical. Females can become pregnant again almost immediately after giving birth, meaning a single breeding pair can produce several generations in a single year. House mice reproduce at a similar pace, with slightly smaller litters but an even earlier onset of sexual maturity. This rapid turnover means that a small population can explode in weeks if food and shelter are available.
Diseases They Carry
Commensal rodents are reservoirs for a range of infectious diseases that can spread to humans. The major ones include rat bite fever, leptospirosis, hantavirus, lymphocytic choriomeningitis virus (LCMV), tularemia, salmonellosis, campylobacteriosis, and giardiasis.
Transmission routes vary. Rat bite fever spreads through bites, scratches, or contact with rodent urine, saliva, and feces. It can also be acquired by eating or drinking contaminated food or water. Tularemia spreads the same ways but can additionally be transmitted by infected ticks or by inhaling aerosolized particles from rodent waste. Hantavirus, LCMV, and leptospirosis are typically shed in the urine of infected animals, and people become infected by inhaling, accidentally ingesting, or having direct skin contact with contaminated material. Notably, rodents carrying these pathogens usually show no visible signs of illness themselves. Salmonellosis, campylobacteriosis, and giardiasis spread through contact with or accidental ingestion of rodent fecal matter.
The sheer volume of waste these animals produce amplifies the risk. Norway rats and house mice leave 20 to 50 droppings per day. Rats excrete roughly half an ounce of urine per night, and mice deposit hundreds of micro-droplets along their travel routes. In a kitchen, pantry, or food storage facility, this contamination accumulates quickly.
Economic and Structural Damage
Beyond disease, commensal rodents cause significant economic harm. An adult rat consumes about 50 pounds of grain per year, while a mouse eats about 2 pounds. Although grain consumption by rodents in storage is estimated at around 1% of the total, severe infestations can cause substantial profit loss, especially when you factor in the larger problem: contamination. The USDA tolerates only one rodent dropping per 1,000-gram sample of corn grain, and any amount of urine contamination can render stored food unmarketable.
Structural damage is equally serious. Rodents gnaw constantly to wear down their continuously growing incisors, and they chew through wood, plastic, soft metals, and electrical wiring. This last habit is particularly dangerous. Rodents are estimated to cause 20 to 25 percent of all fires of unknown origin in the United States by chewing through electrical wiring and gas lines. A gnawed wire can create an arc fault that ignites surrounding insulation or wood framing, often in wall cavities where the damage is invisible until the fire starts.
Resistance to Rodenticides
Chemical control of commensal rodents has grown more complicated over the decades. The first widely used anticoagulant rodenticide, warfarin, was introduced in the mid-20th century. By 1958, resistant Norway rat populations were already documented in Scotland. Resistance spread rapidly through Europe and the United States, driven by the fact that resistance genes confer a high degree of immunity to warfarin.
Second-generation anticoagulants were developed specifically to overcome this resistance. They work through the same basic mechanism (blocking the body’s ability to recycle vitamin K, which is essential for blood clotting) but are far more potent. However, resistance to several of these newer compounds has now been reported in certain areas of Europe and elsewhere. This ongoing arms race between chemistry and genetics is one reason that modern rodent control relies less on poisons alone and more on integrated strategies.
Integrated Pest Management
Effective control of commensal rodents combines multiple tactics rather than relying on any single method. The process starts with a detailed inspection: identifying the species present, locating nests and entry points, and understanding why rodents are there in the first place. Signs to look for include droppings, gnaw marks, footprints, and sebum marks, which are oily brown smudges that accumulate along walls and beams where rodents travel repeatedly.
Exclusion is the most durable form of control. This means sealing gaps in foundations, walls, and rooflines with materials rodents cannot chew through, such as steel wool, hardware cloth, or metal flashing. Even small openings matter, since a house mouse can fit through a hole the size of a dime.
Sanitation removes the incentive for rodents to stay. Store food in chew-proof containers. Clean up spills promptly, including hard-to-reach spots like the sides of stoves and the backs of pantry shelves. Fix leaky faucets and eliminate standing water. House mice in particular will build nests indoors near a food source, favoring dark, warm, undisturbed spaces like the gaps behind appliances, inside stove walls, or near refrigerator motors.
Trapping offers several advantages over baiting. You know exactly where a rodent has died, which avoids the problem of carcasses decomposing inside walls and attracting secondary pests. Traps can also be placed systematically to reveal high-activity areas and entry points. Use the correct trap size for the species: rat traps and mouse traps are not interchangeable. For rats, which are neophobic, place baited but unset traps first and let the rats take bait two or three times before actually setting the trap. Then reduce the bait to a small amount so the rat must engage the trigger mechanism to reach it.
Bait stations containing rodenticides remain an option for large or persistent infestations, but they carry risks of secondary poisoning to pets and wildlife, and they don’t solve the underlying problem. Without exclusion and sanitation, new rodents simply move in to replace the ones that were killed.