Rats are highly successful urban and suburban mammals, and the Norway rat (Rattus norvegicus) is the primary species responsible for digging extensive ground burrows in human environments. Burrows are necessary for survival, serving as a secure harbor from predators and human interference. They provide a stable microclimate, offering insulation from extreme heat and cold, important for maintaining body temperature. The subterranean design is also used for nesting, creating a safe location to bear and raise multiple litters of young.
Burrow Architecture and Function
The physical structure of a rat burrow begins with an entrance hole, typically a round opening measuring between two and four inches in diameter. Freshly excavated burrows often have a fan-shaped pile of loose dirt near the entrance, but established burrows feature packed earth and smooth, worn edges from repeated use. The entrance leads into a tunnel system that can extend horizontally for several feet and reach depths of 12 to 18 inches below the surface, though burrows dug near foundations can be as deep as four feet.
These tunnel systems are more complex than a simple hole, containing specialized chambers and multiple exit points. Nesting chambers are lined with shredded materials (paper, cloth, or insulation) providing a soft, warm environment for the female and her young. The system also includes food storage areas and multiple secondary openings known as “bolt holes.” Bolt holes are small, concealed escape routes allowing rats a quick exit if the main entrance is threatened or the burrow floods. The architecture functions as a protected base, enabling nocturnal rats to forage for food before returning to the safety of the underground network.
Typical Colony Size in a Single Burrow System
The number of rats inhabiting a single burrow system is highly variable and depends on the system’s maturity and the local availability of resources. A newly established burrow may house a family unit of a few adults and their immediate offspring, potentially numbering between five and twelve individuals. However, a “burrow” often refers to an interconnected system of tunnels and chambers that collectively house an entire colony.
In environments with abundant food and shelter, such as urban areas or landfills, these systems support much larger, highly social colonies. Mature systems can contain dozens of rats, with large colonies potentially exceeding 150 members. The social structure is complex; while each female typically has her own nest chamber, multiple related females may share the communal burrow and raise their young together. At high population densities, the social system becomes despotic, meaning a dominant male controls the group. This hierarchy is constantly tested as the population expands and resources are stretched.
Indicators of Large Rat Colonies
The severity of a rat infestation can be determined without excavating the burrow by looking for specific external signs of high density. A reliable indicator of a large, established colony is the number of active burrow entrances, suggesting an extensive and well-used tunnel network. Continuous travel creates visible “runways,” which are well-worn paths through grass and low vegetation marked by oil and dirt smears.
The sheer volume and variety of droppings also signals a dense population, as a single active rat can produce up to 40 droppings per day. Finding droppings of various sizes in one location indicates a mix of older and younger rats, confirming a reproductive and established colony. The presence of rats during daylight hours is a strong sign of a large population and resource scarcity. Rats are primarily nocturnal and only risk daytime activity when forced by hunger. Auditory signs, such as constant gnawing, scurrying, or squeaking noises coming from walls or beneath structures, also point to a significant number of animals.