Rats are common pests, and for several species, creating underground shelters is fundamental to their existence. Burrowing is an organized behavior driven by the need for protection and reproduction. These subterranean networks are complex, serving as the primary habitat for entire colonies. Understanding which species burrows and the structure of their tunnels is the first step toward identifying and managing their presence.
Species That Burrow and Their Motivation
The two most common commensal rat species in North America, the Norway rat (Rattus norvegicus) and the Roof rat (Rattus rattus), have different habits regarding burrowing. Norway rats are the primary burrowing species, preferring to live at ground level or below, which is why they are sometimes called sewer rats or brown rats. Their stout bodies are well-suited for digging and navigating tight, horizontal spaces.
In contrast, the slender, more agile Roof rat is an arboreal species that favors vertical spaces, such as attics, walls, and elevated nests in trees. While they can occasionally use existing ground cover, they do not typically excavate extensive burrow systems like their Norway rat counterparts. The Norway rat’s motivation for digging is threefold, centered on survival needs.
Burrows provide immediate protection from predators, offering a safe retreat from hawks, cats, and other threats. They also serve in thermal regulation, offering shelter from extreme weather, keeping the rats cool in the summer and insulating them during winter months. Finally, the underground chambers are used for nesting, creating a secure environment to raise their litters.
Identifying Active Rat Burrows
Identifying an active rat burrow relies on observing specific physical characteristics and surrounding signs of use. A typical Norway rat burrow entrance appears as a round hole in the soil, usually measuring between two and four inches in diameter. The location of the burrow is often strategic, found along solid structures like building foundations, under concrete slabs, or beneath dense cover such as woodpiles, overgrown vegetation, or riverbanks.
An active burrow is distinguished by the smooth, worn edges of the entrance, polished by the rat’s repeated movement. Freshly excavated dirt often forms a fan-shaped pile of loose soil outside the opening, indicating recent digging activity. Conversely, a hole covered in cobwebs, leaves, or debris is likely inactive.
Other signs of activity include visible runways, which are worn-down paths in the grass or soil leading to the entrance, often marked by greasy track marks. A simple method to confirm activity is the “tracking test”: sprinkle a thin layer of flour or talc near the entrance; fresh footprints the next day confirm the burrow is currently in use. The presence of fresh, capsule-shaped droppings near the hole is a strong indicator of a current infestation.
Structure and Complexity of Rat Burrows
Rat burrows are intricate, multi-chambered tunnel systems designed for colony life and safety. These networks can extend several feet horizontally and typically reach a depth of 12 to 18 inches below the surface, though they may be excavated up to four feet deep near building foundations. The tunnel segments allow only one rat to pass at a time and often branch off into various sections.
A complex burrow system features a main entrance and multiple secondary exit holes known as “bolt holes.” These bolt holes are usually less obvious and serve as emergency escape routes when the main entrance is threatened. Bolt holes often lack the excavated soil pile found at the primary entry point. Within the network, the rats excavate specialized chambers.
One or more chambers are dedicated to nesting, where the rats build soft beds lined with materials like shredded paper, grass, or fabric to house their litters. Other chambers are utilized for food storage when the colony is provisioning for the young. The entire structure is dynamic, with rats continuously digging new tunnels, extending chambers, and modifying the network to accommodate the colony’s growth.