The Norway rat (Rattus norvegicus), also known as the brown rat or sewer rat, is highly capable of swimming and diving underwater. This species, closely associated with human infrastructure, possesses physical and physiological adaptations that allow it to navigate water-filled environments, including short periods of complete submersion. Its comfort in aquatic settings is a key factor in its global success and its ability to utilize complex systems like municipal sewers for travel and shelter.
How Rats Navigate Water Surfaces
The Norway rat is an adept surface swimmer with high endurance. Propulsion is achieved through synchronized paddling of all four limbs, while the long tail acts as a rudder for balance and direction. The rat’s body is naturally buoyant, allowing it to float and conserve energy.
This buoyancy is enhanced by dense, coarse fur, which traps air bubbles close to the skin. The trapped air provides insulation against cold water and aids flotation, enabling the rat to tread water for long periods. Rats can maintain themselves on the water’s surface for up to 72 hours before exhaustion. This endurance allows them to cross significant bodies of water, with individuals recorded swimming distances of up to 400 meters.
Physiological Limits of Submersion
A rat’s ability to swim underwater is linked to its capacity for breath-holding, which it can sustain for several minutes when motivated. Brown rats can hold their breath for 90 seconds to three minutes. This short-term diving is an intentional action, distinct from prolonged water survival.
The physiological response enabling this is the mammalian diving reflex, triggered when the face contacts cold water. This reflex causes an immediate reduction in heart rate (bradycardia) and shifts blood flow away from the extremities toward the brain and heart. The oxygen saved powers the rat through short, tight obstacles, such as water-filled pipes.
The rat’s overall survival time is defined more by the onset of hypothermia and exhaustion than by breath-holding. While they can tread water for days, prolonged exposure to cold water rapidly lowers the body’s core temperature. Sustained immersion can be lethal when the rat’s core temperature drops below a certain point, a limit reached faster than the maximum treading endurance.
Why Rats Enter Aquatic Environments
Rats enter aquatic environments due to necessity and opportunity, often utilizing water as a travel network. The municipal sewer system is the most common aquatic habitat for the Norway rat, offering an ideal environment for survival. Sewers provide a constant, warm temperature, protection from predators, and a continuous food source from discarded waste.
This system acts as an extensive highway, allowing rats to travel efficiently throughout urban areas. Aquatic entry can also be a reactive behavior, such as escaping a predator or a sudden environmental change. Flooding, construction, or overcrowding can force rats to leave burrows and use water-filled pipes to seek new territory or shelter.
Clarifying Common Myths About Rats and Plumbing
The Norway rat’s swimming and diving abilities fuel the common myth that they frequently enter homes through toilet plumbing. While physically possible, this scenario is not a common occurrence. The rat’s flexible skeletal structure allows it to compress its body to fit through openings the size of a quarter, aiding navigation through narrow pipes.
The most difficult barrier is the water-filled U-bend, or P-trap, which prevents sewer gases from entering the home. A rat must dive and hold its breath to pass through this curve, a task achievable due to its three-minute breath-hold capacity. The risk of a rat emerging in a toilet is highest when the P-trap is dry, such as in vacant properties or infrequently used fixtures, which removes the water barrier entirely.