Mammals
Brown Rat
Rattus norvegicus · also called common rat, Norway rat, sewer rat, lab rat
The brown rat is one of the most successful mammals that has ever lived. It followed humans out of the Asian grasslands and onto every inhabited landmass and thousands of islands, thriving in sewers, subways, farms, and ships — and along the way it became both a serious pest and the single most important animal in medical research.
Overview
Brown rats are commensal with humans — they live in, on, and around our structures and food, and populations away from people are rare. They occupy cellars, sewers, drains, warehouses, rubbish tips, farm buildings, river and canal banks, port areas, and ships, digging burrow systems and using the routes and shelter that human infrastructure provides. They are most active at night, live in loose social groups with dominance hierarchies, and communicate by scent, posture, and ultrasonic calls.
Built to exploit
The brown rat’s success comes from an extreme generalist toolkit:
- Diet. It eats essentially anything humans eat, plus what we throw away, and needs only a small amount of water.
- Reproduction. Females can breed within hours of giving birth and raise litter after litter; a pair’s descendants can number in the hundreds within a year under good conditions.
- Gnawing. Ever-growing incisors and powerful jaw muscles let rats chew through wood, plastic, cabling, soft metals, and cinder block.
- Senses and agility. Weak eyes are more than offset by acute smell, hearing, and whisker-touch; rats swim well, climb, jump, squeeze through gaps the size of their skull, and fall several storeys unhurt.
A surprisingly rich inner life
Decades of laboratory work — and the fact that the brown rat is the origin of the domesticated “fancy rat” and the “lab rat” (Rattus norvegicus domestica) — have revealed a cognitively and emotionally complex animal. Rats learn rules and generalise them, navigate by mental maps, show empathy (freeing trapped cage-mates and sharing food), display something resembling regret, dream about mazes they have run, and emit laughter-like ultrasonic chirps when playing or being tickled.
Impact
As a pest, the brown rat contaminates and consumes stored food on a huge scale, damages buildings and wiring (a cause of fires), and can carry pathogens including Leptospira, Salmonella, and hantaviruses. On islands, introduced brown rats are among the worst invasive species, having driven seabirds, land birds, reptiles, and invertebrates to extinction; clearing rats from islands is one of the highest-impact conservation interventions available.
As a research animal, it has been central to the development of most modern medicine — from vaccines and cancer treatments to the entire fields of behavioural neuroscience and psychology.
Status
The brown rat is Least Concern and increasing, with a global population that likely rivals or exceeds that of humans. Control focuses on denying food and harbourage (“rat-proofing”) rather than poisoning, which is increasingly recognised as ineffective at scale and harmful to predators that eat poisoned rats.
Brown Rat: common questions
Are rats actually intelligent?
Very. Brown rats rapidly learn complex mazes and abstract rules, use tools in experiments, plan routes, and remember social relationships. Studies show they display empathy — a rat will free a trapped companion before taking a food reward, and share the food — and behaviour that looks like regret after making a poor choice. They also show individual "personalities" and can be trained, like the giant pouched rats used to detect landmines and tuberculosis (a different species, but the point stands for rat cognition generally).
Do rats laugh?
They make ultrasonic chirps at around 50 kHz — inaudible to us — during play, when anticipating a reward, and when tickled by a familiar human, and the same brain regions light up as in human laughter. Young rats are the most ticklish, they chase the hand that tickles them wanting more, and the chirping stops if the rat is anxious. It is considered the closest thing to laughter documented in a rodent.
How did brown rats spread around the world?
By stowing away. The brown rat originated in the grasslands of northern China and Mongolia and spread west across Asia and into Europe by the 1500s–1700s, then travelled on sailing ships to the Americas, Africa, Australia, and thousands of islands, arriving wherever humans and their cargo went. On islands with no native ground predators it has been devastating to seabirds, reptiles, and plants, and rat eradication is now a major conservation activity.
What is the difference between a brown rat and a black rat?
They are different species. The brown (Norway) rat is larger and heavier, with a blunter nose, smaller ears, and a tail shorter than its body; it prefers to live at ground level and below — burrows, sewers, basements, lower floors. The black (roof) rat is slighter, with a pointed nose, large ears, and a tail longer than its body; it is a better climber and tends to live in roofs, upper storeys, and trees. The brown rat has largely displaced the black rat across cooler regions.