Brown rat

The brown rat, known scientifically as *Rattus norvegicus*, is one of the most successful rodent species on Earth.

It is a large member of the murid family, which includes many types of mice and rats. This species is often called the common rat, street rat, or sewer rat. It has become naturalized in almost every part of the world, except for Antarctica. Because they live so close to people, they are found in nearly every urban environment. They are highly adaptable, allowing them to thrive in cities and various natural habitats.

Physically, the brown rat is built for survival and agility.

They typically have brown or dark grey fur with lighter colored underparts. An adult can weigh anywhere from 250 to 500 grams. Their bodies are quite long, though their tails are usually slightly shorter than their head and body combined. The brown rat has several specialized senses to help it navigate. They possess an acute sense of hearing and a highly developed olfactory sense, which is their sense of smell. While their vision is relatively poor, they are dichromats, meaning they perceive colors similarly to a human with red-green colorblindness. Interestingly, they also have UV receptors, which let them see ultraviolet light that humans cannot see.

These rodents are true omnivores, meaning they eat a wide variety of both plant and animal matter.

Their diet is often determined by what is available in their specific environment. In cities, they rely heavily on discarded human food. In other areas, they may hunt for different things. For example, rats near a hatchery in West Virginia catch small fish. Some colonies along the Po River in Italy have even learned to dive for mollusks. This ability to learn new ways to find food is a sign of social learning within the species. They may also eat birds, mice, or small lizards.

One of the most remarkable traits of the brown rat is its ability to reproduce at an incredible speed.

A female can produce up to five litters in a single year. The gestation period, or the time the babies spend developing inside the mother, is only 21 days. A single litter can contain up to 14 pups, though seven is a more common number. Females can even become pregnant again immediately after giving birth. This means a mother can nurse one litter while she is already pregnant with the next. Under ideal conditions, a small group of just two rats could grow to 15,000 in only one year.

Communication in brown rats involves sounds that humans often cannot hear. They use ultrasonic vocalizations, which are high-pitched sounds above the range of human hearing.

Pups use these cries to direct their mothers to help them in the nest. Adult rats also use these sounds to respond to predators or perceived danger. Some rats even produce a high-frequency "chirping" sound during play. This chirping has been compared to laughter and is linked to positive emotions. Rats also make sounds humans can hear, such as "bruxing." Bruxing is a teeth-grinding noise that usually signals happiness or can be used for self-comfort during stress.

There is a long history of confusion regarding the true origins of the brown rat.

In the 18th century, some people in England called them "Hanover rats" to link them to the House of Hanover. The name *Rattus norvegicus* suggests they come from Norway, but this is a misnomer. The naturalist John Berkenhout popularized this name because he believed the rats arrived on Norwegian ships in 1728. However, by the 19th century, scholars realized the rats were not native to Norway. Even the novelist Charles Dickens noted that while people thought they arrived in timber ships from Norway, the rats were actually unknown in Norway at that time. Modern science now shows they likely originated in central Asia or China.

Today, the brown rat plays many different roles in human society. Through selective breeding, humans have created the fancy rat, which is kept as a pet.

We have also developed the laboratory rat, a domesticated subspecies called *Rattus norvegicus domestica*. These rats are essential model organisms used in biological research. Because they are so similar to other mammals, they help scientists understand how biology works. Even in the wild, they show complex traits. Studies in New York City have shown that different rat populations can evolve distinct genomic profiles over time. This means they slowly develop different genetic traits based on the specific neighborhoods where they live.