Brown rat

The brown rat is an example of a species that has spread to many parts of the world with the help of humans. It first came to Europe from Asia as a stowaway on merchant ships during the Middle Ages, and to Sweden in the 18th century. It then completely displaced the black rat that was previously present here. Brown rats are highly adaptable and live in sewage systems, storage spaces, barns and rubbish dumps, but also in the wild – often near water. Today they are found throughout Sweden except in the far north.

Rapid reproduction

The brown rat’s fur is coarse and is usually brown or dark grey, while the underside is light grey or light brown. Some weigh almost a kilogram, but the most common weight is around 550 grams for males and 350 grams for females.

The brown rat reproduces rapidly, and a few rats can become thousands in a year. The brown rat can breed throughout the year, given the right conditions. A female can have 7–15 pups five or six times a year, and these in turn quickly give birth to their own pups. The female has to take good care of them, as they are blind, naked and deaf when they are born. Rats that are related live together. They groom each other and defend themselves against other rats that are not part of the family.

Able to hear ultrasound

Brown rats are mostly nocturnal. They have excellent hearing, and can hear ultrasound and other sounds that are so high in pitch that humans cannot hear them. They also have a very good sense of smell and see colours much like we do, although they cannot distinguish between red and green or see ultraviolet light. With such highly developed senses, rats are very shy animals. Rats can scream to warn each other about danger. They have to watch out for many different predators, such as humans, cats, eagle-owls and snakes. Rats are omnivores, eating everything from plants to insects and waste discarded by humans.

Skilled swimmers

Rats are skilled swimmers, including underwater. This is how they are able to enter houses through sewerage pipes. They are also good at digging, building long systems of tunnels in the ground where they live for several generations. Rats can learn to find their way through very difficult mazes and solve complicated tasks.

Pests

Brown rats have long been hated and persecuted by humans, having often invaded our homes. They have also moved through rubbish dumps and sewers, spreading both dirt and diseases such as plague and salmonella. When brown rats enter our homes, they can chew and destroy building materials, furnishings, electrical wiring and clothing. They have also caused problems for humans in food stores and granaries, where they have helped themselves to food that had been saved for the winter.

People have tried to reduce the number of rats by catching them with traps or even killing them with rat poison. Several dog breeds have also been bred to be good ratters, such as terriers.

Pets and research animals

At the same time, humans have also bred brown rats to serve them. Rats have proven to be the most important research animals used in medical and environmental experiments as they reproduce so quickly. They have also become popular as pets, as they tend to be affectionate and very tame. They love to be petted and enjoy climbing on their owners.