The brown rat originated in Asia. It arrived in North America via Europe in the 18th century. Rats have followed humans to every continent except Antarctica.

Averaging about 40 cm (16 in.) in length, these animals live in organized social colonies, usually dominated by the larger males. They are active at all times of the day and night. Rats are not fussy about their food: they will eat almost anything, including spilled grain, garbage, chickens, and even lambs and piglets.

They can do significant damage to human artefacts, not only in food destroyed or polluted, but in the destruction of electric and telephone cables, water pipes, furniture and so on. They are also known to carry diseases that can be transmitted to humans, among them the bubonic plague. The brown rat is fierce and aggressive, and humans and other predators have not had significant effect in reducing the size of their population. The first step in controlling these animals around human habitation is good sanitation and proper disposal of garbage, together with the rat-proofing of food storage areas.

Laboratory rats are albino brown rats. These gentle creatures make very dutiful mothers and are credited with helping scientists discover many cures for human disease.