Of the nearly 120 kinds of rats, the best known are the black rat and the brown rat. The latter is most common throughout the United States. Brown rats tend to live under floors, within walls, in piles of garbage or in the ground. Like other types of rats, they have a keen sense of smell and can sense danger very quickly. They tend to be more aggressive than their black cousins.
Rats tend to live in large groups that have dominant individuals.
They feed mainly at night and sometimes band together to attack larger animals such as chickens and piglets. Interestingly enough, brown and black rats originally lived only in Asia. They spread to Europe by ship or overland and later traveled aboard ships to North and South America. Medical researchers frequently use white rats in studies of disease, drug effects, heredity, nutrition, learning and other aspects of behavior. They're also used in the preparation and testing of new drugs.