According to the University of California at Berkeley, the single largest group of mammals is the rodentia, which includes the rat. There are over 1,500 different rodent species, including hamsters, guinea pigs, mice and hamsters.
Rats are considered mammals because they are warm blooded and they feed their young with their milk from concealed mammary glands. Rats and other rodents can cause millions of dollars worth of damaged crops each year, and some of them carry diseases such as the bubonic plague, typhus and Hanta fever. In some parts of the world rodents are used for their fur or as a food source.