The names associated with a great number of species of mammals in a number of different families of the order Rodentia. The rodents constitute the largest order of mammals, and they presently include 2277 species, or about 42% of the known mammalian species of the world. Most of the smaller rodents are called rats or mice. There is no fundamental taxonomic difference between mice and rats; they differ only in size. (Smaller species are generally called mice, larger ones rats.) Both mice and rats are often in the same family, and there are mice and rats in a number of different rodent families.