The wild mice and rats of the Americas are not members of the true "mouse family", the Muridae, but instead belong to the Cricetidae, a name which translates as "hamster family". The vast majority of species in the family are not, however, hamsters, since the group includes both the voles (of which there are many) and the aforementioned New World mice and rats. Many of these latter are found in South America, with plenty more in Mexico and Central America. Most of them are mouse-sized, an example of the great success of the mouse body-plan where, despite having diverged from the true mice over 30 million years ago, such creatures as the American deer-mice differ from the house mouse by, at best, having a different colour pattern.
Just as some members of the mouse family developed larger size, evolving into such things as the familiar sewer rat (which, of course, has been introduced to America, even though it's not native there) so to did some of their New World counterparts. In the southern US, three different lineages of such creatures can be found: the muskrat (which is actually a giant vole), the cotton rats (which are related to the vast collection of South American rats and mice), and a third group variously called pack rats or woodrats.
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