Rats

Rats are members of the order Rodentia, which also encompasses beavers, mice, hamsters, and porcupines. Two major families of rats and mice are recognized: the Sigmodontinae; the New World rats and mice, comprising 369 species in 73 genera, and the Murinae, the Old World rats and mice, comprising 408 species in 89 genera. The major taxonomic difference between the two subfamilies is the presence of a functional row of tubercles on the inner side of the upper molars in the Murinae.