Members of the New World rats and mice have occupied different ecological niches. Because of this, they have developed different forms. Most of them resemble rats or mice in appearance, but some look like voles, moles, gerbils or shrews. The biggest of them are Kunsia, with about 16–30 cm body length, and about 8–16 cm tail. Among the smallest are Baiomys, with about 5–8 cm body length, and about 5 cm tail.