The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, muskrats, and New World rats and mice. At almost 608 species, it is the second-largest family of mammals, and has members throughout the Americas, Europe and Asia. Characteristics The cricetids are small mammals, ranging from just in length and in weight in the New World pygmy mouse up to and in the muskrat. The length of their tails varies greatly in relation to their bodies, and they may be either furred or sparsely haired. The fur of most species is brownish in colour, often with a white underbelly, but many other patterns exist, especially in the cricetine and arvicoline subfamilies. Like the Old World mice, cricetids are adapted to a wide range of habitats, from the high Arctic to tropical rainforests and hot deserts. Some are arboreal, with long balancing tails and other adaptations for climbing, while others are semiaquatic, with webbed feet and small external ears. Yet others are burrowing animals, or ground-dwellers. Their diets are similarly variable, with herbivorous, omnivorous, and insectivorous species all being known. They all have large, gnawing, incisors separated from grinding molar teeth by a gap, or diastema,Molar teeth may be bunodont like in cricetines or lophodont like in arvicolines or some new world mice. Although a few exceptions occur, the dental formula for the great majority of cricetids is: Cricetids' populations can increase rapidly in times of plenty, due to a combination of short gestation periods between 15 and 50 days, and large litter sizes relative to many other mammals. The young are typically born blind, hairless, and helpless. Evolution and systematics The cricetids first evolved in the Old World during the Early Oligocene. They soon adapted to a wide range of habitats, and spread throughout the world. The voles and lemmings arose later, during the Pliocene, and rapidly diversified during the Pleistocene. The circumscription of Cricetidae has gone through several permutations. Some members of the family as currently defined have been placed in the family Muridae. Some muroids have historically been placed in Cricetidae, such as mouse-like hamsters (subfamily Calomyscinae, family Calomyscidae), gerbils (subfamily Gerbillinae, family Muridae), the crested rat (subfamily Lophiomyinae, family Muridae), zokors (subfamily Myospalacinae, family Spalacidae), the white-tailed rat (subfamily Mystromyinae, family Nesomyidae), and spiny dormice (subfamily Platacanthomyinae, family Platacanthomyidae). Multigene DNA sequence studies have shown the subfamilies listed