The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, and New World rats and mice. At almost 600 species, it is the second largest family of mammals, and has members throughout the New World, Asia, and Europe.
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 diet is 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. Although there are a few exceptions, 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 period of between 15 and 50 days, and large litter sizes relative to many other mammals. The young are typically born blind, hairless, and helpless.
The Cricetids first evolved in the Old World during the Miocene, from ancestors that more closely resembled modern Sciurids. 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 term Cricetidae has gone through several permutations, and members of the family as currently defined are often placed in the family Muridae along with all other muroid subfamilies. Many other muroids have been called members of the Cricetidae in the past 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 that the subfamilies listed below are related (i.e. form a monophyletic group), and that the other muroids should not be included in the Cricetidae.