commonly some shade of brown in color, although many have black, grey, or white markings. Murids generally have excellent senses of hearing and smell. They live in a wide range of habitats from forest to grassland, and mountain ranges. A number of species, especially the gerbils, are adapted to desert conditions and can survive for a long time with minimal water. They consume a wide range of foods depending on the species, with the aid of powerful jaw muscles and gnawing incisors that grow throughout life. The dental formula of murids is . Murids breed frequently, often producing large litters several times per year. They typically give birth between twenty and forty days after mating, although this varies greatly between species. The young are typically born blind, hairless, and helpless, although exceptions occur, such as in spiny mice. Evolution As with many other small mammals, the evolution of the murids is not well known, as few fossils survive. They probably evolved from hamster-like animals in tropical Asia some time in the early Miocene, and have only subsequently produced species capable of surviving in cooler climates. They have become especially common worldwide during the current geological epoch, as a result of hitching a ride commensally with human migrations. Classification The murids are classified in five subfamilies, around 150 genera, and about 834 species. Subfamilies Deomyinae (spiny mice, brush furred mice, link rat) Gerbillinae (gerbils, jirds and sand rats) Leimacomyinae (Togo mouse) Lophiomyinae (maned rat or crested rat) Murinae (Old World rats and mice, including vlei rats) In literature Murids feature in literature, including folk tales and fairy stories. In the Pied Piper of Hamelin, retold in many versions since the 14th century, including one by the Brothers Grimm, a rat-catcher lures the town's rats into the river, but the mayor refuses to pay him. In revenge, the rat-catcher lures away all the children of the town, never to return. Mice feature in some of Beatrix Potter's small books, including The Tale of Two Bad Mice (1904), The Tale of Mrs Tittlemouse (1910), The Tale of Johnny Town-Mouse (1918), and The Tailor of Gloucester (1903), which last was described by J. R. R. Tolkien as perhaps the nearest to his idea of a fairy story, the rest being "beast-fables". Among Aesop's Fables are The Cat and the Mice and The Frog and the Mouse. In James Herbert's first novel, The Rats, (1974),