Marsupials represent the characteristic group of mammals of Southern hemisphere. They are diverse in Meganesia and in South America, but in Pleistocene opossums had penetrated for the second time into North America where they lived up to Miocene, and in Neocene they widespreaded up to Greenland, having evolved a separate . One more northern advanced post of marsupials is at Hawaii where these animals had appeared due to human activity and could go through the epoch of people domination on the Earth. But all the same at southern continents marsupials are more diverse, rather than free or involuntary immigrants to the north. They lead various habits of life and occupy diverse habitats: from humid tropical forests up to snow-covered tops of Andes and cold deciduous forests of Tierra del Fuego. Representatives of caenolestid family are tiny insectivorous forms from South America. One species of this group lives even in wool of huge groundsloth rodents. One more unusual representative of this family lives in areas of temperate and cold climate of South America – it is fat-tailed marsupial dormouse. It is very large representative of family: an adult individual is guinea pig-sized one, but it looks larger because of long fluffy tail which exceeds length of other parts of body. Head of this mammal is large and bears small auricles and large bulging eyes. Fat-tailed marsupial dormouse leads tree-climbing habit of life and lives in forested areas. Because of rather short paws it is almost unable to jump in branches and prefers to climb, clinging by hooked claws. The tail of this marsupial is partly prehensile – its tip can twist around of branches, and the animal can hang, fixing only on tail. The fur of animal has beautiful ash-grey color with white underfur. On head and shoulders fur is darker. The mouth of this marsupial is full of pointed teeth: fat-tailed marsupial dormouse is omnivore and has obvious tend to carnivory. Its food includes small mammals and birds, and amphibians and lizards in summer season. Also it willingly eats the carrion, also even frozen one. Similarly to the majority of animals of such kind, it is a solitary species of primitive behaviour, displaying aggression to relatives almost the year round, except for time of hibernation and courtship season. Fur of fat-tailed marsupial dormouse is dense, short and velvety in summer and growing much longer to winter. This animal is strongly eaten off in autumn; in tail and buttocks fat is laid (hence the specific name “crassipyga”, meaning “fat back”). With approach of steady cold weather this marsupial falls into the true hibernation. For this purpose fat-tailed marsupial dormouse makes the shelter – it covers a hollow with dry leaves and constructs grass-made fuse to stop up an entrance of the hollow. During the hibernation the body temperature of animal falls up to a minimum; breath and palpitation are slowed down in great degree. In such condition animal spends approximately two thirds of winter. In second half of winter at the female cubs are born. Shortly before the birth moment physiological processes of female become more active. She clears pouch and licks wool. Female gives birth up to ten underdeveloped cubs independently reaching her pouch. This species has only six nipples, therefore the part of cubs perishes within one hour after birth. When all nipples are occupied by cubs, the female falls asleep again, but dream is less deep. She produces milk which is periodically sprinkled in mouths of cubs. They gradually grow and develop, and the female wakes up after winter dream having already well advanced cubs. Within the summer cubs grow and pass to adult diet. At the beginning of an autumn they become independent and reach sexual maturity at the third year of life. Life expectancy of this species is up to 30 years.