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Gestation Periods of Mammals
Gestation period is the time from fertilization to the actual birth in animals. In humans this period is 266 days or approximately 9 months.
The shortest gestation period known in mammals is approximately 12 days. It is shared by the American or Virginian opossum (Didelphis marsupialis); the rare water opossum, or yapok (Chironectes minimus) of northern South America and the eastern native cat (Dasyurus viverrinus) of Australia. All three of these marsupials give birth to their young while they are still at the embryonic stage. The young then find their way to a small pouch where they mature, like a kangaroo. This pouch stage lasts up to two months after which they stay close to their mothers for another month or so, usually holding tight on the mother’s back.
The longest gestation period known in mammals is that of the African elephant (Loxodonta Africana). These elephants carry their babies in the womb for about 660 days or 22 months. Camels and giraffes carry their babies for about 400 days.  Other animals that have gestation periods closest to that of humans are: dolphins - 276 days, chimpanzee - 237 days and cattle - 280 days.
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