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The mammals included are only viviparous (marsupials and placentals) as some mammals, which are monotremes (including platypuses and echidnas) lay their eggs. A marsupial has a short gestation period, typically shorter than placental. For more information on how these estimates were ascertained, see Wikipedia's articles on gestational age.
The latter subclass is divided into two infraclasses: pouched mammals (metatherians or marsupials), and placental mammals (eutherians, for which see List of placental mammals). Classification updated from Wilson and Reeder's "Mammal Species of the World: A Taxonomic and Geographic Reference" using the "Planet Mammifères" website.
Smaller species normally have a shorter gestation period than larger animals. [2] For example, a cat's gestation normally takes 58–65 days while an elephant's takes nearly 2 years (21 months). [3] However, growth does not necessarily determine the length of gestation for all species, especially for those with a breeding season. Species that ...
Gestation, called pregnancy in humans, is the period of time during which the fetus develops, dividing via mitosis inside the female. During this time, the fetus receives all of its nutrition and oxygenated blood from the female, filtered through the placenta , which is attached to the fetus' abdomen via an umbilical cord .
Marsupials of North America — marsupial mammals of North America, within the Didelphimorphia order that is endemic to the Americas. Pages in category "Marsupials of North America" The following 9 pages are in this category, out of 9 total.
All extant marsupials are endemic to Australasia and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a pouch . Well-known marsupials include kangaroos , wallabies , koalas , opossums , wombats , Tasmanian devils , and the extinct thylacine .
Due to the fact that placental mammals and marsupials nourish their developing embryos via the placenta, the ovum in these species does not contain significant amounts of yolk, and the yolk sac in the embryo is relatively small in size, in comparison with both the size of the embryo itself and the size of yolk sac in embryos of comparable developmental age from lower chordates.
The maximum litter size is, therefore, eight, since marsupial young are attached to the teat during development, although two to four young per litter is a more typical number. The gestation period of peramelids is the shortest among mammals, at just 12.5 days, the young are weaned around two months of age, and reach sexual maturity at just ...
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