Pouch (marsupial)

The pouch is a distinguishing feature of female marsupials, monotremes[1][2][3] and possibly most extinct non-placental mammals including eutherians like Zalambdalestes[4] (and rarely in the males as in the water opossum[5] and the extinct thylacine); the name marsupial is derived from the Latin marsupium, meaning "pouch". Marsupials give birth to a live but relatively undeveloped fetus called a joey. When the joey is born it crawls from inside the mother to the pouch. The pouch is a fold of skin with a single opening that covers the teats. Inside the pouch, the blind offspring attaches itself to one of the mother’s teats and remains attached for as long as it takes to grow and develop to a juvenile stage.

Kangaroo joey inside the pouch
Female Eastern Grey with mature joey in pouch

Variations

Pouches are different amongst different marsupials, two kinds distinguishable (on the front or belly): opening towards the head and extending the cavity under the skin towards the tail (forward, or up) or opening towards the tail and extending towards the front legs (to the rear, backward or down). For example for quolls and Tasmanian devils, the pouch opens to the rear and the joey only has to travel a short distance to get to the opening (resting place) of the pouch. While in the pouch they are permanently attached to the teat and once the young have developed they leave the pouch. The kangaroo's pouch opens horizontally on the front of the body, and the joey must climb a relatively long way to reach it. Kangaroos and wallabies allow their young to live in the pouch well after they are physically capable of leaving, often keeping two different joeys in the pouch, one tiny and one fully developed. In kangaroos, wallabies and opossums, the pouch opens forward or up.

Female koalas have been described as having a ‘backward-opening’ pouch like wombats, as opposed to an upward-opening pouch like kangaroos. However, that's not true. When a female Koala first gives birth to young her pouch opening faces neither up nor down, although it is located towards the bottom of the pouch rather than at the top. It faces straight outwards rather than ‘backwards’. It sometimes appears to be ‘backward-facing’ because when the joey is older and leans out of the pouch, this pulls the pouch downwards or ‘backwards’. The pouch has a strong sphincter muscle at the opening to prevent the joey from falling out.

In wombats and marsupial moles, the pouch opens backward or down. Backwards facing pouches would not work well in kangaroos or opossums as their young would readily fall out. Similarly, forward-facing pouches would not work well for wombats and marsupial moles as they both dig extensively underground. Their pouches would fill up with dirt and suffocate the developing young. Kangaroo mothers will lick their pouches clean before the joey crawls inside. Kangaroo pouches are sticky to support their young joey. Koalas are unable to clean out their pouches since they face backwards, so just prior to giving birth to the young koala joey, a self-cleaning system is activated, secreting droplets of an anti-microbial liquid that cleans it out.[6][7] In a relatively short time, the cleansing droplets clean out all of the crusty material left inside, leaving an almost sterile nursery ready to receive the tiny joey.

Some marsupials (e.g. phascogales) lack the true, permanent pouches seen in other species. Instead, they form temporary skin folds (sometimes called "pseudo-pouches") in the mammary region when reproducing.[8][9][10] This type of pouch also occurs in monotremes (platypus and echidnas).[11]

See also

References
  1. Patricia J. Armati; Chris R. Dickman; Ian D. Hume (17 August 2006). Marsupials. Cambridge University Press. ISBN 978-1-139-45742-2.
  2. Biology of Marsupials. Macmillan International Higher Education. 17 June 1977. ISBN 978-1-349-02721-7.
  3. C. Hugh Tyndale-Biscoe (2005). Life of Marsupials. Csiro Publishing. ISBN 978-0-643-06257-3.
  4. Rose, Kenneth D.; Archibald, J. David, eds. (2005). The Rise of Placental Mammals: Origins and Relationships of the Major Extant Clades. Johns Hopkins University Press. ISBN 9780801880223.
  5. Nogueira, José Carlos, et al. "Morphology of the male genital system of Chironectes minimus and comparison to other didelphid marsupials." Journal of mammalogy 85.5 (2004): 834-841.
  6. Anna Salleh (2004-08-05). "Koala pouch may have its own bug buster". Australian Broadcasting Corporation. Retrieved 2017-02-28.
  7. Gabrielle Bobek, Elizabeth M Deane (2002-03-26). "Possible antimicrobial compounds from the pouch of the koala, Phascolarctos cinereus". Letters in Peptide Science. 8 (3–5): 133–137. doi:10.1023/A:1016257920016. S2CID 41112802.
  8. "Brush-tailed phascogale videos, photos and facts – Phascogale tapoatafa". Arkive.org. Archived from the original on 2014-01-09. Retrieved 12 February 2017.
  9. "Dasyurids". Marsupialsociety.org. Retrieved 12 February 2017.
  10. P. A. Woolley; M. F. Patterson; G. M. Stephenson; D. G. Stephenson1 (2002). "The ilio-marsupialis muscle in the dasyurid marsupial Sminthopsis douglasi: form, function and fibre-type profiles in females with and without suckling young" (PDF). The Journal of Experimental Biology. 205 (Pt 24): 3775–3781. PMID 12432001. Retrieved 2017-02-28.
  11. "Short-beaked echidna (Tachyglossus aculeatus)". Arkive.org. Archived from the original on 2009-08-13. Retrieved 2017-08-21.
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