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BIOLOGY:
- REPRODUCTION AND DEVELOPMENT -
(page 5)
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Gestation period:

    The gestation period for all marsupials is relatively short by comparison to their placental counterparts, as their young complete by far the greater part of their development externally, within the mother's pouch or marsupium.  Few marsupials have a gestation period longer than their oestrous cycle.  The gestation period for the thylacine is unknown.  Only educated comparisons can be made with the thylacine's smaller cousins, such as the Spotted-tailed quoll (Dasyurus maculatus), and the Tasmanian devil (Sarcophilus harrisii), with estimates varying from 21 to 35 days.

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Species Gestation (days) Fur adult pattern, detach from teat, eyes open Venture from
pouch
Permanently leave pouch Weaned
Spotted-tailed quoll
(Dasyurus maculatus)
21 7 wks 7 wks 12 wks 20 wks
Tasmanian Devil
(Sarcophilus harrisii)
31 12-13 wks 13-15 wks 16 wks 32 wks
Thylacine
(Thylacinus cynocephalus)
21-35 
(estimates)
10-11 wks 11-12 wks 16 wks 36 wks
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Gestation period and comparative pup development of three dasyuromorphian marsupials.
Source: Dr. Stephen Sleightholme.

Pregnancy and birth:

    With the majority of marsupials, and the thylacine is no exception, there are no external signs that the females are pregnant.  However, as birth nears, an expectant marsupial mother can be seen cleaning her pouch.  The pouch secretes a waxy compound (which may have antibacterial / antifungal properties) that dries into a dark scale when the pouch has been unoccupied for some time.  The female licks this accumulated scale away prior to the birth of her young.  The cleaning commences around 1-2 days before birth, and is most vigorous 1-2 hours before birth occurs. 
 

    In marsupials with a backward-facing pouch like the thylacine, the mother normally births in a standing position, with her hips raised so that the urogenital sinus is positioned directly above the pouch.  Just before birth, a small yellow yolk sac appears at the urogenital opening, then a clear allantois, and finally the joey in the amnion is born.  The newborn is motionless for a matter of a few seconds before commencing its journey from the sinus in a sticky flow of mucus to the pouch.  In all marsupials, the birthing process is rapid, and completed in 2-4 min.  Once the young have made
contact with the teats, they rapidly expand, resulting in the oversized teats being firmly clamped inside the joeys' mouths.  This attachment process ensures that the developing young do not fall out of the pouch.

    Although the newborn is blind and has many underdeveloped features, it has several characteristics that assist its survival.  The nostrils (1) of the neonate are well developed to allow it to engulf air as it travels to the pouch, as are the forearms (2) to enable the young to pull itself along the mother's belly to the pouch by grasping hairs with its forelimbs. The hindlimbs, on the other hand, are mere paddles (3) and functionally of little use in assisting the newborn's transit to the pouch.  The heart, kidneys and lungs are barely functional and the brain is at a very early ontogenetic stage. 

    Jason A. Lillegraven (1975), in his paper "Biological Considerations of the Marsupial-Placental Dichotomy", suggests that marsupial young must be born at an early stage before the mother's immune system can respond to the presence of foreign tissue in the form of a developing embryo.  Most development therefore takes place in the pouch, safe from maternal immune system attack.

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newborn Tammar wallaby
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Newborn Tammar wallaby (Macropus eugenii).
Teat (T), nostril (1), forearm (2), hindlimb (3).
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    Nothing is known about the total number of young that the thylacine produces per litter.  The Tasmanian devil gives birth to between 20 and 30 young, but with only 4 teats, the majority do not survive and are eaten by the mother.  Although no direct comparison can automatically be assumed, it is likely that thylacines give birth to more than four young.  It is highly unlikely that a female thylacine in her first breeding season would carry the maximum of four joeys.  It is far more reasonable to assume that two young would be the norm, with the litter size increasing in the peak breeding years and then diminishing as the animal ages.

Joey / pup development:

    Like all other marsupials, thylacine joeys are underdeveloped at birth, being approximately two centimetres long and less than 1 gram in weight.  W. Boardman (1945), in his paper "Some points on the external morphology of pouch young of the Marsupial, Thylacinus cynocephalus", compared the external morphological features of early and late stage pouch pups.  Boardman's study specimens were a near full-term pouch pup at around 3 months in the Australian Museum collection (AM 762), and four early stage pouch pups aged around 1 month in the Museum Victoria collection (C5754-C5757).

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thylacine pouch pup (aged approx. 3-4 months)
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Thylacine pouch pup (aged approx. 3 months).
Australian Museum Specimen P762.
Source: ITSD, 4th Revision, 2011.
thylacine pouch pup (aged approx. 1 month)
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Thylacine pouch pup (aged approx. 1 month).
Museum Victoria Specimen C5754.
Source: ITSD, 4th Revision, 2011.
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References
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