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Gestational length in the koala, Phascolarctos cinereus

Gifford, A, Fry, G, Houlden, BA, Fletcher, TP & Deane, EM 2002, Animal Reproduction Science, vol. 70, no. 1, pp. 261-266.

A koala born at Taronga Zoo was found to have a gestational period of at least 50 and up to 77 days, considerably longer than the typical gestational length of 30-35 days.

  The birth of the koala was noted 127 days after a male was removed from a female colony. The minimum gestational period based on the time between the removal of the male and the observation of the pouch young was 50 days; however, data on the joey’s developmental milestones suggest that the gestational length could have been as long as 77 days. The sire was confirmed from DNA samples.

  Two possibilities for the extended gestational period observed in this study are apparent. The first possibility is that the sperm was stored for between 15 and 42 days after mating before fertilisation occurred, much longer than previous research has indicated is possible. The second possibility is that the young was a result of a mating from the previous breeding season and embryonic diapause occurred, though this has never been recorded to occur in a koala. Although the female had previously had her back young removed, the timing of its removal did not align with the date at which development of the embryo would have continued after hiatus. It is, therefore, more likely that the dormant blastocyst was activated when the female entered seasonal oestrus.

  This unique finding is significant for managers of captive koala populations as it raises the possibility that female koalas may bear the young of mates from previous breeding seasons. Accounting for this possibility, managers may need to consider lengthening the time between removing males from and introducing new males to female colonies, and tools for paternity typing will assist in keeping accurate genealogical records.

 

Summarised by Joanna Horsfall

 

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