Growth and mortality of koala pouch and back young
O’Callaghan, P 1996, Proceedings from the Conference on the Status of the Koala in 1996, Brisbane, Australian Koala Foundation, pp. 101-109.
The systematic management and monitoring of captive koala populations presents a unique opportunity for learning about the growth and mortality of koala pouch and back young. In this report, a number of learnings about the factors affecting the development of juvenile koalas from the Lone Pine Koala Sanctuary koala population are documented.
The commencement of koala breeding season aligns with the increased day lengths and temperatures of August and continues through to April. The oestrous cycle can repeat up to four times within one breeding season, allowing multiple mating opportunities. An oestrus female will mate only once during that cycle, even if the mating was not successful. The average rate of conception is 57%, the average gestation period is 33 days. The one- to two-minute journey of the koala joey from the cloaca to the pouch is the first challenge for the young. The highest number of births observed by a female in a single breeding season was three. Although pouch young demonstrate no preference for the left or right teat, a joey that selects the same teat as that selected by a previous offspring has a slightly lowered chance of survival. The average age at which pouch young were observed to push the head out of the pouch was 187 days, followed by full body emergence within another month. Emergence from the pouch coincides with the production of faecal pap which the young consumes to prepare its alimentary tract for digesting eucalyptus leaf. On average, a juvenile will have 3.3 pap feeds over a period of 27.6 days. After as few as one pap feed, juveniles demonstrate a great interest in eucalyptus leaf. The average age at which a koala can break off, chew and swallow a eucalyptus leaf successfully is 228 days, an event typically followed by a spurt in growth. This increasing feeding independence and growth spurt enables the juvenile to leave the pouch permanently at approximately 238 days of age and begin to explore its surroundings away from its mother at 262 days of age. Koalas were observed to be independent or weaned at an average weight of 2.2kg and age of 353 days. Mortality rates of juveniles vary according to season, age and individual, but averaged at 33% over the eight years preceding this publication. Miscarriage is reported to be the most common cause of young mortality, followed by septicaemia. The following practices are suggested to reduce young mortality rates: separation of the sexes to reduce stress in females that may lead to young mortality; grouping of young of the same age to allow females to raise offspring cooperatively; isolating females that are stressed or have a history of young mortality; regular observation of the condition of the pouch and the young; and, if necessary, transferring the young to a surrogate mother.
The information presented in this report contributes greatly to our understanding of the factors and processes that influence the growth and mortality of koala pouch and back young. It is not known, however, to what extent the observations reported here correspond to reality for free-ranging koala populations. Factors affecting the growth and mortality of juvenile koalas in the wild would, therefore, be a relevant focus for future research.
Summarised by Joanna Horsfall
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