Studies of the oestrous cycle, oestrus and pregnancy in the koala (Phascolarctos cinereus)
Johnston, SD, McGowan, MR, O’Callaghan, PO, Cox, R & Nicolson, V 2000, Journal of Reproduction and Fertility, vol. 120, no. 1, pp. 49-57.
This study described the oestrous cycles of both non-mated and mated female koalas in terms of relationships between oestrus, oestrous behaviours, the appearance of the external genitalia, and concentrations of the reproductive steroids oestradiol and progestogen.
The mean duration of the oestrous cycle for non-mated females was 32.9 + 1.1 days and of oestrus 10.3 + 0.9 days. High oestradiol concentrations coincided with commencing oestrous behaviours; however, no changes in the appearance of the genitalia were observed. Between oestrus periods, progestogen concentrations remained at minimum levels, indicating that non-mated oestrous cycles may be non-luteal and anovulatory. After twelve females were mated, six gave birth after a mean gestation of 34.8 + 0.3 days, and the remaining six that did not produce young returned to oestrus after 49.5 + 1.0 days. Once mating occurred, oestrous behaviours stopped, and both pregnant and non-parturient females entered a luteal phase induced by mating as demonstrated by increased progestogen concentrations. During the luteal phase, however, progestogen concentrations were significantly higher in pregnant than non-parturient females. During birth, concentrations of progestogen decreased to a level slightly higher than the minimum concentration, dropping to the minimum concentration after seven days.
The duration of non-mated oestrous cycles varied considerably among individuals, and sometimes even within individuals. The duration of oestrus in non-mated koalas was also variable, ranging from 0 to 19 days. Despite consistently producing high concentrations of oestradiol at the commencement of oestrus, there was also notable variation in the oestradiol profiles of the non-mated koalas. The 49.5-day period between mating and the next oestrus for non-parturient females indicates the combined duration of the koala’s luteal and follicular phases. During this period, there were no significant differences between the oestradiol profiles of pregnant and non-parturient females, except during the period in which non-parturient females returned to oestrus. Although there were no detectable changes in the appearance of the females’ external genitalia after mating, the koalas did demonstrate increased pouch secretions and sensitivity during the late luteal phase regardless of pregnancy status. An increased progestogen concentration is associated with the luteal phase of the oestrous cycle. The low progestogen concentrations exhibited by non-mated koalas during the 32.9-day oestrous cycle suggests that this cycle may be anovulatory and with no luteal phase. The production of progestogen at high concentrations after mating by all females provides the first evidence of a mating-induced luteal phase in a marsupial species.
The findings of this study are critical to improving our understanding of the reproductive physiology of the koala which, until now, has been poorly described. Improved descriptions of the oestrous cycle of the koala assist in the development of effective assisted breeding technology protocols as well as conservation management plans more broadly. The unique reproductive patterns of the koala observed in this study even provide a convincing case for classifying the koala in a new reproductive group that is distinct from other marsupials.
Summarised by Joanna Horsfall
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