Levonorgestrel, not etonogestrel, provides contraception in free-ranging koalas
E. F. Hynes A B , K. A. Handasyde A , Geoff Shaw A and Marilyn B. Renfree AA The Department of Zoology, The University of Melbourne, Vic. 3010, Australia.
B Corresponding author. Email: emily_hynes@hotmail.com
Reproduction, Fertility and Development 22(6) 913-919 https://doi.org/10.1071/RD09253
Submitted: 14 October 2009 Accepted: 4 January 2010 Published: 25 May 2010
Abstract
Management of high-density koala (Phascolarctos cinereus) populations is essential because of the browsing damage they inflict on their habitat. We have tested two types of gestagen implant, namely levonorgestrel and etonogestrel, as contraceptives for koalas. Free-ranging female koalas were given either a control, levonorgestrel (70 mg) or etonogestrel (34 or 68 mg) implant before the breeding season. Koalas were monitored every 4–12 weeks for births. Plasma progesterone was measured and a cytological smear of the urogenital sinus was taken. Fertility was high in the control group and the two etonogestrel-treated groups, with approximately 90% of females giving birth. In contrast, no levonorgestrel-treated female produced young during the study. Removal of levonorgestrel implants from six females reversed the contraceptive effect in the next breeding season, whereas the eight females in which the levonorgestrel implants were left in remained infertile for six breeding seasons. Vaginal cytology showed evidence of oestrous cycles during the breeding season in all females from all groups and there was no difference seen in the prevalence of cornified epithelial cells in the oestrous smears. This indirectly suggests that levonorgestrel does not prevent follicular development and oestrous cycling. Plasma progesterone in levonorgestrel-treated females remained low all year, but rose in controls concurrent with the onset of the breeding season. This suggests that levonorgestrel prevents pregnancy by blocking ovulation. Etonogestrel had absolutely no contraceptive effect at the two doses delivered and so is not suitable for controlling koala populations. In contrast, levonorgestrel was effective as a long-term, reversible contraceptive in wild koalas.
Additional keywords: fertility control, gestagen implants, marsupial, Phascolarctos cinereus, wildlife management.
Acknowledgements
The authors thank the Parks Victoria rangers on French Island (Michael Douglas, Aaron Ledden, Terry Easy, Scott Coutts, Dave Jackson and Graeme Briggs) who provided transport and accommodation and were of great assistance in the field. The authors thank Professor Lynne Selwood for advice on interpretation of oestrous smears and Dr Terry Fletcher for providing advice on the radioimmunoassay. The authors also thank Leiras Pharmaceuticals (Turku, Finland) and Professor R. V. Short for the kind gift of the LNG implants, and Organon Australia (Lane Cove, NSW, Australia) for the ENG implants. Numerous volunteers helped out during the field trips. The research costs were supported by a grant from San Diego Zoological Society, which, along with The University of Melbourne, jointly funded a PhD scholarship for E.F.H.
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