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The effects of gestagen implants on the behaviour of free-ranging female koalas

Hynes, EF, Handasyde, KA, Shaw, G & Renfree, MB 2011, Applied Animal Behaviour Science, vol. 134, pp. 209-216.

Population control in koalas can be achieved using hormonal contraception; however, hormonal changes can have unexpected effects on koala behaviour. Three groups of free-ranging female koalas were treated with either levonorgestrel implants, etonogestrel implants or control to evaluate fertility and effect on behavior. The most prominent behaviour observed was koalas travelling large distances of up to 11 kilometres. Koalas observed to travel these distances were those receiving levonorgestrel implants and with no dependent young.

  The study was conducted on French Island, Victoria, where the overpopulation of southern koalas is of concern due to its impacts on the ecosystem. There were eight koalas in the control group, 15 in the etonogestrel group (seven receiving 34 mg and eight receiving 70 mg of hormone) and eight in the levonorgestrel group (receiving 70 mg). These hormone implants were placed under the skin. All koalas had young at the time of receiving implants, confirming their natural fertility. Koalas were tagged and re-located by radio-collar tracking. Ranging behaviour was observed for three breeding seasons: the 2003/2004 season in which all females were carrying young and implants were provided, 2004/2005 and 2005/2006. Females in the levonorgestrel-treated group produced no young, while females in the control and etonogestrel groups had a 75-100% reproductive success rate. The koalas did not move far from September to March, but some moved large distances of up to 11 kilometres prior to and during the initial breeding months. Females in the levonorgestrel-treated group all travelled great distances in the second and third breeding seasons monitored, as did three females in the control group and eight females in the etonogestrel-treated group. This effect of the levonorgestrel treatment on the number of animals undertaking long-range movements was significant. There was also a significant effect of the presence of back young on distance travelled, as females without back young were significantly more likely to traverse large distances than those with back young.

  In this study, the levonorgestrel treatment was effective in reducing koala fertility; however, treated koalas continued to exhibit mate-seeking behaviour. This may be because although the hormone impedes the ability to conceive, it does not affect their natural sexual behaviours. Interestingly, koalas rendered infertile as a result of the levonorgestrel implant were observed travelling great distances. This effect is not thought to be a direct result of the implant but, rather, an indirect result of an increase in energy availability for other activities due to not carrying back young.

  This study has demonstrated that levonorgestrel is an effective fertility control hormone in female koalas, but it can trigger koalas to travel long distances when it is potentially not safe to do so outside of managed or fragmented sites. Further research is needed, therefore, to elucidate the mechanism of this behavioural change and its consequences for koala conservation and management.


Summarised by Alexandra Selivanova


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