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

Hormonal contraception is an increasingly important management tool for control of highly abundant populations of wildlife but may have both predictable and unpredictable effects on behaviour, with consequent implications for management and animal welfare. In a study of free-ranging koalas we demonstrated that implants of levonorgestrel, but not etonogestrel, prevent pregnancies. Concurrently the effects these implants had on the behaviour of female koalas were investigated. Free-ranging female koalas were fitted with radio-collars and given either a control, levonorgestrel (70 mg) or etonogestrel (34 mg or 68 mg) implant. Ranging behaviour was compared across groups. During the first breeding season patterns of movement were similar across all groups and most koalas only moved small distances with there being no difference between treatment groups for the average (P = 0.45) or maximum (P = 0.64). However, leading up to and during the second and third breeding seasons some females moved up to 11 km away from their normal range. All (8/8) infertile levonorgestrel-treated females undertook these long-range movements in both (2004/2005, 2005/2006) breeding seasons, but some fertile females also exhibited this behaviour with control (2004/2005: 0/8, 2005/2006: 3/8), 34 mg etonogestrel-treated (2004/2005: 2/6, 2005/2006: 1/7) and 68 mg etonogestrel-treated (2004/2005: 2/8, 2005/2006: 3/7) females also moving large distances in the breeding season. A common characteristic of females undertaking these excursions was that they were not carrying a dependent young, either because they were infertile or had lost their young. Thus this behavioural response seems unlikely to be a result of the direct action of levonorgestrel but rather the absence of a dependent young resulting from levonorgestrel treatment.