Contraceptive effects of levonorgestrel implants in a marsupial

The effect of subcutaneous levonorgestrel implants on reproduction in female tammar wallabies was investigated during the breeding and non-breeding season. Female tammars were given either a control or a levonorgestrel implant and their pouch young were removed to terminate embryonic diapause. Both the control and the levonorgestrel-implant animals treated during the months of May and June gave birth, demonstrating that levonorgestrel does not prevent the reactivation of the diapausing blastocyst or its subsequent development when given at these times. However, none of the levonorgestrel-treated animals mated post partum, whereas all of the control females that gave birth had a post partum oestrus and mated. Control animals gave birth again when the neonate was removed, and continued to breed normally during the following 36 months of the investigation. None of the levonorgestrel-treated animals gave birth again or mated during the next 36 months. Animals given control implants during December did not reactivate or give birth until the normal start of the breeding season in late January. Animals treated with levonorgestrel implants during December did not reactivate with the control animals at the beginning of the breeding season and did not give birth during the next 36 months. There were no effects of levonorgestrel treatment on early lactation. Levonorgestrel implants were removed from six females and four of these animals resumed reproductive activity, confirming that the contraceptive effect of the implants is reversible. Levonorgestrel implants therefore provide a highly effective, reversible and long-term method of contraception for tammar wallabies. This contraceptive system appears to offer a method of population control for the management of overabundant captive and selected wild populations of macropodid marsupials.