Introduction The eastern grey kangaroo (Macropus giganteus) is common throughout its range in Australia. Populations can reach very high densities under favourable conditions, particularly near urban areas. Its status as a protected native species and its high public profile often places socio-political constraints on the management options available, with culling facing increasing public opposition (Adderton Herbert 2004). This has resulted in investigations into fertility control as an alter- native management technique to control kangaroo popu- lation size (Nave et al. 2002; Herbert et al. 2004a). A promising approach to fertility control involves potent agonists of gonadotrophin-releasing hormone (GnRH). Chronic GnRH agonist treatment initially induces a transient increase in LH and FSH concentrations that can last for several days (acute phase), followed by a chronic phase during which the secretion of LH is blocked (D’Occhio et al. 2000). The resulting decreases in plasma concentrations of LH and FSH in females lead to the suppression of ovulation, oestrous cycling and gonadal steroidogenesis (Fraser 1993). Slow-release implants containing the GnRH agonist deslorelin have proved to be an effective contraceptive agent in female tammar wallabies (Herbert et al. 2004a) and eastern grey kangaroos (Herbert et al. 2006). In the latter, treatment with deslorelin implants successfully inhibited reproduction for 559 ± 111 days (mean ± s.e.m, n = 6) and 651 ± 21 days (n = 5) after administration of one or two 10-mg implants respectively. There was no effect on lactation or growth of a pouch young and contraception was reversible in a high proportion of females following the cessation of deslorelin release from the implants (Herbert et al. 2006). The GnRH analogue, deslorelin, provides a potential biotechnological solution for achieving a controlled, reversible suppression of fertility in female eastern grey kangaroos. But the practicality of using this agonist is depen- dant on effective inhibition of reproduction without negative physical or behavioural side-effects. To date, there have been no significant undesirable physical side-effects documented following long-term contraception using GnRH agonists. However, it is possible that the reduction in gonadal steroido- genesis caused by chronic GnRH agonist treatment may bring about behavioural changes, principally to sexual behaviour and dominance hierarchies. In wild populations, such changes may be undesirable as changes in dominance may confer altered access to resources within the environ- ment (Tuyttens and Macdonald 1998). Only two studies on the effects of GnRH agonists on wildlife behaviour have been published. These studies did not identify any significant behavioural side-effects of treatment, but were limited to observations of sexual interactions in wapiti (Baker et al. Wildlife Research, 2006, 33, 47–55 10.1071/WR04114 1035-3712/06/010047 © CSIRO 2006 R. Woodward A,B , M. E. Herberstein A and C. A. Herbert A,C,D A Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia. B Current address: Taronga Zoo, PO Box 20, Bradleys Head Road, Mosman, NSW 2088, Australia. C Current address: School of Biological, Earth & Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia. D Corresponding author. Email: cathherbert@unsw.edu.au Abstract. In recent years fertility control has been proposed as an ethically acceptable alternative to lethal control techniques when managing overabundant kangaroo populations. A promising non-steroidal, non-immunological approach to contraception in female kangaroos involves the use of slow-release implants containing the gonadotrophin-releasing hormone (GnRH) agonist deslorelin. The practicality of using deslorelin implants as a management option is dependant on its effective inhibition of reproduction without negative physical or behavioural side-effects. This study investigated the behavioural effects of deslorelin implants in female eastern grey kangaroos. Treatment had no detectable effects on crepuscular activity. Alterations in the frequency of sexual interactions were observed in deslorelin-treated females, with a behavioural oestrus induced ~3 days after combined removal of pouch young and deslorelin administration. Copulation was observed during this early oestrous period, but conception was not achieved and pouch young were not observed in any treated females. Control females gave birth within 69.6 ± 10.4 days (mean ± s.e.m., n = 9) of placebo implant administration. The first births observed in treated animals were on Days 510, 637 and 643 after treatment. The remaining seven treated animals had not bred by the end of the study, a period of 647 days. Fertility control in female eastern grey kangaroos using the GnRH agonist deslorelin. 2. Effects on behaviour www.publish.csiro.au/journals/wr CSIRO PUBLISHING