The effect of testosterone suppression on health and parasite burden in male eastern grey kangaroos (Macropus giganteus)

Testosterone has a dualistic effect in males by promoting sexual ornamentation at the cost of immune defence. This trade-off has been demonstrated in several taxa, such that males often host a greater parasite burden compared with female conspecifics. We suppressed testosterone in wild male eastern grey kangaroos (Macropus giganteus) for 10 weeks using a novel gonadotropin-releasing hormone (GnRH) vaccine, Bopriva. We evaluated the impact of testosterone suppression on testes width, parameters of health, tick and worm burden in kangaroos using a before-after-control-impact (BACI) experimental design. Given the potential impact of animal movement on parasite burden, core area use of a subset of males was also determined. Bopriva significantly reduced testosterone in male kangaroos as well as reducing the combined size of testes by 9.6% (P = 0.01). There was no detectable effect of testosterone suppression on parasite burden and core area use in Treated kangaroos compared to Control (placebo) and Before treatment animals. Our results suggest that a duration of suppression longer than 10 weeks may be required to observe changes in parasite burden. Overall, this study provides a suitable framework for future studies to test whether reproductive hormones influence energy allocation, parasitism, and reproductive strategies in marsupials.