Effects of bilateral ganglionectomy and melatonin replacement on seasonal rhythm of testicular activity in Zembra Island wild rabbits (Oryctolagus cuniculus)

This study examined the effects of subcutaneous melatonin implants on testi cular activity (testis volume and plasma testosterone) in wild rabbits (Ory ctolagus cuniculus) captured on Zembra Island (North Tunisia) and maintaine d in natural temperature and photoperiod conditions. The period of testicul ar activity was 2 months longer in intact animals with melatonin implants t han in intact animals without melatonin implants. After bilateral superior cervical ganglionectomy, testicular activity was inhibited and no spontaneo us recrudescence in gonadal function was observed during the 13 months of t he experiment, demonstrating the absence of an endogenous circannual rhythm of reproductive function in this species. Renewed testicular activity was observed 2 months after the insertion of melatonin implants in ganglionecto mized animals. These results confirm that testicular activity is stimulated by short days in late autumn and that melatonin reactivates the reproducti ve axis in this species. This finding is in contrast to that in continental populations of wild rabbits in which reproduction is inhibited by short da ys or melatonin. These results are discussed in terms of insularity and may reflect the geographical isolation of this population.