Effects of exogenous hormones on the reproductive behaviour of adult male domestic ducks II. Correlation with morphology and hormone plasma levels.

The behavioural effects observed following repeated injections of pregnant mare serum (PMS), human chorionic gonadotropin (HCG), testosterone propionate (TP), oestradiol benzoate (OB) or progesterone (P) were accompanied by significant effects on reproductive morphology and on plasma levels of pituitary-gonadal hormones. The testes weight was decreased by TP and P treatment, FSH secretion was inhibited by the three steroid preparations (TB, OB, P) but LH was not suppressed following OB injections (only TP and P were effective in this respect). The behavioural effects previously described are discussed in the light of the present results and a possible unitary mechanism is proposed for the inhibition of social display by TP, OB and P. Correlations between individual variations of hormone levels and of behaviour are presented: these are shown to be significant in only a few cases. Finally, a tentative model is proposed to explain the endocrine basis of the annual reproductive cycle of male ducks.