EFFECT OF THE RAM ON DURATION OF OESTRUS IN THE EWE

Summary. A series of experiments is reported in which the effect on the length of heat of varying degrees of association of the sexes was studied in Merino sheep. Length of heat in all ewes was determined by teasing with vasectomized rams at 4-hr intervals. When present throughout oestrus, mating behaviour of rams and also of stilboestrol-treated (`masculinized') ewes, reduced heat length in experimental ewes by some 30 to 50%. The sight, smell and sound of the rams were not sufficient to reduce the duration of oestrus. However, neither mounting nor intromission were necessary components of the male behaviour for the effect to be fully expressed. Oestrous periods synchronized by injections of progesterone were as effectively shortened by the continuous presence of the rams. The ewes were most sensitive to the effect of ram activity between 4 and 8 hr after the beginning of oestrus, but oxytocin treatment during this time did not reduce oestrous duration.