Photoperiodism and sexual activity in Merino ewes. I. The effect of continuous light on the development of sexual activity

Abstract

Sixteen Merino ewes were subjected to continuous light made up of daylight during the day and artificial light at night for a period of 3 years from the time the ewes were 2–3 months old. Sexual activity in these exes was compared with that in 16 control ewes held in natural daylight. Criteria of sexual activity mere the occurrence of oestrus and ovulation. Oestrus was inferred from the presence of colour marks on the rumps of the ewes which were run with vasectomized rams painted on the brisket with sheep-branding fluid. Ovulation was inferred from the occurrence of characteristic changes in the vaginal contents. Compared with the activity exhibited by the control ewes, the occurrence of oestrus, but not of ovulation, was apparently partly suppressed by continuous light during the first year and was more variable during the second year of the experiment. There was little difference between the two groups in the third year. Despite the initial suppression of oestrus, most ewes under continuous light exhibited sexual activity which was seasonal in nature and little different from that exhibited by the control ewes. Subsequently the continuously lighted eves from the first experiment were subjected variously to continuous light of constant intensity alone or to continuous light of constant intensity on which daylight was superimposed, with and without exposure to rain and wind. Compared with the activity exhibited by control ewes, continuous light of constant intensity alone permitted, if anything, an increase in sexual activity. The results do not fit any existing hypothesis for photoperiodic control of sexual activity in Merino ewes, and several alternative suggestions are discussed.