Photoperiodism and sexual activity in Merino ewes. II. The effect of equinoctial light on sexual activity

Abstract

Merino ewes were subjected to equinoctial light for 2 years; some continued to exhibit seasonal variation in sexual activity while others exhibited continuous sexual activity. Continued seasonal variation in sexual activity in equinoctial light is at variance with existing hypotheses for photoperiodic control of sexual activity in ewes, and alternative explanations for such behaviour are discussed. Two groups, each of five Merino ewes, were subjected to a light environment of 12 hr light : 12 hr dark, the one group from the autumn, the other from early in the spring of the same year. In the first group, all ewes failed to experience anoestrus during their first spring, but two ewes experienced anoestrus during the spring of the second year. In the second group, three ewes experienced anoestrus later in the spring in which treatment commenced, and the same three experienced anoestrus during the spring of the second year of the experiment. In these five ewes which exhibited seasonal variation in sexual activity, the anoestrous periods were of shorter duration, but occurred at the same season as those in ewes of two control groups.