THYROIDECTOMY PROGRESSIVELY RENDERS THE REPRODUCTIVE-SYSTEM OF STARLINGS (STURNUS-VULGARIS) UNRESPONSIVE TO CHANGES IN DAYLENGTH

If starlings (Sturnus vulgaris) are transferred to long days soon after being thyroidectomized, they show gonadal maturation but no subsequent photorefractoriness, suggesting a specific role for thyroid hormones in the development of photorefractoriness. This study tests an alternative hypothesis: that following thyroidectomy all gonadal responses to photoperiodic changes are progressively inhibited. Photoperiodic responses of starlings were examined at two different times after thyroidectomy. Transfer of thyroidectomized birds to long days 4 weeks after thyroidectomy caused testicular growth but at a slower rate than in intact birds, whereas, in thyroidectomized birds kept on short days, testicular growth was greater than in corresponding intact birds. Thirteen weeks after thyroidectomy, there was little or no response to an increase or a decrease in daylength in thyroidectomized birds, whereas intact birds showed marked testicular growth or regression respectively. The results support the hypothesis that the reproductive system gradually becomes unresponsive to changes in daylength during the weeks following thyroidectomy. This long-term effect may mean that earlier conclusions regarding the effects of thyroidectomy need to be reassessed.