The involvement of thyroxine and daylength in the development of photorefractoriness in European starlings

Abstract

AbstractThyroidectomized starlings on long days do not become photorefractory. Four experiments were done to investigate the roles of thyroxine and daylength in the development of photorefractoriness. 1) Pharmacological doses of thyroxine administered to thyroidectomized starlings on long days induced photorefractoriness and molt, whereas the same treatment to birds on short days was ineffective. 2) Maintenance of intact starlings on a daylength long enough to cause full gonadal maturation but not quite long enough to induce photorefractoriness caused no progression toward photorefractoriness; subsequent gonadal regression after transfer to long day days occurred at the same rate as in intact birds transferred directly from short days to long days. 3) In thyroidectomized birds held on long days and then treated with thyroxine, gonadal regression occurred at the same time as in thyroidectomized birds held on short days and then treated with thyroxine while simultaneously being transferred to long days. 4) Thyroidectomized birds held on long days and treated with different doses of thyroxine became photorefractory at the same rate. These results demonstrate that thyroxine does not drive the process leading to photorefractoriness. For photorefractoriness to occur there has to be a temporal coincidence of daylength in excess of a critical daylength and the presence of thyroxine; thyroxine specifically permits the process to respond appropriately to ambient daylength.