Changes in plasma prolactin in male starlings during testicular regression under short days compared with those during photorefractoriness.

Male starlings which had fully developed testes under 11-h daily photoperiods were subjected for 10 weeks to daylengths either increasing or decreasing by 30 min per week. In both cases testicular regression occurred; in the former case this was complete due to photorefractoriness, and was accompanied by a postnuptial moult and a marked (P less than 0.01) rise in plasma prolactin concentration. In the latter case the rate of gonadal regression was slow and variable and no moulting was observed. Plasma prolactin remained very low. Eventually the birds which had experienced decreasing photoperiods were transferred to long (16 h) days; they were shown to be photosensitive since their testes redeveloped rapidly. There was also a rise in plasma prolactin, the birds eventually showed the full testicular regression characteristic of photorefractoriness, and moulting occurred. These results show that raised prolactin secretion is not associated with short-day-induced testicular regression but only with long-day-induced photorefractoriness.