Pinealectomy affects the circannual testicular rhythm in european starlings (Sturnus vulgaris)

Summary Pinealectomized and sham-operated European starlings were maintained for 16 months under a constant 12-h photoperiod and constant temperature conditions. In all birds, testicular width was measured at about monthly intervals and the onset and end of molt was determined. Shortly after the beginning of the experiment, the sham-operated birds went through a cycle of testicular growth and regression which was followed by a complete molt; subsequently most individuals initiated a second testicular cycle. Most of the pinealectomized birds, in contrast, failed to go through a second testicular cycle. Moreover, during the first cycle their testes regressed earlier than in the sham-operated birds and the subsequent molt was relatively advanced. In these respects the pinealectomized birds behaved like intact starlings under a 13-h photoperiod. Since pinealectomy probably changes the phase-relationship between circadian rhythms and the entraining light-dark cycle it is proposed that pinealectomy in the present experiment might have altered the phase-relationship between a circadian rhythm of photosensitivity and the light-dark cycle in such a way that the birds interpreted the 12-h photoperiod as a 13-h photoperiod.This work was supported by the Deutsche Forschungsgemeinschaft SPP Mechanismen biologischer Uhren.