The annual reproductive cycle in mallards.

In wild mallard drakes plasma FSH and LH levels were elevated at the height of the breeding season in spring, decreased towards the end of the reproductive phase and were low during the time of photorefractoriness in summer, when the testes were regressed. In contrast plasma prolactin concentrations increased only towards the end of the breeding season and were at their annual height during the summer. Castration of photorefractory mallards caused a steep rise of plasma LH levels indicating that the testes were involved in the maintenance of the low LH concentrations during the refractory period. In these drakes LH titers remained high throughout the year and, thus, in the absence of the testes the birds did not become photorefractory. In photorefractory drakes removal of 90% of the testicular tissue not only increased plasma LH levels but induced complete spermatogenesis in the testicular remains within 6 weeks. Therefore it seems that photorefractoriness in mallards is due to an increased sensitivity of the hypothalamo-hypophyseal system to the negative feedback action of testicular androgens. After injection of tritiated testosterone the hypothalamus, anterior pituitary and testes, but not other organs studied, took up more radioactivity during the refractory phase than at the height of the breeding season. It is speculated that the increased sensitivity of the hypothalamo-hypophyseal unit to the negative androgenic feedback during photorefractoriness depends on an increased number of androgen receptors in these structures which, again, could be due to the elevated prolactin levels during this phase of the annual cycle.