Photorefractory period of the Muscovy duck (Cairina moschata): endocrine and neuroendocrine responses to day length after a full reproductive cycle

Abstract

1. The Muscovy duck, of equatorial origin, is photosensitive and its sexual maturation can be advanced by long days. The aim of this study was to investigate the photorefractory nature of the seasonal sexual rest in this species. 2. Sixty males were allocated to three groups of 20 birds each after completion of one full reproductive cycle in long days (16L:8D). Group A was transferred to longer days (20L:4D). Group B remained in 16L:8D. Group C was transferred to short days (6L: 18D) for 9 weeks and then returned to 16L:8D. 3. Group B spontaneously redeveloped their testes and recrudescence, indicated by increased plasma concentrations of luteinising hormone and testosterone, started between the 4th and 10th week after total regression. Longer days (in group A) accelerated recrudescence. Shorter days (in group C) delayed it, but the return to long days restored reproductive function. 4. In group C, the transfer to short days increased the hypothalamic content of luteinising hormone‐releasing hormone (LHRH) and secretion of the neuropeptide in vitro. The pituitary sensitivity to LHRH was increased in that group after the return to long days. 5. These effects show that the birds were photosensitive when daylength was changed 8 weeks after the completion of regression, and that short days were not necessary for the birds to regain photosensitivity. 6. The shortness of the regressed period and the spontaneous recrudescence in long days are more typical of equatorial birds than temperate zone species, which are more dependent on photoperiodic changes. The Muscovy duck probably retains features of its wild ancestor, which, living in equatorial latitudes, was more subject to non‐photic synchronisers.