Photoperiodic control of sexual maturation in Muscovy drakes.

The influence of photoperiodic control on reproductive development was investigated in Muscovy drakes by the use of night-interruption photoschedules consisting of a main photofraction of 6 hr and a secondary photofraction of 1 hr, interrupting the dark period 6 hr (Group A), 8 hr (Group B), 10 hr (Group C), and 12 hr (Group D) after dawn. Sexual maturity, assessed by the increase in plasma concentrations of luteinising hormone (LH) and testosterone, was achieved in all groups by Week 34. However, significant differences in the precocity of the rate of sexual maturation were evident on Week 27. At this time, plasma testosterone concentrations, relative to those at Week 15, had increased by 2.02, 0.29, 0.89, and 2.68 ng/ml in Groups A, B, C, and D respectively. Increases in LH concentrations followed a similar pattern. The more pronounced increases in LH and testosterone concentrations observed from Group B to Group D indicate that long days stimulate reproductive development in the Muscovy duck via a diurnal rhythm of photosensitivity. However, the nonphotostimulated increase in hormonal concentrations observed in Group A implies that a factor other than photostimulation may be involved. The dissipation of juvenile photorefractoriness by long-term short-day exposure and subsequent passive sexual maturation may explain these results. We hypothesize that the rate of sexual maturation results from a balance between the dissipation of juvenile photorefractoriness by short days and the relative strength of the photoperiodic drive as the result of long days.