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Seasonal changes in the concentrations of plasma gonadotropins and prolactin in wild mallard drakes.

Seasonal changes in the concentrations of plasma luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin were measured in serial samples taken from seven captive wild mallard drakes exposed to natural lighting and temperature in Kiel, West Germany (54 degrees N), for 20 months. The seasonal pattern of plasma LH levels was characterized by high titers during the reproductive phase in the spring, a steep decrease toward the end of this phase (May/June), low levels during the summer, and a second annual peak in the fall. Plasma FSH levels increased during February and March, the period of rapid testicular growth, and reached the highest values at the end of March/beginning of April. Later in the spring FSH levels decreased and remained low for the rest of the year. The concentrations of plasma prolactin increased progressively during April and May, reaching their highest values at the end of the breeding season, coinciding with the steep fall in the levels of plasma gonadotropins. Prolactin concentrations fell during July and August and were at their lowest level in the autumn. It is concluded that the development of photorefractoriness is associated with an increase in the concentrations of plasma prolactin.