Vernal increases in prolactin levels of nonbreeding male and female canvasbacks ( Aythya valisineria)

Abstract

Serum levels of prolactin were determined in nonbreeding male and female canvasbacks ( Aythya valisineria) from mid-April to late June to evaluate the temporal changes occurring during spring. During this time interval, serum prolactin levels increased 1.5- to 2-fold in both males and nonlaying females ( P < 0.01). Serum prolactin levels of males and females exposed to natural lighting and temperature at Delta, Manitoba (50°11′N, 98°19′W), steadily increased from mid-April when the daylength was 14 hr light and peaked in mid-June when daylength was 16.25 hr light. Administration of exogenous estradiol via slow-release subdermally implanted capsules in females with intact reproductive systems had a marginal effect on reducing the seasonal increase in prolactin. Exogenous testosterone in similar slow-release capsules implanted subdermally in males had no influence on circulating prolactin levels.