Serum levels of luteinizing hormone (LH), prolactin, estradiol, and progesterone in laying and nonlaying canvasback ducks ( Aythya valisineria)

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Temporal changes in the levels of serum luteinizing hormone (LH), prolactin, estradiol, and progesterone associated with the reproductive patterns of 53 wild captive canvasback ducks were measured. The reproductive endocrinology of both laying and nonlaying females was compared in this 3-year study. Females that remained sexually inactive had ovaries with small, undeveloped follicles. Nonlaying ducks also had lower serum levels of LH ( P < 0.01), prolactin ( P < 0.05), estradiol, and progesterone than those of laying ducks in mid-April (during prelay), mid-May (on the fourth day of egg production), and mid-June (during postlay and incubation). Prolactin levels of both layers and nonlayers increased over this time interval ( P < 0.01) but levels of nonlayers were significantly lower than those of layers for the three blood-sampling dates. The low prolactin levels demonstrate that reproductive failure was not a result of inhibition by high serum prolactin levels. Intravenous injections of luteinizing hormone-releasing hormone (LHRH) in female canvasbacks resulted in significantly elevated ( P < 0.01) serum LH on the prelay blood sampling date. Lack of reproduction in nonbreeding canvasbacks was thus associated with low circulating serum LH levels but with LHRH-sensitive pituitary pools. These data suggest that lack of reproduction was a result of the failure of the hypothalamus to release releasing factors. The serum hormones of laying canvasbacks varied temporally with stages of the nesting cycle. LH levels increased prior to egg laying and fluctuated during the laying period. LH levels decreased at the onset of incubation but increased after loss of clutch, with renesting activity. Serum prolactin levels of layers were low prior to egg laying and increased gradually through laying of the first clutch, the renesting period, and laying of the second clutch. The highest prolactin levels occurred in ducks incubating their eggs. Prolactin levels decreased in ducks that failed to incubate their eggs. Serum estradiol levels increased sharply between 2 and 5 weeks prior to egg laying and remained high until the second day of egg production. Estradiol levels decreased when the fourth egg was produced, and remained low through the laying of the first clutch, the renesting period, and laying of the second clutch. Progesterone fluctuated widely through the nesting cycle, showing several major peaks before laying and another during incubation.