Seasonal variation in pituitary responsiveness to luteinizing hormone-releasing hormone of mallards and canvasbacks.

Endocrine profiles of wild mallards (Anas platyrhynchos) and canvasbacks (Aythya valisineria) were determined during the spring and the summer. Males and females of each species were treated with a single intravenous injection of 12 micrograms of synthetic luteinizing hormone-releasing hormone (LHRH). Preinjection and postinjection blood samples were collected and LH levels were determined by radioimmunoassay. LHRH evoked a brisk and significant elevation in serum LH in all ducks indicating the pituitary glands of both species contain readily releasable pools of LH. In March, the preinjection baseline levels of LH in the two groups did not differ. However, mallards responded to LHRH with a twofold increase in serum LH, while canvasbacks responded with a fourfold increase (F = 17.1, P less than 0.001). At the summer blood sampling period, mallards had significantly higher circulating levels of serum LH than canvasbacks (F = 35.9, P less than 0.001). Both species responded to LHRH with a twofold increase in circulating LH. These data indicate that seasonal variations in reproduction shown by the two species in captivity may be associated with differences in LHRH-sensitive pools of LH.