Endocrine changes in the incubating and brooding turkey hen.

Turkey hens were allowed to incubate eggs and to hatch and rear young. Plasma prolactin (Prl) levels increased prior to the start of continuous incubation and rose sharply as incubation progressed to reach a peak of 1178.2 +/- 221.8 ng/ml (mean +/- SEM) just before hatching. Prl levels then fell precipitously before the hens left the nest, and returned to preincubation levels (36.8 +/- 3.4 ng/ml) by the time the poults were 2 weeks old. These results show that the high plasma concentrations of Prl found during incubation are not initiated or maintained only by the stimulus of nesting. We suggest that the decline in Prl levels at the end of incubation could be related to the pipping and hatching of eggs, and the consequent shift to maternal behavior. Plasma growth hormone (GH) levels were significantly increased in hens which were brooding poults, but not in hens incubating eggs. An elevenfold, 1-day increase in plasma GH was observed immediately after the hens left the nests. Mean plasma GH levels rose from 12.0 +/- 4.7 ng/ml on the day that the hens left the nests to 133.0 +/- 32.0 ng/ml on the following day, and then declined to 23.1 +/- 9.6 ng/ml after an additional day. There were no significant changes in plasma thyroxine levels during laying, incubation and brooding. Plasma glucose concentration was significantly depressed during incubation.