Daily variations in the concentrations of plasma prolactin in broody bantams

Abstract

Daily variations in the concentration of plasma prolactin were measured in bantam hens exposed to 14 hr light/day when they were laying, between 1 and 6 days before starting to incubate their eggs, 1 or 2 days after incubation had begun, and when they were brooding their chicks, 9–10 days after hatching. The mean concentration of plasma prolactin over 24 hr increased three- to fourfold at the onset of incubation, from 24.8 ± 1.2 ng/ml (mean ± SE, n = 7) to 100.3 ± 23.5 ng/ml ( n = 7) but fell to 41.6 ± 6.9 ng/ml ( n = 7) in brooding birds. The concentration of plasma prolactin did not vary over 24 hr in laying or incubating hens although in the former, the highest values consistently occurred during the night. A significant ( P < 0.001) daily variation in the concentration of plasma prolactin occurred in hens which were within 6 days of the onset of incubation or which were brooding chicks: prolactin levels increased steeply immediately after the lights went out and remained high during the dark period. Observations on individual bantams showed that about 6 days before incubation began, the secretion of prolactin increased at night and during successive days, this increase persisted for longer periods into the light period until on the first day of incubation, the concentration of plasma prolactin was high throughout the light and dark periods. These observations support the view that in the hen, an increase in the secretion of prolactin plays a key role in the induction of incubation behaviour. Further, a nocturnal increase in prolactin secretion may be involved in the maintenance of brooding behaviour.