Melatonin release from the pineal cells of chicks, pigeons and crows (diurnal birds) in vitro was compared with that from owls (a nocturnal bird). The pineal cells of diurnal birds secreted large amounts of melatonin during the dark period, whereas owl pineal cells released virtually no melatonin over 24 h and did not respond to exogenous stimulant agents. Histological examination revealed that the owl pineal gland is very small and has a poor vascular network. These results suggest that the pineal gland of owls may have degenerated and is not involved in the circadian clock mechanism in this species.
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