Abstract
Development of daily rhythms in the chick heart rate was investigated during the post-hatching period. The measurements were made in free-moving chicks at one-hour intervals under different light conditions. Clear daily rhythms were seen 3 days after hatching under LD (12:12 hr.) whereas no daily rhythm was observed under either constant illumination or constant dark. The present study demonstrated that while heart rate increased with age during the post-hatching period, the light-time heart rate increased more rapidly than the dark-time heart rate. It was also found that the daily rhythms in heart rate were not formed after food removal even under LD. These results indicate that the light and feeding play an important role in the development of the daily rhythms in chick heart rate.
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