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Abstract
This study investigates the relative strengths of food and light zeitgebers in synchronization of circadian rhythms of Indian weaver birds and the role of the pineal gland in food-induced synchronization of the circadian activity rhythms. Two experiments were performed. In the first experiment, six birds were concurrently exposed for 10 days to PA 12/12 (12 h food present: 12 h food absent) and LD 12/12 (12 h light: 12 h dark). Then, the PA 12/12 cycle was reversed: food was present during the dark period of the LD 12/12 cycle. After 15 days, birds were released into constant dim light (LLdim). During exposure to overlapping light and food availability periods, birds were active only during the daytime. When light and food availability periods were presented in antiphase, two of six birds became night active. However, with the removal of the light zeitgeber (i.e., under LLdim), all birds were synchronized with reversed PA 12/12; hence, they were active during the subjective night (i.e., the period corresponding to darkness [ZT12-0] of the preceding LD 12/12). The second experiment examined whether the pineal contributed to the food-induced synchronization. After two weeks of concurrent PA 12/12 and LD 12/12 exposure, six birds were released into LLdim for 2.5 weeks. Under LLdim, five of six birds were synchronized to PA 12/12 with the circadian period (tau, τ) = 24 h. The LD 12/12 was restored, and after seven days, birds were pinealectomized (pinx). After 2.5 weeks, pinx birds were again released into LLdim for 2.5 weeks. Under LLdim, pinx birds did not become arrhythmic; instead, they appeared synchronized to PA 12/12 with τ = 24 h (n = 4) or ∼24 h (n = 2). We conclude that both food and light act as zeitgebers, although light appears to be the relatively stronger cue when the two are present together, as in the natural environment. We also found that the pineal is not necessary for food-induced synchronization. The findings suggest that food cycles could act as the synchronizer of circadian rhythmicity in biological functions in individuals held in an aperiodic environment.