Chapter 34 - Circadian Rhythms

Abstract

Due to the daily rotation of the Earth on its axis, nearly all organisms are exposed to daily cycles of environmental factors, including the intensity and quality of light, temperature, and other abiotic or physical factors, as well as changes in biotic factors such as presence of predators, competitors, and food. Because of this, most organisms have evolved endogenous biological clocks that synchronize internal processes with external cycles and coordinate intrinsic processes. In birds, these biological clocks are particularly evident partly due to the fact that they are mostly diurnal and visible and also to the fact that these organisms strictly time all aspects of their lives. The biological clocks of birds comprise multiple circadian oscillators whose complex interactions are critical for self-sustained rhythmicity. These include the pineal gland, which influences the system through the secretion of the hormone melatonin, the retinae, and the hypothalamic suprachiasmatic nuclei. This circadian system is, in turn, synchronized or entrained to light–dark cycles via retinal and extraretinal photoreceptors in the pineal gland, lateral septum, preoptic area, and tuberal hypothalamus. This multi-oscillatory system synchronizes multiple behavioral and physiological processes that include brain metabolism, body temperature, endocrine processes, sleep–wake cycles, bird song, migration, and so on. The molecular mechanisms underlying these rhythms are currently being elucidated and appear to contain homologous transcriptional/translational feedback loops involving “clock genes” shared by all animals. Future prospects of integrating molecular with physiological and behavioral research depend on the development of transgenic technologies.