In chronobiology, the circannual cycle is characterized by biological processes and behaviors recurring on an approximate annual basis, spanning a period of about one year. This term is particularly relevant in the analysis of seasonal environmental changes and their influence on the physiology, behavior, and life cycles of organisms. Adaptations observed in response to these circannual rhythms include fur color transformation, molting, migration, breeding, fattening[1] and hibernation, all of which are inherently driven and synchronized with external environmental changes.[2][3]
The regulation of these cycles is linked to internal biological clocks, akin to the circadian rhythm, which respond to external cues such as variations in temperature, daylight length (photoperiod),[1] and food availability. Such environmental signals enable organisms to anticipate seasonal variations and adjust their behaviors and physiological states, thereby optimizing evolutionary fitness and reproductive success.[3]
Circannual rhythms are evident in a range of organisms, including birds, mammals, fish, and insects, facilitating their adaptation to the cyclical nature of their habitats. Circannual cycles can be defined by three primary characteristics: persistence in the absence of apparent time cues, the capacity for phase shifting, and stability against temperature fluctuations.[3] Classified as an infradian rhythm, it occurs less frequently than a circadian rhythm. This cycle was first discovered by Ebo Gwinner and Canadian biologist Ted Pengelley.[3][4]
Derived from Latin, the term circannual combines circa, meaning approximately, with annual, referring to a period of one year.
- ^ a b Wood SH, Christian HC, Miedzinska K, Saer BR, Johnson M, Paton B, et al. (October 2015). "Binary Switching of Calendar Cells in the Pituitary Defines the Phase of the Circannual Cycle in Mammals". Current Biology. 25 (20): 2651–62. Bibcode:2015CBio...25.2651W. doi:10.1016/j.cub.2015.09.014. PMC 4612467. PMID 26412130.
- ^ "Circannual rhythm". Oxford Reference.
- ^ a b c d Murphy BA (May 2019). "Circadian and Circannual Regulation in the Horse: Internal Timing in an Elite Athlete". Journal of Equine Veterinary Science. 76: 14–24. doi:10.1016/j.jevs.2019.02.026. PMID 31084748. S2CID 87820119.
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