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.
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.
The birds were exposed to these conditions for eight years and consistently molted at the same time as they would have in the wild, indicating that this physiological cycle is innate rather than governed environmentally.
They started hibernating in mid August and early April respectively for the following two years, displaying a circannual rhythm with a period of about 10 months.
[6] Gwinner observed the willow warbler (Phylloscopus trochilus) which is a bird species that migrates seasonally to tropical and southern Africa.
Gwinner observed that even through a lack of environmental cues for migration, the willow warblers followed precise schedules attributed to their circannual rhythm.
Lastly, to clarify the significance of the circannual control of the A. verbasci life cycle, larvae were reared under natural photoperiod and temperature from the various times of the year.
The sensitivity to these environmental influences reflect adaptations to migration patterns that could serve as further insight to the cost-and-benefit of transportation and risk of predation.
If they begin their preparations too early or too late they risk not being pollinated, competing with different species, or other factors that might affect their survival rate.
Having a circannual cycle may keep them from making this mistake if a particular geographic region experiences a false spring, where the weather becomes exceptionally warm early for a short period of time before returning to winter temperatures.
As another example, studies of the Pied Flycatcher (ficedula hypoleuca) have shown that their spring migration timing is triggered by an internal circannual clock that is fine tuned to day length.
[11] In short, even if each individual species can easily live with elevated temperatures, disruptions of phenology timing at ecosystem level may still imperil them.
To put this into perspective, a two-week experiment for a circadian biologist would take fourteen years for a circannual researcher, in order to achieve the same level of data robustness for the conclusions.