A circannual cycle is a biological process that occurs in living creatures over the period of approximately one year. This cycle was first discovered by Ebo Gwinner and Canadian biologist Ted Pengelley. It is classified as an Infradian rhythm, which is biological process with a period longer than that of a circadian rhythm, less than one cycle per 24 hours. These processes continue even in artificial environments in which seasonal cues have been removed by scientists. The term circannual is Latin, circa meaning approximately and annual relating to one year. Chronobiology is the field of biology pertaining to periodic rhythms that occur in living organisms in response to external stimuli such as photoperiod.
Cycles come from genetic evolution in animals which allows them to create regulatory cycles to improve their fitness. Evolution for these traits comes from the increased reproductive success of animals most capable of predicting the regular changes in the environment like seasonal changes and adapt capitalize on the times when success was greatest. The idea of evolved biological clocks exists not only for animals but also in plant species which exhibit cyclic behaviors without environmental cues. Plentiful research has been done on the biological clocks and what behaviors they are responsible for in animals, circannual rhythms are just one example of a biological clock.
Rhythms are driven by hormone cycles and seasonal rhythms can endure for long periods of time in animals even without photoperiod signaling which comes with seasonal changes. They are a driver of annual behaviors such as hibernation, mating and the gain or loss of weight for seasonal changes. Circannual cycles can be defined by three main aspects being that they must persist without apparent time cues, be able to be phase shifted, and should not be changed by temperature. Circannual cycles have important impacts on when animal behaviors are performed and the success of those behaviors. Circannual cycles can be defined by three main aspects being that they must persist without apparent time cues, be able to be phase shifted, and should not be changed by temperature.
The location of the physical circannual timer in organisms and how it works are almost entirely unknown.
Examples
In one study performed by Eberhard Gwinner, two species of birds were born in a controlled environment without ever being exposed to external stimuli. They were presented with a fixed Photoperiod of 10 hours of light and 14 hours of darkness each day. 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.
Researchers Ted Pengelley and Ken Fisher studied the circannual clock in the golden-mantled ground squirrel. They exposed the squirrels to twelve hours of light and 12 hours of darkness and at a constant temperature for three years. Despite this constant cycle, they continued to hibernate once a year with each episode preceded by an increase in body weight and food consumption. During the first year, the squirrels began hibernation in late October. 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.
An annual rhythm has been observed in humans diagnosed with obsessive compulsive tic disorder (OCTD). The study focused on observing the patients’ seasonal patterns and how the cycle of seasons affected their behaviors. They observed that there was a statistically significant annual rhythm in patients with OC symptoms but not in patients with tic symptoms. As a result of the study, the researchers concluded that treatments for this disorder can be implemented following an observation of the patient’s cycle and annual rhythm that they follow.