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Circannual Cycle
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, so less than one cycle per 28 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.

The location of the physical circannual timer in organisms and how it works are almost entirely unknown.

Challenges for Scientific Study
One caveat of gathering information on circannual cycles is the longevity of these studies. The ratio of the period length of a circannual cycle to the length of the productive life of a scientist makes this branch of chronobiology very difficult. It takes an entire year to get one data point which makes it difficult to really see how these cycles adjust in time. To put this into perspective, a two week experiment for a circadian biologist would take fourteen years for a circannual researcher to come to the same magnitude of conclusions.

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. This lends to the argument that their physiology is innate rather than learned.

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.

Biological Advantage
A multitude of experiments have been done on birds and other organisms to help understand how and why this cycle exists. Generating these biological rhythms internally helps organisms anticipate important changes in the environment before they occur, thus providing the organisms with time to prepare and survive. For example, some plants have a very strict time frame in regards to blooming and preparing for spring. If they begin their preparations too early or too late they risk not being pollinated, competing with different species and other factors that might affect their survival rate. For example, the Parus major closely times the hatching of their chicks to the emergence of the protein-rich winter moth caterpillar. These birds are a single-brood bird, meaning they breed once a year with about nine chicks per brood. If the birds and caterpillars emerge at the right time, the caterpillars eat the new oak leaves and their population increases dramatically. This, hopefully will coincide with the arrival of the new chicks, allowing them to eat. The circannual cycle in some plants 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. Bird plumage and mammal fur change with the approach of winter and is triggered by the shortening photoperiod of autumn.

The circannual cycle can also be useful for animals that Migrate or Hibernate. For example, studies of the Pied Flycatcher, or ficedula hypoleuca, have shown that their spring migration timing is triggered by an internal circannual clock that is fine tuned to daylength. These particular birds overwinter in dry tropical forest in Western Africa and breed in temperate forests in Europe, over 4,500 km away. From 1980-2000 temperatures at the time of arrival and the start of breeding increased significantly. However, the birds have not advanced the spring arrival on their breeding grounds. They have, though, advanced their mean laying date by ten days. The reason they have not advanced their spring arrival is because it seems their circannual clock is triggered by daylength rather than temperature.

Many animals' reproductive organs change in response to changes in photoperiod. Male gonads will grow during the onset of spring to promote reproduction among the species. These enlarged gonads would be nearly impossible to keep year round and would be inefficient for the species. Many female animals will only produce eggs during certain times of the year.

Interaction with Changing Climate
Changing climate will make it difficult for species to evolve at an effective pace. Warming temperatures will cause some species to be unable to time important biological functions properly, thus making it difficult for them to thrive and survive. Other species behaviors that are influenced by time of day alone will not be affected too severely. That means that even if each individual species can adapt, there may still be disruptions at ecosystem level because the global food chain is such a delicate system.

One study performed by Menzel et al., analyzed 125,000 phenological records of 542 plant species in 21 European countries from 1971 to 2000. These researchers found that 78% of all plants studied advanced in flowering, leafing, and fruiting. Only three percent were sugnificantly delayed. They determined that the average advance of spring and summer was 2.5 days per decade in Europe.