Circannual cycle

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.[1]

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.[3]

A classic example in insects is the varied carpet beetle.

Generating biological rhythms internally helps organisms anticipate important changes in the environment before they occur, thus providing the organisms with time to prepare and survive.[1] 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, 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.

Similarly, bird plumage and mammal fur change with the approach of winter, and is triggered by the shortening photoperiod of autumn.[4] The circannual cycle can also be useful for animals that Migrate or Hibernate. 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.[3]

Changing climate may unravel ecosystems in which different organisms use different internal calendars. Warming temperatures may lead to earlier blooms of flora in spring. For instance, one study performed by Menzel et al., analyzed 125,000 phenological records of 542 plant species in 21 European countries from 1971 to 2000 and found that 78% of all plants studied advanced in flowering, leafing, and fruiting while only three percent were significantly delayed. They determined that the average advance of spring and summer was 2.5 days per decade in Europe.[5] Meanwhile, fauna may breed or migrate based on the length of day, and thus might arrive too late for critical food supplies they co-evolved with.

For example, the Parus major closely times the hatching of their chicks to the emergence of the protein-rich winter moth caterpillar, which in turn hatches to meet the budding of oaks.[3] These birds are a single-brood bird, meaning they breed once a year with about nine chicks per brood. If the birds and caterpillars and buds all emerge at the right time, the caterpillars eat the new oak leaves and their population increases dramatically, and this hopefully will coincide with the arrival of the new chicks, allowing them to eat. But if plants, insects, and birds respond differently to the advance of spring or other phenology changes, the relationship may be altered.

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.[3] 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 have warmed significantly. They have advanced their mean laying date by ten days, but have not advanced the spring arrival on their breeding grounds because their migration behavior is triggered by photoperiod rather than temperature.[4]

In short, even if each individual species can easily live with elevated temperatures, disruptions of phenology timing at ecosystem level may still imperil them.[3]

One reason for the paucity of research on circannual cycles is the duration of required efforts. 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 difficult.[3] It takes an entire year to get a time series which makes it difficult to see how these cycles adjust over the years. 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.