So far, I have directed all my attention to daily - circadian - rhythms, and pretty much ignored other rhythms that correspond to other cycles in nature. Another obvious cycle in nature is the procession of seasons during a year.
Just as an environment during the day is different from the same environment during the night and thus requires different adaptations for survival, so the winter environment and the summer environment present very different problems for an organism's physiology to solve. If one thinks of the circadian clock as a relay timer that orchestrates switching on the "night adapatations" in the evening and "day adaptations" in the morning, then what mechanism plays an analogous role over the course of a year?
There are a number of obvious changes in the environment that occur as a year progresses. It is cold in the winter and hot in the summer. It snows in the winter, and there is a lot of rain during spring and fall. It is easy to find food during the warm season, but very difficult during the winter. It makes sense that organisms would evolve physiological mechanisms that allow them to allocate energy-expensive and risky activities (e.g., reproduction, parenting) to the times of the year when food and cover are abundant, while switching to a more energy-saving mode in the winter.
It also seems wise to be able to predict changes before they happen: not starting a big reproductive effort while the weather is good if it is going to turn cold and nasty just at the time the young are born. Interestingly, it has been found that many mammalian species harbor a subset of individuals that do not respond to seasonal cues - they are quite capable of breeding during the winter. This is an evolutionarily risky strategy, but the pay-off is large whenever the winter is particularly mild. Such non-responsiveness has been studied in a number of species, particularly white-footed mice, and was easily artificially bred in Japanese quail.
One such mechanism for governing biological seasonality is the circannual clock (or "calendar"). In many organisms (even when kept in constant conditions in the laboratory), certain events, e.g., reproductive maturation and behavior, occur with a precise rhythm whose period is close to (usually a little shorter than) 365 days. Not much is known about the physiology of circannual rhythms, though. Deletion of the SCN in rodents does not eliminate circannual rhythms, for instance, suggesting that circannual clock is a separate mechanism from the circadian clock and is also located elsewhere in the brain or body of the animal.
Freerunning circannual rhythms have periods too different from 365 days to be accurate on their own. They have to be entrained to the actual year, in a manner similar to the way circadian rhythms need to be entrained to the day/night schedule. Circadian rhythms can be entrained by a large variety of cues (e.g., temperature cycles, noise, social cues, cycles of magnetic field changes, barometric pressure cycles, etc.), but by far the strongest cue is light. Changes of light intensity over the course of 24 hours are the universally most-utilized entraining agents in nature, because they have the greatest predictive power: no other cue is as reliable. How about circannual rhythms?
Depending on the geographic region, a number of environmental cues serve as dominant triggers for annual physiological changes. Rain (e.g., monsoons), temperature and social cues can trigger reproductive maturation and behavior, entry into hibernation, or start of annual migration. These cues are sometimes called "proximal cues" as they more or less directly affect the onset of annual biological events. However, the proximal cues can only work if the organism is already 'prepared' for them.
In very unpredictable environments, e.g., in deserts in which it may not rain for years, the organisms are at a constant readiness - always receptive to proximate cues. A well known example is a finch living in deserts of Australia. When it rains, it is ready to mate and lays eggs within a day or two.
In almost constant environments, e.g., at the poles and at the equator, the plants and animals may follow freerunning circannual rhythms. For instance, elephant seals have a breeding season every ten months, thus falling in a different month every year.
In most areas of the planet, however, there are sharp and predictable changes between the seasons. The "ultimate cue" that prepares the body for the emergence of proximal cues is the gradual change in daylength - photoperiod.
Unlike fluctuations in temperature, humidity, or even light-intensity, photoperiod is a reliable cue. On March 21st of EVERY year, photoperiod is LD 12:12. At any latitude, the photoperiod is always exactly the same on the same date of the year. No other cue can come close to matching such precision. Thus, it is not surprising that almost all organisms, even those living on the equator and the poles, are capable of responding to changes in daylength, at least in the laboratory. Seasonal Affective Disorder (SAD) - or "Winter Blues" - in humans is thought to be a response to changes in photoperiod. It is not cold weather that makes you depressed in winter, it is the short days and long nights.
In many organisms circannual rhythms are too weak, or even undetectable, to be able to drive a seasonal rhythm of responsiveness to the environment, let alone a rhythm of actual behavior. Still, no matter if the circannual rhythm is robust, weak, or undetectable, changes in photoperiod are capable of entraining (robust) or directly driving (weak) circannual rhythms.
Next couple of posts will focus on the phenomenon of photoperiodism and what we have learned about the mechanisms organisms use to translate changes in daylength into seasonal biological events. You should not be surprised that evolution used an existing timer - the circadian clock - to measure the length of the day. More on that later...