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For every animal, there is an annual cycle of events. This may include such regular activities as mating, migration, colour change or even the growth and loss of antlers. The timing of each of these events is very important to the survival of each individual. If a snowshoe hare turns white too early, it will stand out dangerously against the brown colours of autumn.

Most animals possess a form of internal clock which keeps them in tune with the changes occurring around them. This clock follows an almost annual cycle bringing about the seasonal changes and adaptations of each species. It doesn't necessarily correspond with the calendar year as it will vary slightly based on the environmental clues received by the animal. These clues may include things such as photoperiod (length of the dark period of night), temperature and snowfall.

Photoperiod provides one of the primary environmental triggers to change. This changes seasonally and is often used as a trigger for adaptation. For instance it is thought to play an important role in the colour change of animals like the weasel and the snowshoe hare. As the days get shorter, the hare's colour will begin to change to white.

A problem arises when the snows come unusually early or late thus leaving the hares in a contrasting colour scheme and unusually vulnerable. Luckily, in reality this rarely occurs as photoperiod is only one of triggers for colour change. The exact timing is adjusted according to temperature and snowfall. Without this variation, the hares would turn colour around the same time every year regardless of the actual weather conditions.

How does this all occur? The change in photoperiod causes a hormonal change to occur. The hormones produced are responsible for the change in colour but are also affected by temperature and snowfall. This helps to ensure that colour change will not occur too early.

This timing mechanism doesn't always work and it is possible to occasionally come across a brown hare against a white snowpack. The wonders of natural selection generally make sure that this is a fairly rare occurrence as these individuals often find themselves attracting the attention of hungry predators.

Although we've talked mainly of colour change in regards to the animals internal clocks, the same principals apply to most annual behaviour. Birds migration is triggered by photoperiod but varies according to food supply and other environmental conditions (such as open water for waterfowl). Hibernation in ground squirrels also varies according to external conditions.