Several animal activities are restricted in their occurrence to particular times of the year,
which infers that the correct timing of these activities is necessary for the efficient
organization, perhaps even the survival, of the species concerned. A feature common to
many of the animal mechanisms is their concern with the regulation of annual
reproductive cycles to ensure that offspring are produced when seasonal conditions are
most favourable. In many cases this means that the chain of events starts long before the
season itself and therefore a control system is required that anticipates favourable
environmental circumstances. Weather and food supply cannot be predicted in detail, but
the control system most favoured by natural selection is one that leads to the production
of the young at a time of year when the probability of favourable conditions in the
breeding season is greatest.