Some Aspects of Animal Photoperiodism

Many animals have the ability to distinguish between daily cycles of light and darkness that differ in length. This photoperiodic process is clearly one of time measurement since, in effect, it involves the comparison of different intervals of time. The mechanism that controls it can therefore be regarded as a clock of a particular type, namely an interval timer. Although seasonal reactions to day length are common enough, information on the location and mode of operation of the clock is only available for a few groups, among which birds and anthropods are particularly prominent.