Life evolves in a cyclic environment, and to be successful, organisms must adapt not only to their spatial habitat, but also to their temporal habitat. How do plants and animals determine the time of year so they can anticipate seasonal changes in their habitats? In most cases, day length or photoperiod acts as the principal external cue for determining seasonal activity. For organisms not living at the bottom of the ocean or deep in a cave, day follows night, and the length of the day changes predictably throughout the year. These changes in photoperiod provide the most accurate signal for pr ... More

Life evolves in a cyclic environment, and to be successful, organisms must adapt not only to their spatial habitat, but also to their temporal habitat. How do plants and animals determine the time of year so they can anticipate seasonal changes in their habitats? In most cases, day length or photoperiod acts as the principal external cue for determining seasonal activity. For organisms not living at the bottom of the ocean or deep in a cave, day follows night, and the length of the day changes predictably throughout the year. These changes in photoperiod provide the most accurate signal for predicting upcoming seasonal conditions. Measuring day length allows plants and animals to anticipate and adapt to seasonal changes in their environments in order to optimally time key developmental events including seasonal growth and flowering of plants, annual bouts of reproduction, dormancy and migration in insects, and the collapse and re-growth of the reproductive system that drives breeding seasons in mammals and birds. Although research on photoperiodic time measurement originally integrated work on plants and animals, recent work has focused more narrowly and separately on plants, invertebrates, or vertebrates. As the fields have become more specialized there has been less interaction across the broader field of photoperiodism. As a result, researchers in each area often needlessly repeat both theoretical and experimental work. However, over the past decade, intense work on daily and seasonal rhythms in fruit flies, mustard plants, and hamsters and mice, has led to remarkable progress in understanding the phenomenology, as well as the molecular and genetic mechanisms, underlying circadian rhythms and clocks. This book was developed to further this type of cooperation among scientists from all related disciplines.