n. the behavioral and physiological reactions of nonhuman animals and plants to changes in the length of days or in the intensity of light in the environment. Photoperiodism in animals is involved in the timing of seasonal migration behavior, reproductive cycles, changes in plumage or pelage, and hibernation. The shedding of leaves in autumn and winter dormancy are signs of photoperiodism in plants. Behavior affected by photoperiodism is usually in response to the direction of change in day length (i.e., increasing or decreasing) rather than absolute day length.