Activity Cycle

a term in biology denoting periodic variations in animal mobility and behavior in connection with alteration (seasonal, daily) of living conditions and oscillations of metabolic intensity which result from such oscillations. The activity cycle is one manifestation of biological rhythms, which are inherent in all organisms. The seasonal activity cycle, associated with variation in day length, temperature curve, humidity, food supply, and the like, determines the periodicity of reproduction, animal and bird migrations, food storing, and fat accumulation. Certain land mammals and nearly all cold-blooded animals go into hibernation during the cold months of the year. Daily activity cycles result from changes in temperature, illumination, and other factors; with these are associated daily migrations of animals (this is characteristic mainly of plankton, soil organisms, and a few others). On the basis of type of daily activity, animals can be classed as diurnal, nocturnal, crepuscular, and around the clock. Variations in the activity cycle are not uncommon; many animals lead a diurnal existence during winter and a nocturnal existence during summer.