Animal Behavior: Orientation and Navigation Biological Clocks Anyone who has lived for any time in the world recognizes that the world changes in cyclical patterns: day and night, the seasons, and tides. Many animal species have a sort of internal clock, called a biological clock, which predicts cyclical environmental change and prepares the animal to deal with it. Biological clocks can be set by exogenous (external) stimuli, or by endogenous (internal) rhythms. Exogenous stimuli are called Zeitgebers, which means "time giver" in German, and include light, temperature, and length of day. Endogenous clocks are responses to an internal rhythm that is pre- programmed to correspond with the environmental temporal pattern. Circadian Rhythms Circadian (literally "about a day") rhythms are endogenous animal clocks that operate on a daily time schedule. Most circadian rhythms are actually slightly shorter than 24 hours, some are longer, and a few are exactly 24 hours. Jet lag provides a common example of circadian rhythm.
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