Many plants respond to the climatic cycle of their habitat. In particular, photoperiod, temperature, and rainfall are important cues for the commencement and also cessation of dormancy. Some species respond to long-term climatic cycles, while others undergo more immediate facultative responses to ambient temperature or water availability. For animals, single-day torpor can occur rapidly in response to short-term environmental changes, such as inclement weather. It generally occurs on a circadian cycle, corresponding to the normal period of activity/ inactivity. Onset of hibernation or estivation, being seasonal long-term periods of dormancy, is a more prolonged and sometimes programmed response to an impending change in environmental conditions. For example, desert frogs initiate estivation if conditions become dry, by burrowing, forming a cocoon, and initiating intrinsic metabolic depression; this can take 3-4 weeks, but occurs independent of time of year. In contrast, hibernation by some mammals such as ground squirrels is obligate and only occurs at a specific time of the year after a period of preparation (e.g., seeking out or constructing suitable hibernation sites, increased activity and feeding, deposition of energy stores, and changes in body fluid solutes). This pattern of obligate hibernation is controlled on a circannual cycle by cues that include shortening photo-period and decreasing air temperature. Reduced water availability and high Ta are primary cues for estivation.
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