The general roles of torpor, hibernation, and estivation are avoidance of unfavorable short-term or long-term (seasonal) climatic conditions and conservation of energy during this period of inactivity. Seasonal dormancy also has obvious ecological benefits. It allows species to exploit ephemeral environments. Hibernation and estivation enable species to colonize habitats that would otherwise be unsuitable for growth or survival at certain times of the year due to harsh environmental conditions. Timing of active life stages or generations can be optimized. Seasonal dormancy therefore contributes to the fitness of individuals and species.
There would also appear to be costs associated with torpor. Many species do not use or survive torpor, and species capable of torpor do not necessarily use it on a routine basis. There is a fundamental physiological advantage (at least for endotherms and even for many ectotherms) of maintaining a high and stable body temperature, for example, growth, digestion, muscle contractility and immunological defense. There is also a physiological danger ofthermal death or being unable to arouse if the Ta becomes too low (e.g., freezing), or death if energy reserves become insufficient for arousal. Ecological costs of torpor could include vulnerability to predation, competition from conspecifics that do manage to successfully forage, reduced reproductive success, and lower rates of essential activities such as cell division and digestion. There are also similar and additional costs of seasonal dormancy. It can delay reproduction and development, diminish posthibernal reproduction, require that shortlived species survive for longer, and result in sex-biased populations if there is differential survival based on gender. For multiday torpor by endotherms, there appears to be a necessity for periodic arousal, suggesting some physiological requirement for an occasional return to a high Tb (see above).
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