Vertebrates have regular patterns and routines that involve obtaining food and carrying out life-history stages such as breeding, migrating, molting, and hibernating. These are generally regulated by predictable changes in the environment, for example, seasons. Superimposed on these are unpredictable challenges, for example, storms and natural disasters, which have great potential for stress. The concept of allostasis, maintaining stability through change, has been introduced as a fundamental process through which organisms actively adjust to both predictable and unpredictable events. This process considers the predictable and unpredictable components of the environment as a continuum and includes the effects of body condition (parasites and injuries), experience, and habitat configuration. It combines classical homeostasis with anticipatory responses, stress, and social status. By using the balance between energy input and expenditure as the basis for applying the concept of allostasis, 2 types of allostatic overload have been proposed. Type 1 allostatic overload occurs when energy demand exceeds supply, resulting in activation of the emergency life-history stage. This serves to direct the animal away from normal life-history stages into a survival mode that decreases allostatic load and regains positive energy balance. The normal life cycle can be resumed when the perturbation passes. Type 2 allostatic overload begins when there is sufficient or even excess energy consumption accompanied by social conflict and other types of social dysfunction. The latter is the case in human society, and in some situations affecting animals in captivity and possibly in natural conditions as well. In all cases, secretion of glucocorticoids increases with allostatic load, but if it is chronically high, then pathologies develop. Curiously, type 2 allostatic overload does not trigger an escape response, and can only be counteracted through learning or changes in social structure.
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