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The research described in this article was supported by NIH Grant R21-AG029862 (to D.M.) and by an NSF Graduate Fellowship (to C.L.H.). We thank all of our research collaborators including, in alphabetical order, James Ayala, Christopher Coe, Melissa Gerald, James Higham, Adaris Mas-Rivera, Karen Parker, Brian Prendergast, and Angelina Ruiz. We also thank the students and the staff of the Caribbean Primate Research Center who provided assistance with data collection.
In order to survive and function properly, organisms must maintain their physiological parameters (e.g., temperature, blood pressure, glucose, and hormone concentrations in blood) within a certain range of values appropriate for their age, gender, and species. The state of equilibrium in which all of an organism's physiological parameters are within the normal range is referred to as homeostasis. The external environment, which includes both abiotic factors such as ambient temperature and humidity and biotic factors such as other organisms and their behavior, can cause perturbations of homeostasis so that some physiological parameters change and assume values that are either above or below the normal range. Perturbations of homeostasis can also be induced by pathological processes such as infectious diseases or by genetic alterations or physiological malfunction. In response to these perturbations of homeostasis, the organism makes adjustments in its physiological processes so as to bring the values of the parameters that have been altered back into their normal range. This dynamic process through which an organism adjusts its physiological parameters in response to perturbations of homeostasis is referred to as allostasis. Allostatic processes involve feedback mechanisms that detect a deviation from homeostatic equilibrium and trigger the appropriate compensatory responses.
Environmental perturbations of homeostasis are known as stressors. There are many types of stressors, acute or chronic. Responses to acute stressors help the individual survive and reestablish homeostasis. A sudden predator attack and aggression from a conspecific are good examples of stressors that can threaten survival or physical well-being. Areas of the brain such as the amygdala and prefrontal cortex play a crucial role in evaluating the threat and producing an emotional response that will help cope with it. Signals from the brain activate the release of catecholamines (epinephrine and norepinephrine) from the sympathetic-adrenal-medullary axis and of glucocorticoid hormones (cortisol...