Stress: Does it matter? A review of mechanisms and fitness consequences of stress in large herbivores
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The recent discipline of ecophysiology investigates physiological adaptations of organisms to their environment. This opens for testing new research questions, such as more thorough investigations of the link between environmental stressors and physiological responses, or the role of stress as a biomarker of health and fitness. Many studies support the cort-fitness hypothesis, which predicts a negative relationship between stress and fitness, but they also show that this relationship is not linear. Animals need stress to keep themselves alert for hazards, but detrimental effects can occur at high release levels or in chronic situations. This review focuses on causes, mechanisms and consequences of stress on large herbivores, both for individual fitness and population dynamics. Stress is a multidimensional physiological response that challenges internal stability, and can be measured through “stress hormones”. These however fluctuate depending on a range of factors, and can be measured in different ways, depending on the research question. Therefore, caution should be taken when interpreting results. Stress can be described based on duration (acute or chronic) and type of stressor (physical or psychological). Chronic stress inhibits the body from returning to homeostasis, and can have a range of physiological consequences ultimately affecting fecundity, offspring survival and immunity. Although studies on the ecophysiology of wild animals, and especially large herbivores, are scarce, there is evidence that individual effects of stress can scale up to population dynamics. Rangifer, due to its peculiar ecology, is particularly sensitive to the rapidly increasing human disturbance. Although so far only few studies provided physiological measures of stress hormones, these documented the occurrence of nutritional stress, and succeeded in establishing a causal link between human disturbance and physiological stress. This review shows that there is reasons to believe that stress does matter, both at an individual and population level. There is an urgent need for more interdisciplinary studies to establish the link between different type of stressors and stress responses, and to better understand the relationship between stress, individual fitness and population dynamics. In particular, there is a need for studies on wild species that seem particularly sensitive to stress, such as Rangifer, in order to plan sound management and conservation strategies.