EVALUATING ALLOSTATIC LOAD: A NEW APPROACH TO MEASURING LONG-TERM STRESS IN WILDLIFE

Animal welfare, conservation, and stress assessment are all critical components of species survival. As organisms experience stressors, they accumulate physiologic dysregulation, leading to multiple negative health outcomes. This brief review suggests measuring the degree of stress-induced damage, known as allostatic load, and then using allostatic load to evaluate changes implemented to improve animal welfare and conservation efforts. Over the past two decades, human clinical research has developed multiple allostatic load indices constructed from composites of neuroendocrine, cardiovascular, metabolic, and immune biomarkers. These indices are designed to estimate allostatic load in hopes of ameliorating or even negating damaging effects of stress. Among humans, allostatic load is associated with a variety of factors such as age, sex, stressful experiences, personality, social position, and early life history. Despite conservation of stress responses throughout mammalian species, reported allostatic load indices for animals are rare. Because many zoo researchers and field scientists already collect data on multiple biomarkers, constructing allostatic load indices may be a relatively affordable, easily implemented, and powerful tool for assessing relative risks of morbidity and mortality within wildlife. As an example, in a study among zoo-housed gorillas, an allostatic load index constructed using seven biomarkers was associated significantly with age, sex, stressful experiences, rearing history, markers of poor health, and mortality risk. Such results evidence that allostatic load is as applicable to animal populations as it is to humans. By using allostatic load as a predictive tool, human caretakers will be better informed of individuals at greatest risk for health declines. Most importantly, allostatic load may provide earlier opportunity for preemptive care while contributing a transformational tool to animal welfare research. Additionally, allostatic load may be compared between individuals and groups within the same population and allow comparisons of health between and across populations, consequently informing habitat and population protection efforts.