A novel integrative method for measuring body condition in ecological studies based on physiological dysregulation

1. The body condition of free-ranging animals affects their response to stress, decisions, ability to fulfil vitalneeds and, ultimately, fitness. However, this key attribute in ecology remains difficult to assess, and there is aclear need for more integrativemeasures than the common univariate proxies.2. We propose a systems biology approach that positions individuals along a gradient from a ‘normal/optimal’to ‘abnormal/suboptimal’ physiological state based onMahalanobis distance computed from physiological biomarkers.We previously demonstrated the validity of this approach for studying ageing in humans; here, we illustrateits broad potential for ecological studies.3. As an example, we used biomarker data on shorebirds and found that birds with an abnormal condition hada lower maximal thermogenic capacity and higher scores of inflammation, with important implications for theirecology and health.Moreover,Mahalanobis distance captured a signal of condition not detected by the individualbiomarkers.4. Overall, our results on birds and humans show that individuals with abnormal physiologies are indeed inworse condition.Moreover, our approach appears not to be particularly sensitive to which set of biomarkers isused to assess condition. Consequently, it could be applied easily to existing ecological data sets.5. Our approach provides a general, powerful way to measure condition that helps resolve confusion as to howto deal with complex interactions and interdependence among multiple physiological and condition measures. Itcan be applied directly to topics such as the effect of environmental quality on body condition, risks of healthoutcomes, mechanisms of adaptive phenotypic plasticity, and mechanisms behind long-term processes such assenescence