Monitoring the impact of anthropogenic disturbance on species or populations of interest is an important goal of conservation (Van Dyke 2008). Because the effects of environmental alteration often manifest in an organ ism's physiology before changes can be detected at the population level, physiological measures can provide earlier detection of disturbances and greater predictive capacity than traditional demographic methods (Wikel ski & Cooke 2006; Ellis et al. 2012). This mechanistic approach, known as conservation physiology, can also help determine which populations are most susceptible to disturbance, key periods when disturbances may be most detrimental, and whether management techniques are having positive effects (Carey 2005; Wikelski & Cooke 2006). Incorporating physiological biomarkers into pop ulation monitoring also provides the opportunity to in terpret anthropogenic changes from the perspective of the organism rather than the researcher and thus im prove our understanding of which conditions constitute a disturbance. Glucocorticoids (GCs), often referred to as stress hor mones, represent some of the most widely proposed physiological biomarkers (Cooke & O'Connor 2010). GCs (eg, corticosterone and Cortisol) act in 2 distinct and separately measurable ways as determined by their circu lating concentration and the receptors to which they bind (Landys et al. 2006). GCs are best known for their role in enabling individuals to respond to unpredictable events such as extreme weather, predator interaction, or so cial conflict through the acute stress response (McEwen & Wingfield 2003). By increasing within minutes of an acute environmental challenge, GCs act to mobilize stored energy reserves, enhance immune function, pro mote escape behaviors, and suppress nonsurvival activ ities such as courtship or copulation (Sapolsky et al. 2000). However, at baseline levels, GCs promote ener getic balance, by influencing processes such as foraging, and glucose and lipid mobilization, which allows individ uals to meet daily energy requirements and the prolonged energetic expenditures associated with predictable life history events (eg, migration, rearing offspring)(Landys et al. 2006). Nevertheless, prolonged elevation over days to weeks (ie, chronic stress) can negatively affect health and fitness (Sapolsky et al. 2000). Unfortunately, given the general perception of GCs as only stress hormones, much of their application in con servation has been based on the generalized assumption that increases in GCs are always indicative of challenging or stressful environments (Bonier et al. 2009 «; Busch & Hayward 2009). Viewed in this way, the interpretation of changing GC levels is appealing and easily applied. However, mounting evidence suggests GC physiology is much more complex (Romero 2005; Bonier et al. 2009 «; Romero et al. 2009), making this approach controversial. Here we appeal to conservation biologists to take a pre dictive, physiological approach to the application of GCs to conservation goals in their study systems. We focus on 3 specific considerations that will improve conservation based interpretation of GC levels across vertebrate taxa: a focus on baseline GC measures and their role in energetic balance; an understanding of the context-dependent nature of GC levels and their relationship to fitness; and a consideration of intra-individual variation in GC levels.

* email madlige@ uwindsor. ca Paper submitted November 27, 2012; revised manuscript accepted July 5, 2013•