Abstract
Variation in glucocorticoid hormones (GCs) is often interpreted as reflecting ‘stress’, but this interpretation is subject of intense debate. GCs induce gluconeogenesis, and we hypothesized therefore that GC variation can be explained by changes in current and anticipated metabolic rate (MR). Alternatively, GC levels may respond to psychological ‘stress’ over and above its effect on metabolic rate. We tested these hypotheses in captive zebra finches, by inducing an increase in MR using a psychological stressor (noise), and compared its effect on corticosterone (CORT, the primary avian GC) with the effect induced by a decrease in ambient temperature increasing MR to a similar extent. We found the increase in CORT induced by the psychological stressor to be indistinguishable from the level expected based on the noise effect on MR. We further found that a handling and restraint stressor that increased CORT levels also resulted in increased blood glucose levels, corroborating a key assumption underlying our hypothesis. Thus, GC variation primarily reflected variation in energy expenditure, independently of psychological stress. GC levels have many downstream effects besides glucose mobilization, and we propose that these effects can be interpreted as adjustments of physiological functions to the metabolic level at which an organism operates.
Highlights
Glucocorticoid (GC) hormones are considered to play a central role in coping strategies and organismal adjustments to environmental variability
CORT is often interpreted as indicator of ‘stress’ independent of metabolic rate (MR), and we tested this interpretation by comparing CORT between two treatments that both increased MR, with one treatment inducing a rapid response, while the other treatment induced a gradual homeostatic response
We show that variation in CORT levels was explained by variation in MR, independent of the stimulus that caused the increase in MR
Summary
Glucocorticoid (GC) hormones (e.g. cortisol, corticosterone) are considered to play a central role in coping strategies and organismal adjustments to environmental variability (reviewed in[1,2,3]). Ecological and evolutionary physiologists advocated the need to reconsider the causes of GC variation from a wider perspective[2,3,11] In this context, GCs are considered as primary mediators of allostasis (i.e. achieving stability through change3), integrating physiology and associated behaviours in response or anticipation to changing environments[2,3]. Many studies show that GCs are modulated in parallel with factors that can be assumed to affect energy expenditure This is true for both predictable (e.g. seasonal or daily temperature variation, breeding, daily activities[2,3,15,16,17,18,19,20] and unpredictable and/or uncontrollable (e.g. sudden meteorological events, predator threat, human presence or handling21–25); variation in energy demands. As our interpretation of the predicted MR-CORT association is based on the assumption that CORT ensures increasing fuel (i.e. glucose) supply to match higher energetic needs, we tested whether a response to a standardized stressor[20,27,28], which induces an increase in plasma CORT, was accompanied by an increase in plasma glucose
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