Throughout their lives, animals live in a world of change and glucocorticoid hormones (cortisol, corticosterone) play a key role in responses to predictable and unpredictable environmental challenges. Glucocorticoid hormone concentrations increase substantially after an unpredicted challenge and this has led to glucocorticoids being considered ´stress hormones´. However, individuals, populations and species show large variation in their glucocorticoid concentrations, and interpreting the causes and consequences of this variation is still a contentious issue. In this study, we used captive zebra finches living in outdoor aviaries in which we manipulated the degree of environmental challenge during development and in adulthood. We investigated environmental (i.e. temperature, early life adversity, foraging costs) and internal (i.e. sex, body mass, glucose regulation) factors driving glucocorticoid variation, and further tested physiological mechanisms involved. The most relevant driver of glucocorticoid variation we conclude to be metabolic rate, as we found that glucocorticoids are modulated in accordance with changes in energetic demands (e.g. psychological stressors, temperature changes, foraging costs), in both controlled indoor conditions and more naturalistic outdoor aviaries. This raises the question whether glucocorticoids are indicators of ´stress´ beyond the effects of stressors on metabolism, because we found the physiological response to a decrease in temperature to be indistinguishable from the one showed when facing a psychological stressor that increased metabolic rate to the same extent. Our work provides novel insights on how to interpret glucocorticoid variation, and underlies the need to revise the traditional interpretation of glucocorticoids as indicators of ´stress´ and animal welfare.
|Translated title of the contribution||Oorzaken en gevolgen van glucocorticoide variatie in zebravinken|
|Qualification||Doctor of Philosophy|
|Place of Publication||[Groningen]|
|Publication status||Published - 2018|