Abstract
Cortisol is one of the main glucocorticoid hormones involved in both the mammalian stress response, and in fat metabolism and energy regulation, making it of increasing interest as a biomarker for stress, health and overall physiological state. However, transient stress responses to animal handling and sampling may be important sources of measurement artefact when investigating circulating concentrations of this hormone in wildlife. Here, cortisol concentrations were measured in the plasma and, for the first time, in the blubber of live captured adult harbour seals (Phoca vitulina). Plasma cortisol concentrations were positively correlated with capture time, suggesting that they were largely driven by a stress response to the capture event. In contrast, blubber cortisol concentrations were shown not to be significantly affected by capture time and varied significantly by sex and by season, with higher concentrations during natural fasting periods of their life cycle, particularly during the moult. These results suggest that cortisol may play a key role in increased fat metabolism during highly energetically demanding periods, and that blubber concentrations have the potential to be used as physiological state indicators in phocid seals.
Highlights
Wild animals encounter a diverse range of environmental stressors that can be described as any physical or psychological event that disrupts homeostasis[1]
Plasma cortisol concentrations ranged between 102.10–1662.81 ng/ml
The plasma cortisol concentrations showed considerable individual variation with higher concentrations recorded than in other studies on harbour seals published to date[17,19,30,31,32], but within the ranges published for other phocid species[15,20,33,34]
Summary
Wild animals encounter a diverse range of environmental stressors that can be described as any physical or psychological event that disrupts homeostasis[1] Their responses to such stressors are largely hormonally mediated, by the glucocorticoids[2], primarily cortisol[2]. Cortisol concentrations could be used as indicators of overall physiological state and provide vital information on the health and resilience of a population as a whole For these reasons, conservation biologists are increasingly using glucocorticoid hormone assessments to monitor stress and animal health to better inform management efforts[10,11]. Studying natural variation in glucocorticoid hormone concentrations in wild populations is inherently very difficult as the temporary capture and restraint necessary for sampling may alter the physiological parameters of interest. There is often a time delay between capture and sedation which is reflected in the circulating cortisol concentrations post-sedation[15]
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