The connection between health and chronic stress in the harbour porpoise

Abstract

Abstract The aim of this study is to gain insight in chronic stress and welfare in the harbour porpoise in general. For this purpose we will demonstrate a connection between chronic stress and clinical disease. Chronic stress is known to negatively interfere with adaptive capacities, thereby resulting in an impaired welfare. In addition, several studies have shown direct negative influences of chronic stress on health. Although most of these studies have been carried out in other animal models, chronic stress might influence the health status of the harbour porpoise as well. After all, these animals are faced with many chronic stressors every day and are an especially vulnerable species. They can therefore function as sentinels for population and ecosystem health. By measuring biomarkers for chronic stress in these marine mammals, we might identify a possible underlying welfare problem. For this purpose, cortisol levels in blubber tissue and glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) mRNA expression levels in hippocampal brain tissue have been measured and subsequently compared between ‘healthy’ and ‘diseased’ porpoises. Blubber cortisol levels are shown to be higher in diseased porpoises compared to healthy porpoises (p=0,001). Furthermore, blubber cortisol shows a weak interaction with nutritional condition (p=0,052). Despite the absence of significant effects between blubber cortisol and other parameters tested for (sex, age class, decomposition condition, blubber thickness, length and mass), pairwise comparisons show a significant interaction between blubber cortisol and respectively females (p=0,006) and between a medium and fat nutritional condition score with regard to health (p=0,019). Remarkably, hippocampal GR cornu ammonis-1 (CA1) mRNA expression levels are significantly higher in diseased porpoises compared with healthy porpoises (p=0,043). However, no significant interactions between GR CA1 and other factors (sex, age class, decomposition condition and nutritional condition) are observed and no significant correlations between blubber cortisol and GR CA1 have been found. These methods are shown to be useful for the measurement of chronic stress in the harbour porpoise and they both offer many applicabilities for future studies, although larger follow-up studies are highly recommended for the obtainment of more reliable results.