Co-variation between glucocorticoids, behaviour and immunity supports the pace-of-life syndrome hypothesis: an experimental approach

The biomedical literature has consistently highlighted that long-term elevation of glucocorticoids might impair immune functions, while short-term elevation might enhance them. In contrast, in wild and free-ranging animals, patterns are less clear. In the present study, we explored the stress-immunity relationship in controlled conditions, taking into account the potential effects of behavioural profiles. Thirteen captive roe deer (Capreolus capreolus) were monitored over an eight-week period encompassing two capture events. We assessed how changes in baseline faecal cortisol metabolite (FCM) concentrations following a standardised capture protocol and vaccination affected changes in thirteen immune parameters of the innate and adaptive immunity, and whether behavioural profiles were linked to changes in baseline FCM levels and immune parameters. We found that individuals showing an increase in baseline FCM levels over time also exhibited an increase in immunity, for both the innate and adaptive immunity and were characterised by more reactive behavioural profiles (as indicated by low activity levels, docility to manipulation and neophilia). Consequently, our results suggest that immunity of large mammals may be influenced by their stress level, but also by their behavioural profiles, as it is predicted by the pace-of-life syndrome hypothesis. Our analysis therefore highlights the need to consider co-variations between behavioural profiles, immunity and stress hormones in order to gain a better understanding of the unexplained among-individual variability in the glucocorticoids-immunity relationships observed in wildlife, as they may be underpinned by different life-history strategies.