Stress, Santé et Sénescence chez un mammifère longévif, le chevreuil (Capreolus capreolus)

Le résumé fourni par la source

The stress response – a suite of physiological and behavioural adjustments – allows organisms to support heightened energy requirements associated with unpredictable environmental challenges (i.e. stressors). The stress response involves the activation of different physiological systems, among which the most studied is the hypothalamic-pituitary-adrenal axis leading to the enhanced production of glucocorticoids (GCs). At baseline levels, the primary role of GCs is energy mobilisation and regulation. These hormones act as mediators of allostasis, the active process of reaching differential homeostatic states according to physiological demands. Following exposure to a short-term, temporary stressor (i.e. acute stress), increased GC levels favour energy allocation towards functions promoting immediate survival at the expense of other non-immediately essential functions (e.g. growth, reproduction), and is therefore adaptive. On the other hand, individuals repeatedly exposed to stressors, or exposed to stressors that last for longer (i.e. days-weeks) can have elevated baseline GC levels. They enter into states of chronic stress during which resources are deviated away from physiological functions such as growth, reproduction or immunity on the long-term, with expected adverse consequences on fitness. Measuring states of chronic stress in the wild is challenging and associations between GCs and fitness-related traits are equivocal due to the influence of life-history and environmental contexts on such relationships.Throughout my Ph.D., I aimed at evaluating the relationships between baseline GC levels and fitness-related traits reflecting individual health (i.e. body mass, physiological condition, immunity), and their senescence patterns in a long-lived mammal. This work was conducted in two roe deer (Capreolus capreolus) populations experiencing markedly contrasted environmental conditions, and which have been longitudinally followed at the individual-based level.I evidenced that GCs related to health parameters on the short-term (i.e. when measured at the same sampling event), but that GCs did not have carry-over effects on the senescence of those traits. In a first chapter, I showed that GCs were negatively associated to body mass in adults, but not juveniles, of both populations and both sexes, supporting the idea that it is essential to take into account the life-history stage when evaluating relationships between GCs and life-history traits. I also found that the Neutrophil:Lymphocyte (N:L) ratio – another biomarker increasingly used as a proxy of the stress response – related negatively to body mass on the medium term (i.e. change in individual body mass between two consecutive years) during the late growth phase. This suggests that GCs and N:L ratios describe different components of the stress response and are not interchangeable. In a second part, I showed that GCs positively related to creatinine levels (i.e. a proxy of muscle mass and renal efficacy) on the short-term, suggesting an increased catabolic state in individuals with elevated GCs. I also found a sex-specific association between GCs and albumin, the main blood protein, suggesting that sex hormones can modulate GC-life-history trait relationships. In a third chapter, I found that GCs measured during early-life did not modulate patterns of immunosenescence but were only positively associated to the abundance of lung parasites in adults.In the last chapter of this thesis, I discuss the relevance of the present work for the field of ecophysiology. I have investigated the associations between GCs and various fitness-related traits at different temporal scales, using longitudinal data on a free-ranging mammal. This allowed to account for much of the sources of variations identified in the literature. I then explore and develop future research perspectives, both for the roe deer program, and for research in general, and I argue that assessments of chronic stress in wild animals should ideally be done through the construction of indices including several biomarkers reflecting the whole stress response. Furthermore, I explore the physiological mechanisms linking GCs and senescence by incorporating recent insights about the link between GCs and telomere attrition. I advocate to develop such mechanistic approaches to identify relevant biomarkers of chronic stress and to better predict its consequences. Finally, I conclude with some recommendations for researchers that aim at evaluating GC-fitness relationships.

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