Recent findings suggest that immune functions do not unidirectionally deteriorate with age but that a potentially adaptive remodelling where functions of the immune system get down regulated while others get upregulated with age could also occur. Scarce in wild populations, longitudinal studies are yet necessary to properly understand the patterns and consequences of age variations of the immune system in the wild. In the meanwhile, it is challenging to understand if the observed variations in immune parameters with age are due to changes at the within-individual level or to selective (dis)appearance of individuals with peculiar immune phenotypes. Thanks to a long-term and longitudinal monitoring of a wild population of Alpine marmots, we aimed to understand within- and between-individual variation in the immune phenotype with age, in order to improve our knowledge about the occurrence and the evolutionary consequence of such age-variations in the wild. To do so, we recorded the age-specific leukocyte concentration and profile in repeatedly sampled dominant individuals. We then tested whether the potential changes with age were more likely due to within-individual variations and/or selective (dis)appearance. Finally, we investigated if the leukocyte concentration and profiles were correlated to the probability of death at a given age. The leukocyte concentration was stable with age, but the lymphocyte count decreased, while the neutrophil count increased, over the course of an individual’s life. Moreover, between individuals of the same age, individuals with fewer lymphocytes but more neutrophils were more likely to die. Therefore, selective disappearance seems to play an important role in the age variations of the immune parameters in this population. Further investigations linking age variations in immune phenotype to individual fitness are needed to understand whether remodelling of the immune system with age could or could not be adaptive.

Recent findings suggest that immunosenescence should not be considered as a unidirectional deterioration, and that the remodeling of the immune system with age could also be adaptive. Longitudinal studies on immunosenescence in wild populations are scarce, and therefore, processes like selective disappearance cannot be easily torn apart from senescence. Using a long-term and longitudinal monitoring of a wild population of Alpine marmots, we aimed to understand within and between individual variation in the immune phenotype with age, in order to improve our knowledge about the occurrence and the consequence of immunosenescence in the wild. We tested, once controlled for a potential selective disappearance, whether individuals’ immune function only decreases as they age, as expected from the disposable soma theory, or whether remodelling of the immune system does occur. Therefore, we recorded the age-specific leukocyte concentration and counts in repeatedly sampled dominant individuals and we tested the potential changes with age as well as their association with survival probabilities. The overall leukocyte concentration was stable with age, but the lymphocyte count decreased, while the neutrophil count increased, over the course of an individual’s life. The leukocyte counts also predicted survival: at a given age, individuals with fewer lymphocytes but more neutrophils were more likely to die. Longitudinal studies, like the present one, are required to properly understand the patterns and consequences of immunosenescence in the wild.