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Understanding the mechanistic links between individual variation in life history and behavior is a major challenge in evolutionary ecology. Glucocorticoids (GC) play a major role in this link through their baseline levels into the blood and their implication in stress responses to environmental perturbations. However, very few studies have investigated the long-term joint relationships between GC stress reactivity, life history, and behavior in natural conditions. Here, we took advantage of the behavioral and life history differences among individual males and females of a wild population of eastern chipmunks ( Tamias striatus): We investigated how individual exploration, age, and reproduction were linked to level and intra-individual variability (IIV) of fecal cortisol metabolites over a 5-month period. Our analyses revealed that female cortisol levels decreased during gestation and lactation compared with non-reproductive females. We also found that slower exploring females and females with a smaller litter displayed higher IIV in fecal cortisol metabolites. For males, fecal cortisol metabolites level during the mating season increased with the number of offspring produced and decreased with age. Our study highlights the necessity of considering simultaneously seasonal fluctuations in GC level and the dynamics of stress reactivity in the study of life history and behavioral co-adaptations within natural populations. [ABSTRACT FROM AUTHOR]