<p>Seasonally breeding mammals exhibit behavioural and physiological responses regulated by internal and external cues, mediating physiological trade-offs between survival and reproduction. Steroid hormones, such as androgens and glucocorticoids, play key roles in regulating reproductive behaviours, energy utilization, and responses to environmental constraints. Studying hormonal and behavioural responses to internal and external factors, provides insight into adaptive strategies in ungulates inhabiting highly seasonal, unpredictable ecosystems. Using the guanaco (<i>Lama guanicoe</i>), a territorial ungulate, we (1) analysed faecal cortisol (FCMs) and testosterone (FTMs) metabolites in males of different social units during the reproductive season; and (2) assessed the relationship between these hormones and aggression, often associated with territoriality and competition. During the 2014 and 2016 reproductive seasons, we recorded 227 focal observations of territorial (family group, solitary) and non-territorial (mixed and bachelor groups) males across three periods (group-formation, mating, and post-mating). We paired each observation with a fresh faecal sample for hormone analysis via group-specific enzyme immunoassays. FCMs and FTMs increased during the mating period, with significant inter-annual differences, likely related to variations in forage quality. Hormone metabolite levels were similar between social units. We found no relationship between aggression and FCMs, potentially due to the predominance of low-cost, low-aggression level displays. The relationship between FTMs and aggression supports the predictions of the challenge hypothesis for polygynous species, suggesting low androgen responsiveness to male-male interactions due to already elevated baseline testosterone. Our findings highlight how reproductive demands and environmental variability shape hormone secretion, emphasizing the interplay between social and ecological factors in regulating endocrine responses in wild mammals.</p>