CONICET | Buscador de Institutos y Recursos Humanos

Background. Providing the context for the evolution of life-history traits, habitat fea- tures constrain successful ecological and physiological strategies. In vertebrates, a key response to life?s challenges is the activation of the Stress (HPA) and Gonadal (HPG) axes. Much of the interest in stress ecology is motivated by the desire to understand the physiological mechanisms in which the environment affects fitness. As reported in the literature, several intrinsic and extrinsic factors affect variability in hormone lev- els. In both social and non-social animals, the frequency and type of interaction with conspecifics, as well as the status in social species, can affect HPA axis activity, result- ing in changes in the reproductive success of animals. We predicted that a social envi- ronment can affect both guanaco axes by increasing the secretion of testosterone (T) and Glucocorticoid (GCs) in response to individual social interactions and the ener- getic demands of breeding. Assuming that prolonged elevated levels of GCs over time can be harmful to individuals, it is predicted that the HPA axis suppresses the HPG axis and causes T levels to decrease, as GCs increase.Methods. All of the data for individuals were collected by non-invasive methods (fe- cal samples) to address hormonal activities. This is a novel approach in physiologi- cal ecology because feces are easily obtained through non-invasive sampling in animal populations.Results. As expected, there was a marked adrenal (p-value = .3.4e−12) andgonadal (p-value = 0.002656) response due to seasonal variation in Lamaguanicoe. No significant differences were found in fecal GCs metabolites between males/females*season for the entire study period (p-value = 0.2839). Despite the seasonal activity variation in the hormonal profiles, our results show a positive correlation (p-value = 1.952e−11, COR = 0.50) between the adrenal and gonadal system. The marked endocrine (r 2 = 0.806) and gonad (r 2 = 0.7231) response due to seasonal variation in male guanaco individuals highlights the individual?s energetic demands according to life-history strategies. This is a remarkable result because no inhibition was found between the axes as theory suggests. Finally, the dataset was used to build a reactive scope model for guanacos.Discussion. Guanacos cope with the trade-off between sociability and reproductive benefits and costs, by regulating their GCs and T levels on a seasonal basis, suggest- ing an adaptive role of both axes to different habitat pressures. The results presented here highlight the functional role of stress and gonad axes on a critical phase of a male mammal?s life?the mating period?when all of the resources are at the disposal of the male and must be used to maximize the chances for reproductive success.