Litter characteristics and helping context during early life shape the responsiveness of the stress axis in a wild cooperative breeder
Stress responses have evolved to quickly and appropriately deal with environmental stressors in order to secure or restore homeostasis. Since the regulation of stress hormones plays a key adaptive role, the regulatory processes controlling stress hormones levels may be under high selective pressure. The social environment during early life (parents and litter characteristics) strongly affects ontogeny of the hypothalamic-pituitary-adrenal (HPA) axis. In cooperative breeders, offspring are also confronted with helpers but whether and how variation in the helping context can affect HPA axis responsiveness of offspring remains unanswered. Combining dexamethasone suppression and adrenocorticotropic hormone stimulation tests, we investigated the link between the social environment and the characteristics of the HPA axis at the early stages of life in wild Alpine marmots. We show that when raised in the presence of helpers, marmot pups exhibit a greater capacity not only to mount, but also to turn off a stress response. The capacity to mount a stress response was also higher as the pups were raised in large litters. Determining impacts of such social modulation of the HPA axis functioning on individual fitness would make an important contribution to our understanding of the evolution of cooperative breeding.