Adolescence is the gradual transition from childhood to adulthood. In the fields of bio-medical and bio-psychological sciences, this time is regarded as an important, dangerous, and potentially stressful life stage. From an evolutionary perspective, however, adolescence may also be an opportunity for adaptation. Our previous work in guinea pigs shows a conspicious environment-dependent canalization of behavioural profiles and stress responsiveness during this phase of life. Based on these findings, the aim of the present project is to analyse underlying mechanisms and the possible adaptive significance of this phenomenon. We hypothesize that frequency and intensity of social interactions during adolescence trigger neuroendocrine processes that shape the individual's behavioural and hormonal profile and thereby adjust it to the current social situation and maximise its reproductive success. From this approach we expect a significant new contribution to the understanding of how and why social environment, hormones, and behaviour interact during adolescence to shape individual behavioural profiles.