Shaped from an early age: Hormonal and behavioural phenotypes in juvenile male guinea pigs living in distinct social environments

Individuals can adjust to different social environments via shaping of endocrine and behavioural phenotypes. As the social environment can change at any time, individuals need to be able to adjust throughout their lives. Our goal was therefore to examine potential effects of different social environments on the endocrine and behavioural phenotype in male guinea pigs during juvenility, an important developmental phase characterized by prominent changes of the social environment. For this approach, twenty domestic juvenile male guinea pigs (Cavia aperea f. porcellus) were housed in two distinct social environments: while males of both groups lived in heterosexual pairs, males of one group were additionally socially stimulated (e.g., an unfamiliar individual is introduced into the focus males' home enclosure for 10 min) regularly whereas males of the other group were not. This procedure increased the number of social interactions. We hypothesized males from the two social conditions to differ in their hormonal and behavioural phenotype. Indeed, only males with additional social stimulation displayed an initially increased stress responsiveness, enabling them to adequately react to the unpredictable social encounters. Over time, males then conformed to this challenging environment and displayed a decrease in stress responsiveness again. Moreover, only males with additional social stimulation showed a significant increase of courtship and sexual behaviour with age. Taken together, these findings suggest that already in juvenility the social environment induced hormonal adjustments and behavioural changes in male guinea pigs, thereby highlighting how juvenile social experiences can shape individuals' phenotypes.