Shaped by you: The effect of social partner on cortisol and behavior during adolescence in a female rodent

Adolescence is a sensitive phase when cues from the social environment can shape consistent individual differences in behavioral and hormonal profiles, but the effect of the social partner on these phenotypes is not well-understood, especially for females. We therefore aimed to investigate whether the age class and sex of the housing partner affect social behavior and cortisol concentrations in female guinea pigs during adolescence and estimate the repeatability of cortisol concentrations in this life stage. Female guinea pigs were housed upon weaning for 4 weeks with either an adult female, adolescent female, or adolescent male. Social behavior was observed in the home enclosure and a preference test and social interaction test were carried out at the end of the experiment. Cortisol concentrations (baseline and response to a challenge) were measured prior to the start of the experiment and 2 and 4 weeks after pair formation. We hypothesized that the social partner would affect behavior and cortisol concentrations. We found that females housed with an adult female were less aggressive and more consistently displaced by their housing partner than females housed with an adolescent female. Sex of the housing partner did not affect focal female behavior. Regarding hormones, baseline cortisol concentration was neither significantly repeatable nor significantly affected by the housing partner. However, cortisol responsiveness was highly repeatable both 1 h and 2 h after the onset of a challenge. The age class of the housing partner affected cortisol responsiveness, with higher cortisol concentrations measured at 1 h in females housed with an adult female. These results indicate that adolescent female guinea pigs exhibit consistent individual differences in cortisol responsiveness while retaining plasticity to adjust to cues from the social environment. Taken together, we show in a female rodent that the social environment can significantly shape behavioral and endocrine parameters during adolescence.