Social isolation affects partner-directed social behavior and cortisol during pair formation in marmosets, Callithrix geoffroyi

► The social instability associated with establishing a social pairing led to a transient increase in cortisol levels. ► Social isolation resulted in a prolonged HPA axis dysregulation in the pairing period. ► Social isolation increased contact behavior with a new social partner. ► Female social contact behavior was associated with higher cortisol levels at the onset of pairing.