The typical response to a stressor in Vertebrates involves the hypothalamic-pituitary-adrenal (HPA) axis. It consists of a complicated series of hormonal cascade that results in the release of glucocorticoïds, as cortisol and corticosterone, from adrenal cortex to prepare the animal for a “fight or flight” response (Sapolsky et al., 2002).
For members of a social species, one of the most profound moderators of function in the HPA axis during times of stress may be the presence of significant social partners (Hennessy, 1997; Rukstalis and French, 2005). Indeed, the presence of significant social partners down-regulates activity in the HPA axis and hence serves to buffer the individual against stressful stimuli. The study of Hennessy (1997) in guinea pig shows that offspring exposed to stressful situations in the presence of the mother display significantly reduced behavioural and glucocorticoid responses to those stressors, relative to offspring exposed to the same stressors in the absence of the mother. Moreover, in visually obscured environments, isolated individuals may attempt to maintain the positive effects of social support by using a single vocal individually signature signal of their social partner rather than having to rely on a combination of several signals (olfactory, visual and auditory). Therefore in marmosets, individually specific communication signal can decrease the magnitude of a physiological stress response (i.e. decrease of urinary cortisol excretion) in a manner analogous to the physical presence of a social partner, a process that the authors term “vocal buffering” (Rukstalis and French, 2005).
