Abstract
Vocalizations are important components of social behaviour in many vertebrate species, including our own. Less well-understood are the hormonal mechanisms involved in response to vocal cues, and how these systems may influence the course of behavioural evolution. The neurohormone oxytocin (OT) partly governs a number of biological and social processes critical to fitness, such as attachment between mothers and their young, and suppression of the stress response after contact with trusted conspecfics. Rodent studies suggest that OT's release is contingent upon direct tactile contact with such individuals, but we hypothesized that vocalizations might be capable of producing the same effect. To test our hypothesis, we chose human mother-daughter dyads and applied a social stressor to the children, following which we randomly assigned participants into complete contact, speech-only or no-contact conditions. Children receiving a full complement of comfort including physical, vocal and non-verbal contact showed the highest levels of OT and the swiftest return to baseline of a biological marker of stress (salivary cortisol), but a strikingly similar hormonal profile emerged in children comforted solely by their mother's voice. Our results suggest that vocalizations may be as important as touch to the neuroendocrine regulation of social bonding in our species.