'Love hormone' promotes social behavior among infant rhesus monkeys

Researchers have suggested that the hormone oxytocin appears to increase social behaviors in newborn rhesus monkeys.

Working with infant rhesus monkeys, the NIH researchers, the University of Parma in Italy, and the University of Massachusetts Amherst, found that oxytocin increased two facial gestures associated with social interactions- one used by the monkeys themselves in certain social situations, the other in imitation of their human caregivers.

The researchers began by gauging the ability of rhesus macaques to imitate two facial gestures: lip smacking and tongue protrusion. In lip smacking, the lips are protruded and open, then smacked together repeatedly. The study authors wrote that rhesus mothers will engage in this facial gesture with their infants in the first month after giving birth.

Tongue protrusion involves a brief protrusion and retraction of the tongue. Although this gesture is seen in other primates and typically not seen in macaques, macaques will imitate it when their human caregivers display it, the study authors added.

By observing the monkeys' ability to imitate the two gestures, the researchers sought to determine if oxytocin could promote social interaction through a gesture that was natural to them as well as through a gesture not part of their normal communication sequence.

The researchers tested the infants in the first week after birth. Three times a day, every other day, the caregivers would demonstrate the facial gestures in sequence to the infant monkeys, while the animals' responses were recorded on video. At this phase of the study, the researchers found that some of the monkeys mimicked their caregivers' gestures more frequently than did other monkeys. The researchers referred to the monkeys who gestured more frequently as strong imitators.

Beginning in the second week of life, the researchers tested the monkeys on two separate days. The infant monkeys inhaled an aerosolized dose of oxytocin in one session, and a dose of saline in the other. In each session, the dose was delivered through an inhalation mask held gently over the animal's face.

Overall, the monkeys were more communicative after receiving oxytocin, more frequently making facial gestures, than they were after receiving the saline.

The study has been published online in Proceedings of the National Academy of Sciences.