Presence of companion moderates arousal of monkeys with restricted social experience.

Squirrel monkeys were reared on inanimate maternal surrogates in individual cages. The surrogates were removed at 8 months, and at approximately 1 year of age each animal was paired with a similarly-reared peer for 4 weeks prior to testing. When placed in a novel environment for 30 min, the juveniles emitted significantly more high-pitched vocalizations and were significantly more active when tested alone than they were when tested in the presence of their cagemates. A significant elevation of plasma cortisol over resting levels was observed only when animals were exposed to the novel environment alone. Separation from the peer in familiar surroundings had no effect on any measure. These results indicate that the capacity to use social partners to moderate arousal can develop in squirrel monkeys raised from shortly after birth without a mother or physical interaction with other conspecifics, and with relatively little social experience overall.