Allomaternal vocal behavior in squirrel monkeys.

Three groups of squirrel monkeys, containing 1-3 infants (N = 7) and 4 adult females each, were observed weekly for the first 12 weeks of the infants' lives. Infants received approximately 100 vocalizations, mostly "caregiver" calls, per waking hour from their mothers and the other adult females with whom they were housed (allomothers). Mothers vocalized very little to their own infants during the first few weeks of life, when infants remain on the mother's back full-time. Instead, allomothers (who were often carrying their own dependent infants) vocalized copiously to others' very young infants. Infants responded vocally to these allomaternal caregiver calls as early as Day 1 but were less responsive to mothers. When infants began leaving the mothers' backs, mothers' rates of calling increased five-fold as they used caregiver calls to retrieve separated infants. Early vocalizing to infants involves them in their first social exchanges and is probably performed by allomothers rather than mothers because the infant rides dorsally in this genus.