Whitham said that "while intently watching infants, females excitedly wag their tails and emit long strings of grunts and girneys."
When the actual mother of the infant approached, one or more other females would sometimes approach the mom and initiate a mutual grooming session.
Although the researchers do not think the sounds communicate specific information, Maestripieri said they believe the calls "are signals used in friendly interactions, to signal that the vocalizer means no harm."
The scientists also believe the sounds help to attract an infant's attention.
"In fact, it could be that humans and monkeys emit sounds that infants find attractive," said Maestripieri. "Babies like to hear what we consider to be baby talk."
He added that humans, and particularly women, the world over use this melodic way of speaking to infants, no matter what the native language of the speaker is.
The main difference between human and monkey motherese, however, is that humans will often use baby talk with their own kids, but rhesus moms do not. That is probably because the monkeys carry their infants on their chest, so "the calls aren't necessary with such constant face-to-face contact," Maestripieri said.
The study adds to research conducted by University of California at Los Angeles anthropologist Joan Silk.
Silk and her team also noticed that grunts and girneys are "effective in facilitating grooming and in reducing aggression."
Baby talk may even help to ease spats between adult female baboons.
After analyzing such conflicts, Silk's group pointed out that one of the baboon combatants might sweetly grunt while approaching a former opponent and her infant. After a few gentle caresses of the baby, all seemed to be forgotten and forgiven.
Related Links:
Parenting: How to Speak Baby
Rhesus Macaque Factsheet
The University of Chicago Behavioral Biology Laboratory
