Puerto Rico's female rhesus monkeys make unique vocalizations to interact with infants to get their attention.
"These female monkeys are definitely excited about looking at babies, and their vocalizations convey that excitement," said study co-author Dario Maestripieri, an associate professor in comparative human development at the University of Chicago.
One particular monkey sound, known as a "girney," seems especially designed for infant ears. "When infants are around they use [the girney] a lot more, and they also do other things like wag their tails to the babies—which they don't do in other circumstances," Maestripieri said.
Maestripieri and colleagues published their findings in an issue of the journal Ethology.
Baby talk, also called "motherese," is practiced by people all over the world. No matter what language they speak, people baby-talk in the same way, with a raised pitch and a swooping, sing-song style. Everyone does it because babies respond to it, and scientists believe the practice has a useful biological origin.
But those looking for a monkey motherese translation will be disappointed.
"You can't ask What does it mean?" Maestripieri said. "It doesn't mean anything. It's the intonation that matters."
But the sounds appear to serve a key purpose.
"They don't have a meaning linked to a representation of an item or object, but they may perform a very important social function to bring individuals together," said Lisa Parr of Yerkes National Primate Center at Atlanta's Emory University. Parr was unaffiliated with the research.
For instance, adult females use motherese to speak to infants other than their own.
This makes the infants' mothers more receptive to social overtures, such as grooming. It also promotes social interaction among the group's females.
Though it may seem odd that monkey moms don't use motherese on their own offspring, it may be that they simply don't need to.
"Moms carry their own infants on their chest almost all the time," study co-author Maestripieri said.
"So [their babies] are not as novel to them as they are to other females. They also don't need to do anything special to get their attention because they are almost constantly face-to-face."
"In terms of auditory signals this may be one of the first articles that speaks directly to the verbal communication between adults and infants," Parr said.