Female rhesus monkeys use special vocalizations while interacting with infant monkeys, the way human adults use motherese, or "baby talk," to engage babies' attention, new research at the University of Chicago shows.
"Motherese is a high pitched and musical form of speech, which may be biological in origin," said Dario Maestripieri, an associate professor in Comparative Human Development at the university's Behavioral Biology Lab, who led the research team.
"The acoustic structure of particular monkey vocalizations called girneys may be adaptively designed to attract young infants and engage their attention, similar to how the acoustic structure of human motherese, or baby talk, allows adults to visually or socially engage with infants," he explained.
But unlike human mothers, the rhesus macaque mothers did not direct grunts or girneys toward their own offspring. It could be that the monkey mothers are familiar with their own offspring and use the vocalizations with other babies because they are excited about the novelty of seeing a new infant, Maestripieri said.
The researchers studied a group of free-ranging rhesus macaques living on an island off the coast of Puerto Rico. Dr. Melissa Gerald, a researcher at the University of Puerto Rico, was also a co-author of the study.
They studied the vocalizations exchanged between adult females and found that grunts and girneys increased when a baby was present.
They also found that when a baby wandered away from its mother, the other females looked at the baby and vocalized, suggesting that the call was intended for the baby.
"Adult females become highly aroused while observing the infants of other group members," explains lead author of the article, Jessica Whitham, a recent Ph.D. graduate of the University of Chicago, who investigated this topic as a doctoral student at the university and currently works at Brookfield Zoo near Chicago.
"While intently watching infants, females excitedly wag their tails and emit long strings of grunts and girneys," she said.
"The calls appear to be used to elicit infants' attention and encourage their behavior. They also have the effect of increasing social tolerance in the mother and facilitating the interactions between females with babies in general," Maestripieri and his colleagues write in the article, "Intended Receivers and Functional Significance of Grunt and Girney Vocalizations in Free-Ranging Rhesus Macaques" published in the current issue of the journal "Ethology."
"Thus," they write, "the attraction to other females' infants results in a relatively relaxed context of interaction where the main focus of attention is the baby."
Researchers have long been interested in the noises that non-human primates make and how they are used for communication.
Monkey vocalizations could be carrying information that the sender expects the recipient to understand, or they could be noises that the recipient can draw inferences from, but are not intended to carry information.
A human sneeze, for instance, is a noise that people understand may be associated with a cold, but it did not develop to convey information.
The study by Maestripieri's team showed that the grunts and girneys emitted by the rhesus macaques fall into the category of vocalizations not intended to convey specific information, and appear to be used to attract other individuals' attention or change their emotional states.
When females vocalize to young infants, however, the infants' mothers infer that the females simply want to play with the infants and are unlikely to harm them.
Therefore, these vocalizations may facilitate adult females' interactions not only with infants, but with the infants' mothers as well. They found, for instance, that the grunts and girneys were sometimes followed by an approach and grooming of the mothers.
Maestripieri, a native of Italy, became interested in animal behavior as a child and became interested in primates while in college, when he conducted a research experiment with them. "What I noticed was the great variation in their personalities," he said, "6just like in humans."