Chicago ? Adult rhesus macaques make high-pitched, sing-songy vocalizations when they encounter infant monkeys – just the kind of sounds humans seem to naturally fall into using around infants, scientists reported this week.

Seeing the adult monkeys use a form of baby talk suggests the behavior in humans may be biological in origin, said Dario Maestripieri, associate professor of comparative human development at the University of Chicago.

In monkeys the vocalizations are called “girneys,” and they are a sound completely different from any other the animal makes, Maestripieri said.

Previous studies by other scientists had concluded that when a rhesus monkey approached a mother and its baby, it directed the girney vocalization at the mother to signal the caller’s intentions were benign and harmless – that it was not threatening mother or child.

But after watching 19 adult females in a family of 65 rhesus monkeys on the Puerto Rican island of Cayo Santiago, co-author Jessica Whitham said she saw something quite different was happening.

“Actually, they are looking at the baby when they are producing these vocalizations, so we think they are intended for the baby, not the mother,” said Whitham, a recent Ph.D. student of Maestripieri’s.

The intent seemed simply to get the infant to look over and make eye contact with the caller, she said, then to amuse the infant, much as a human might try to get a baby to smile.

A rhesus macaque wanting to charm a tot has an advantage over a human: Besides babbling in baby tongues, it can wag its tail. The monkeys often wag their tails in the presence of a monkey infant, like a human using a rattle to entertain the little one.

Maestripieri and Whitham published a paper on their findings today in the research journal Ethology.