Grunts prep babies for talking.

When Lorraine McCune began monitoring language development in infants, she never paid much attention to their grunts. A researcher at Rutgers University in New Brunswick, N.J., McCune was hoping to learn how to predict when a baby would begin to talk. Most of the children she studied began picking up a few words in preparation for speech. But one just grunted.

Curious about his grunting, she listened again to recordings collected from other babies. She and her colleagues were surprised to find that grunts made up 10 to 45 percent of the infants' vocalizations, she says.

Grunts arise spontaneously in many animals when an increase in oxygen demand leads to a change in the larynx that causes air to reach deeper into the lungs. When exhaled air rushes through the resulting constriction, it creates a sound - - ultrasonic in rats but audible in larger animals.

"Nobody has to start [doing] it; it's in the physiology," says McCune. Lots of animals, including monkeys, chimpanzees, and newborn rats use these utterances to communicate.

Babies begin to grunt at about age 3 months because of the effort they make to hold up their heads, she says. Three to 6 months later, they "hmm" and "uh" as their brain cells work to figure out the world around them. Finally, around their first birthday - about the same time they are babbling and forming their first words - babies begin purposeful grunting.

Data analyzed from five children studied by McCune indicate that once they figure out how to communicate with grunts, these babies quickly pick up and use many more words. "Communicative grunts are actually a developmental phase in children's transition to language," she concludes.

McCune thinks that babies associate the metabolic changes and the constricted feeling with what is happening around them and with the sounds that emerge from their throats. "They get the insight that a particular sound goes with a particular experience," she adds. "It's sort of a sound-meaning connection."