Call of the left brain.

For most people, parts of the brain's left hemisphere handle basic aspects of language. Adult rhesus monkeys display a similar cerebral setup, with the left half of the brain often taking responsibility for vocalizations intended to signal aggression, fear, and friendliness, two anthropologists report in the April 26 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

Scientists have yet to confirm whether rhesus monkeys' vocal calls convey information in a structured, languagelike way or purely through the emotional quality of specific calls, note Marc D. Hauser of Harvard University and Karin Andersson of Radcliffe College in Cambridge, Mass.

The researchers studied monkeys living on Cayo Santiago, Puerto Rico. When an animal sat down at one of three open-air food dispensers on the island, a speaker hidden in vegetation behind it played one of the three categories of rhesus calls or a seabird alarm call frequently heard by the monkeys.

Overall, 61 of 80 adult animals turned the right ear (which sends acoustic information to the left hemisphere) toward the speaker in response to rhesus calls; they usually turned the left ear toward the seabird call. In contrast, 37 rhesus infants showed no ear preference for monkey or seabird calls.

Left-hemisphere structures able to discern what various calls mean may not emerge in rhesus monkeys until at least 2 years of age, Hauser and Andersson propose.