The scientists focused on female macaques because they are the main coo callers. The monkeys usually produce these sounds in relaxed situations, such as when feeding, grooming, or resting.
Since it's possible that calls vary acoustically during different types of behavior, researchers only analyzed vocalizations produced by the monkeys when feeding.
The result showed that the Yakushima monkeys had a tone about 110 hertz higher, on average, than the Mount Ohira group.
The difference is probably linked to the monkeys' habitats. The island group lives in an evergreen forest with high trees that tend to block sound. The Ohira monkeys reside in an open, gravelly range with little vegetation.
"Higher sound is likely to transmit more effectively in the forest than lower sound," Masataka explained.
Until now, monkeys and apes were thought to be poor vocal imitators, said Frans de Waal, director of the Living Links Center at Emory University in Atlanta, Georgia.
Humans were considered to be the only primates able to mimic each other's vocalizations, such as when learning how to pronounce new words.
"This exciting new study suggests that monkeys from the same genetic stock may, in a couple of decades, develop different call features," de Waal said.
"It shows greater vocal flexibility than assumed for primates beforeperhaps even cultural variation, such as in human dialects."
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES