Separate groups of monkeys have different calls that might be the simian version of local dialects. Now Nobuo Masataka of Kyoto University in Japan says the differences can be a response to the demands of the local environment.
Masataka and two colleagues recorded and analyzed the calls of two groups of Japanese macaques originally from the southern Japanese island of Yakushima. One group was removed from the island in 1956 and transported more than 400 miles to Ohirayama in central Japan, where they have lived partially wild ever since. The Ohirayama macaques' "coo" calls, which the animals use to keep track of each other while foraging, were a third of an octave lower on average than those of the macaques that remained on Yakushima.
Since the two groups are genetically identical, Masataka suspects that the dialects are behavioral adaptations. The Yakushima group lives in a temperate forest, while the Ohirayama group lives on gravelly open ground. The latter may have developed deeper calls because bass sounds travel better in the open. The monkeys apparently do not learn their local dialect until they are about 6 months old, which is the equivalent of 3 or 4 years old in humans. "At 6 months there is no difference between calls in the two groups, but at 1 year, they are quite different," Masataka says.
The three researchers spent hundreds of hours over eight years trailing and recording female monkeys, which make more calls than males. On average it took four months of pursuit just to be able to approach close enough to the monkeys without scaring them away. The researchers are now playing taped calls over speakers in the two habitats to see whether vegetation also affects the dialects of the macaques.