The research team analyzed voice tones of two groups of the same species of primates, the Japanese Yakushima macaque also known as Macaca fuscata yakui, between 1990 and 2000.

• 其中一群猕猴有23只，栖息在日本九州岛以南的屋久岛上；另一群猕猴共30只，它们的父辈1956年时从屋久岛迁移到了日本中部的大平山。