Search

Recently, the acoustic features of coo calls were reported to differ between two populations of Japanese macaques (Macaca fuscata yakui), namely the Ohirayama and Yakushima populations. We hypothesized that this difference may arise through differences in the acoustic environments of the two habitats, and measured the degrees of transmission of pure tones (250-8000 Hz) and the coo calls of the two populations in each habitat. In the Ohirayama habitat, lower frequencies were transmitted more efficiently, and the low-pitched coo calls of the Ohirayama population showed significantly better transmission than the high-pitched calls of the Yakushima population. In the Yakushima habitat, the degrees of transmission of the calls of the two populations did not differ significantly. Therefore, the calls of the Ohirayama population possess acoustic features that allow better transmission in their own habitat, suggesting that the habitat acoustics may be a factor contributing to the population difference between the calls.

Vocal exchanges of coo calls in female Japanese macaques were observed in two populations. Temporal patterns of occurrence of these vocalizations during vocal interaction were studied by analyzing inter-call intervals between two consecutive coos. When the second call was uttered by a different caller from the first (DC sequence), most of the second calls occurred at intervals shorter than approximately 0.8 sec, and the remaining ones at intervals longer than approximately 0.8 sec. These results indicate that the second coos of DC sequence are of two different types, and that second calls separated by short intervals may occur in response to the first call, whereas second calls separated by a relatively longer interval may occur independently of the preceding call. When an animal responds to a coo given by another, she appears to do so within a certain period of time. When two consecutiove calls were uttered by the same caller (SC sequence), the second coos rarely occurred within 0.8 sec, but mostly at longer intervals. These results suggest that when an animal utters a coo spontaneously, it remains silent for a short interval and when no response occurs, she is likely to give further coos addressing groups members. Acoustic analysis of two consecutive coos in DC sequence showed that the second coos given by a different caller and occurring within 0.8 sec significantly correlated with those of first coos with respect to several acoustic parameters. This correlation was not observed when coos occurred at intervals of more than 0.8 sec. This phenomenon was confirmed by a playback experiment. The results suggest the possibility that an animal is able to alter acoustic features of responding coos and matches those of preceding coos.