Variations in fundamental frequency peak position in Japanese macaque (Macaca fuscata) coo calls

Japanese macaques (Macaca fuscata) produce coo calls whose use in various contexts is correlated with differences in the relative temporal position of the peak value of a call's fundamental frequency (Green, 1975). Studies have produced conflicting results about both the location of the category boundary between smooth early high and smooth late high coo subtypes and macaques' perceptual sensitivity to variations in peak position. In this study, fundamental frequency peak positions were measured in 578 coos produced by 8 captive adult female Japanese macaques in order to test whether calls with peak positions close to either of 2 hypothesized boundaries occurred at low rates. Overall, such calls were found to occur at rate equal to or higher than predicted by chance. Peak position varied more consistently between animals than by behavioral context. The results may indicate that peak position in coos does not form 2 distinct categories.