Pyow but not hack calls of the male putty-nosed monkey (<i>Cercopithcus nictitans</i>) convey information about caller identity

Individual differences within the acoustic structure of vocalisations have the potential to inform signal receivers about the identity of the caller. Such differences can often be explained by morphological differences of the signaller’s sound production apparatus. Natural selection may have favoured individual variation within call types, especially if identity cues enhance call function. In addition, animals may modify their vocalisations such that they sound more similar to, or more distinct from those of neighbouring conspecifics. We recorded pyow and hack vocalisations from five recognised male putty-nosed monkeys (<i>Cercopithecus nictitans</i>) in Gashaka Gumti National Park, Nigeria. We analysed the temporal and spectral features of both call types to investigate whether the calls contained identity cues, and whether calls of neighbouring males were less or more different in their acoustic structure than expected by chance. More parameters were found to vary significantly between individuals within pyows than hacks, and whilst pyows could be correctly assigned to individual callers more often than would have been expected by chance, hacks could not. We found no relation between geographic distance and acoustic similarity of pyows and hacks.