Writing in the original Old World Monkeys volume, Struhsaker (1970) demonstrated for the first time how the loud calls of primates, analyzed with sound spectrograms, can be used in phylogenetic analysis, particularly to understand better the relationships within a genus or species-group. Struhsaker's study built on the work of Marler (1957) and others who had used vocalizations in the study of non-primate taxa. Struhsaker's analysis has been followed by many other primate studies, including those of Wilson and Wilson (1975) on leaf-monkeys (Presbytis), Marshall and Marshall (1976) and Mitani (1987) on gibbons (Hylobates), Oates and Trocco (1983) on black-and-white colobus, Snowdon et al. (1986) on lion tamarins (Leontopithecus), Gautier (1988) on guenons (Cercopithecus), Zimmermann (1990) on galagos (Galagonidae), and Whitehead (1995) on howler monkeys (Alouatta).
These spectrographic studies have found that primate vocalizations, particularly male loud calls, are both relatively stable characters within species, and also measurably different between species. However, most of these previous studies have used limited samples (a few individuals from a few sites) and rarely have they been repeated, or their conclusions tested by examining samples from additional animals or new sites.
The work we present here is a reappraisal of the analysis and conclusions of the study of black-and-white colobus male loud calls by Oates and Trocco (1983). It involves both a new analysis of the original sample of recordings using different methods, and the addition of new recordings from several sites not included in the earlier analysis.