Sequential exchanges seem to be constituted by two inter-related subsystems of calls that aid muriquis to coordinate activities, despite dispersion of group members and poor visibility in the habitat. Staccatos and neighs containing a large proportion of short elements are preferentially used in the coordination of nearby individuals. Since the occurrence of a great group cohesion is relatively rare, such calls are most frequently used in exchanges containing a relatively low number of participants.
Staccatos may also obey motivational-structural rules (Morton 1982), and work as a mechanism for competition avoidance. In some primates, the degree of harshness and pitch of calls relates to tendencies of spatial proximity or spatial dispersion (Robinson 1982, Boinski 1991). In short-range exchanges of muriquis, different forms of short elements could be emitted according to the availability of resources, and/or tendencies of spatial proximity. Calls dominated by harsh elements, for example, could represent a potential conflict, and result in the maintenance or increase of distances among nearby listeners. Calls dominated by higher and more tonal elements, on the other hand, were more often produced by dispersed individuals, or individuals at sites with abundant food. The relative frequency of emission of short elements in "single source" and "feeding" contexts seems to support this hypothesis.
When the group is dispersed, muriquis may communicate with nearby individuals through staccatos, or emit neighs dominated by long elements, which reach a larger number of group members. In this way, exchanges containing neighs, especially those with relatively small numbers of short elements, present on average larger number of participants.
The specificity of referents in short and long-range neigh production is also relevant. Short-range interactions may be accompanied by redundant visual cues. Callers and receivers of long-range vocalizations may, on the other hand, benefit by more specific information, such as the sexual receptivity of female callers, and on the proximity of neighbor groups.
Variables related to the spatial distributionof individuals are important determinants of vocal production in primates' intragroup vocalizations (Snowdon and Hodun 1981, Robinson 1982, Boinski and Mitchell 1992, Palombit 1992, Boinski 1996, Rendall et al. 1999). Exchange calls are, in most species, used at relatively short distances. The sequential exchanges of muriquis are remarkable for their great variability in distances between participants. Some calls are restricted to nearby individuals (i.e. muriquis at a 50 meters range), while others include participants dispersed over larger areas. A single exchange may include both short-range and long-range calls of different acoustic properties. This flexibility in structure and usage indicates the possibility of differences in the function and information content of calls, a possibility that deserves further attention.
The research counted with the collaboration of Charles T. Snowdon and Karen B. Strier, funds from National Geographic Society (CTS and KBS), CNPq, N.S.F. (KBS), FAPESP, C.I.-Brazil, and logistic support from Fundação Biodiversitas.