The Northern Muriqui Brachyteles arachnoides hypoxanthus, hereafter simply called muriqui, is a neotropical monkey found exclusively in the Atlantic Forest region of Brazil (Hilton-Taylor and Rylands 2002). Members of social groups are frequently out of visual range of one another. The high degree of dispersion, and the habitat's dense vegetation, may have favored the evolution of complex vocal signals in this species (Nishimura et al. 1988, Mendes 1997).
Part of the species repertoire is a set of combinatory calls that are produced in a particular pattern of turn-taking, named sequential exchanges. In a sequential exchange, one individual vocalizes, and different group members usually respond one at a time with a single call each, and with little or no temporal overlap between adjacent calls. Such sequences occur throughout the day, in many different contexts, and are among the most frequent vocal expressions of muriquis (Mendes 1995).
Sequential exchanges are composed of a variety of acoustic forms that can be qualitatively discriminated from other vocalizations of the species' repertoire. Strier (1986, 1999) distinguished vocalizations occurring in sequences as "short" and "long" neighs, and Nishimura et al. (1988) noted that neighs, whinnies and screams usually evoked responses from other individuals. These labels account for part of the acoustic repertoire present in sequential exchanges. Less conspicuous calls composed of short pulsed sounds are also frequently exchanged by group members. Longer and louder "run-on" utterances are also used, in combination with pulsed sounds. Some combinations give a neigh-like or scream quality to the vocalization, but both pulsed and run-on sounds are combined in a variety of ways (Mendes 1995, Ades and Mendes 1997).
We will present and discuss results taken from a broader study (Mendes 1995) that indicate a relationship between sequential exchanges and the coordination of intragroup spacing.