Vocal Coordination of Troop Movement in Squirrel Monkeys (Saimiri oerstedi and S. sciureus) and White-Faced Capuchins (Cebus capucinus)

Abstract

Troops of New World monkeys in the wild are often first detected by field workers from the tumult of routine intra-group vocal communication. Prodigious numbers of calls can be produced by an individual, and especially a troop, within a brief time period. This vocal barrage usually represents a few types of vocalizations emitted at high rates (Smith et al.,1982; Boinski et al.,1994). Although a species’ vocal repertoire usually comprises a limited set of distinct vocalizations, each type of vocalization may encompass much individual and population within-call variation in acoustic structure (Newman 1985; Snowdon 1982, 1989). Yet many vocalizations produced by wild New World monkeys are not associated with any overt social interaction, foraging activity, or predation threat. This prompts inquiry into why individual monkeys indulge in such varied, apparently non-essential vocal communication. The cumulative costs of these intra-group vocalizations are likely significant in terms of energy expenditure and enhanced exposure to predators, even if the cost of each individual vocalization is trivial (Krebs & Dawkins 1984; Jürgens & Schriever 1991).