Behav Ecol Sociobiol(1992) 31:339 347 Behavioral Ecology and Sociobiology © Springer-Verlag 1992 Prey-capture benefits in a mixed-species group of Amazonian tamarins, Saguinus fuscicollis and Carlos A. Peres* Dept° de Zoologia, Museu Goeldi, Cx. Postal 399, Bel~m-Parfi,66040, Brazil ReceivedJune 10, 1991 / Accepted June 16, 1992 S. mystax Summary. Tamarins of the genus Saguinus feed on a wide range of arthropods and small vertebrates, which compose a critical component of their diet. This paper examines the foraging patterns and capture success of the Avila-Pires saddle-back (S. fuscicollis avilapiresi) and the red-capped moustached tamarin (S. mystax pileatus) in very stable mixed-species groups, and whether and how any foraging benefits for either species resulted from their association. Moustached tamarins actively searched for prey items which were mainly well exposed on the midstorey foliage. Saddle-back tamarins, on the other hand, foraged at lower heights, largely by manipu- lating a variety of microhabitats potentially concealing embedded prey. The foraging activity of the numerically dominant and larger-bodied moustached tamarins often resulted in prey items escaping to lower substrates, usual- ly the forest leaf-litter. The "beating effect" of this species substantially facilitated captures of large, mobile prey items by saddle-backs, which were highly adept at locat- ing and retrieving flushed prey. It is estimated that, while saddle-backs obtained 66-73% of their prey biomass from flushed items, this proportion was substantially lower (2-9%) for moustached tamarins. Commensal in- sectivory appears to involve a highly asymmetric benefit to saddle-backs, and a low cost to moustached tamarins, which partly explains the stability of mixed-species groups. Introduction Interspecific associations are a widespred phenomenon in several vertebrate taxa including primates (Struhsaker 1981; Terborgh 1983), ungulates (Leuthold 1977), birds (Munn and Terborgh 1979; Wiley 1980), and fish (Alevi- *Present address: Sub-Department of Veterinary Anatomy, Uni- versity of Camridge, Tennis Court Road, Cambridge CB2 1QS, England Correspondence to the present address zon 1976). The ecological advantages promoting the evo- lution and maintenance of stable mixed-species groups may be similar to those of monospecific social groups, and are usually viewed in terms of benefits derived from increased predator-avoidance, increased foraging effi- ciency, or both (e.g. Terborgh 1990). In primates, stable interspecific associations are pri- marily restricted to two of the most diverse genera: guen- ons (Cercopithecus spp.) in Africa (Cords 1987; Gautier- Hion 1988) and tamarins (Saguinus spp.) in South Ameri- ca (Terborgh 1983; Garber 1988). Opportunities for asso- ciations arose within these species-groups following their high rates of speciation, which resulted in the co-occur- rence of congeners throughout their geographical ranges (Hershkovitz 1977; Kingdon 1988). Competition theory predicts that, under resource-limited conditions, asso- ciated heterospecifics (which potentially encounter equal units of the same resources at equal rates) should segre- gate ecologically to a certain extent, or else their long- term coexistence could be threatened by competitive ex- clusion (e.g. May 1974). Yet patterns of resource use by each species in mixed-species groups cannot be too divergent because costs of forming and maintaining their association would become prohibitive. Overlap in re- source use then provides room for potentially intricate interspecific interactions ranging from mutually benefi- cial, to commensal, to competitive. The level of stability of an association, given its initial changes of occurring at all, may then reflect a balanced trade-off between these interactions. In this paper I examine the very stable interspecific associations between two small-bodied insectivorous pri- mate species occurring in a central Amazonian forest site: the Avila-Pires saddle-back tamarin (Saguinus fusci- coIlis avilapiresi) and the red-capped moustached tamar- in (S. mystax pileatus). I describe the animal component of tamarins' diet, the characteristics of their prey items, and the techniques they used to search, capture, and handle prey. I emphasise ecological differences between these two species, as dictated by parameters intimately related to prey acquisition. The costs and benefits of