INTERACTIONS BETWEEN PRIMATE SPECIES IN A NEOTROPICAL FOREST: MIXED-SPECIES TROOPS OF SAGUINUS MYSTAX AND S. FUSCICOLLIS (CALLITRICHIDAE) (AMAZON BASIN, ECOLOGY, PERU, NICHE OVERLAP)

The geographic distribution of the saddle-backed tamarin monkey (Saguinus fuscicollis) encompasses much of the western Amazon Basin. Four closely related Saguinus spp. inhabit mostly allopatric geographic ranges within the distribution of S. fuscicollis. Much documentation suggests that two tamarin species form stable mixed-species troops within their respective regions of sympatry. This study addressed ecological and behavioral mechanisms of coexistence in mixed-species troops of S. fuscicollis and S. mystax. The 1982 to 1983 field study took place at two field sites in northeastern lowland Peru.;Troop coordination was maintained by exchanging long distance vocalizations and sleeping in nearby, or the same, sleeping trees at night. Both subtroops participated in territorial defense activities against conspecifics in neighboring troops, which occurred approximately every other day. Both species gave birth to twins at approximately eight month intervals and infant survival appeared to be very high. (No hybridization has been documented between these species.).;Niche overlap averaged greater than 70% in quantitative measures of locomotion, postures, vertical height used in the forest and diet. Aggression between species was rare, occurring in the form of supplantation while feeding in the same tree.;Two factors were thought to positively influence the stability of mixed-species troops--high population density of both species with all known areas of the habitat occupied by mixed-species troops and a relatively high degree of niche overlap. Although this is rather counterintuitive from the perspective of competition theory in ecology, it would not be unexpected within the framework of interspecific mutualism. The major benefit of mixed-species troops is thought to be stability of territorial boundaries. It is hypothesized that in a population of sympatric tamarins, mixed-species troops compete for space more effectively than single-species troops.;A mixed-species troop was defined as a reproductively active, territorial troop of between five and ten members of each species. Individuals of both species travelled together on 65% to 100% of the sample days--S. mystax troop members occupied the upper middle forest canopy and S. fuscicollis travelled through the lower middle portion of the forest. All known tamarin troops (N = 10) were mixed-species troops.