Mechanisms promoting stability in mixed Saguinus mystax and S. fuscicollis troops.

Mixed-species troops composed of two species of tamarin monkeys, and rarely other ceboids, occur throughout much of western Amazonia and are notable for their long-term stability. This study identified several variables that appeared to promote mixed-species troop cohesion and yet maintained spatial segregation between species. Three variables enhanced in terspecific spatial segregation: differences in support use, vertical stratification, and interindividual spacing. In contrast, early morning vocalizations and well-coordinated movement patterns contribute to cohesion within a mixed-species troop. The combination of these variables may help explain how two congeneric species accommodate one another with little apparent interspecific aggression. The exact nature of benefits and costs to each individual in a mixed troop is still elusive; however, a comparison of population densities of sympatric and allopatric tamarins suggests that population growth of one partner (Saguinus fuscicollis) is enhanced in the presence of a congeneric species.