• titi monkeys;
  • squirrel monkeys;
  • stranger confrontations;
  • social relationships;
  • cardiac activity;
  • cortisol


Comparisons were made of the behavioral and physiological responses of heterosexual pairs of two closely related New World primates, Callicebus moloch and Saimiri sciureus, to confrontations with unfamiliar pairs. Subject pairs were exposed to strangers in two situations differing in the presumed degree of arousal they imposed: obligatory (full visual exposure) and facultative (one part of the test cage allowed visual withdrawal). Behavioral reactions were evaluated employing frequencies of affiliative behaviors, agonistic behaviors, and spatial distribution. Physiological reactions were evaluated employing heartrate and plasma corticosteroid levels. Callicebus males and females were similarly affected by unfamiliar pairs. Both sexes behaved agonistically towards strangers; pairmates drew closer together and behaved in a coordinated fashion. Display behaviors declined, and spatial withdrawal increased over time. Heartrate in both sexes increased during confrontations, particularly during obligatory exposure. The reactions of Saimiri to strangers differed between the sexes. Males reacted agonistically toward unfamiliar conspecifics, as reflected in displays and spatial adjustments. In contrast, females showed a strong attraction to unfamiliar females. Despite the sex differences in behavior, heartrate did not differentiate confrontation conditions clearly in either sex. Plasma cortisol measures failed to differentiate between experimental conditions in either species.