Familiarity with Intruders Modulates Agonism towards Outgroup Conspecifics in Wied's Black‐tufted‐ear Marmoset (Callithrix kuhli: Primates, Callitrichidae)

Encounters between individuals in established groups have important implications for demographic processes. While most intergroup encounters in callitrichid primates (marmosets and tamarins) are characterized by intense agonistic exchanges, some interactions occur in the absence of aggression. This paper investigates the role of familiarity among partners in determining the quality and intensity of aggression during stranger encounters in Wied's black‐tufted‐ear marmoset (Callithrix kuhli). Resident male and female pairs were presented on several occasions with a single male or female intruder. Half of the intruders were housed in cages in colony rooms distant from the residents, and the other half of the intruders resided in cages in the same room as the residents. Aggression was higher in male residents than in females, and males showed significantly higher rates of agonistic interactions with unfamiliar intruders of both sexes. The behaviour of the intruder did not differ according to its degree of familiarity to the residents, although aggression by resident males and submission by intruders were highly correlated in encounters with unfamiliar, but not familiar, intruders. Resident pairs approached each other more often after intruder trials than before. The results of this experiment suggest that familiarity among individuals in neighbouring groups is sufficient to modulate aggression during encounters. Since home‐range overlap among free‐ranging black‐tuftedear marmosets is extensive, differential familiarity among groups may have important implications for dispersal‐related processes. 1995 Blackwell Verlag GmbH