FAMILIARITY WITH INTRUDERS MODULATES AGONISM TOWARDS OUTGROUP CONSPECIFICS IN WIEDS BLACK-TUFTED-EAR MARMOSET (CALLITHRIX KUHLI, PRIMATES, CALLITRICHIDAE)

Encounters between individuals in established groups have important im plications for demographic processes. While most intergroup encounters in callitrichid primates (marmosets and tamarins) are characterized b y intense agonistic exchanges, some interactions occur in the absence of aggression. This paper investigates the role of familiarity among p artners in determining the quality and intensity of aggression during stranger encounters in Wied's black-tufted-ear marmoset (Callithrix ku hli). Resident male and female pairs were presented on several occasio ns with a single male or female intruder. Half of the intruders were h oused in cages in colony rooms distant from the residents, and the oth er half of the intruders resided in cages in the same room as the resi dents. Aggression was higher in male residents than in females, and ma les showed significantly higher rates of agonistic interactions with u nfamiliar intruders of both sexes. The behaviour of the intruder did n ot differ according to its degree of familiarity to the residents, alt hough aggression by resident males and submission by intruders were hi ghly correlated in encounters with unfamiliar, but not familiar, intru ders. Resident pairs approached each other more often after intruder t rials than before. The results of this experiment suggest that familia rity among individuals in neighbouring groups is sufficient to modulat e aggression during encounters. Since home-range overlap among free-ra nging black-tufted-ear marmosets is extensive, differential familiarit y among groups may have important implications for dispersal-related p rocesses.