Intergroup interactions in wild common marmosets, Callithrix jacchus: territorial defence and assessment of neighbours

Abstract

Group territoriality associated with aggressive intergroup interactions is characteristic of most cooperatively breeding species. Neighbours, however, are not only competitors but also potential mates. Intergroup interactions might provide a direct mechanism for the assessment of breeding opportunities in neighbouring groups. I studied 228 intergroup interactions in wild cooperatively breeding common marmosets to understand how animals might balance their cooperation in territorial defence with their assessment of neighbours as potential mates. Intergroup interactions included both agonistic and nonagonistic behaviour. Aggression was directed primarily to same-sex individuals. Reproductive females' involvement in intergroup interactions decreased during the last 2 months of pregnancy and during the first 2 months of lactation. Subordinate females participated more often than reproductive females in territorial defence but they were also more involved in nonagonistic interactions with animals from other groups. Intergroup copulations involved mainly subordinate females, and occurred both during forays into other groups and during agonistic intergroup encounters in the subordinate female's territory. Eldest helper females were more likely than younger females to chase away other individuals, but were also more likely to engage in extragroup copulations. Cooperation by helpers in territorial defence might decrease the cost of defence for the breeders. Furthermore, our data indicate that assessment of neighbours does not occur solely during individual forays into other groups' territories. Helpers' participation in intergroup interactions in their own territory might provide direct benefits related to dispersal decisions as well by allowing nonreproductive individuals to assess individuals from neighbouring groups, evaluate potential breeding vacancies and increase familiarity with neighbouring groups.