CONICET | Buscador de Institutos y Recursos Humanos

For most howlers species the maximum number of adult males and adult females residing in the same social group appears to be phylogenetically constrained. This constraint in howler sociality and social intolerance is likely related to reproductive competition. In this study we report 8 cases of ambush behavior in Alouatta caraya. This behavior was expressed when peripheral lonely males or females tried to enter in an established group. During this behavioral context either males or females chase, surprise and attack aggressively intruder individuals of the same sex. The study was concentrated in two groups of black and gold howlers that live in a continuous flooded forest on Brasilera Island (27º 20' S and 58º 40' W), in northern Argentina. These two multimale groups were followed from May 2003 to November 2004 and from September 2005 to September 2006, 5 day-month from sunrise to sunset. In 6 out of the 8 cases, the defenders acted coordinately expelling intruders. This behavior may be indicating a strong reproductive competition that makes difficult for peripheral individuals to enter into established groups. In this regard, the eviction of the intruders may result in coordinated and co-operative behaviors that enhance within-group social bonding. Therefore, strong reproductive competition among males and among females may result in co-operative and social bonds between same-sex individuals that promote stability in multimale-multifemale social groups. Funded by The Leakey Foundation, The Wenner- Gren Foundation, American Association of Primatologists, University of Illinois, and Ideawild Inc.

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