The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Same sex aggression and reproductive competition: collective action in Alouatta caraya


1Estacion Biológica de Corrientes-MACN, CONICET-Argentina, 2Department of Anthropology, University of Illinois, Urbana, 3Instituto del Conurbano, Universidad Nac. Gral. Sarmiento-Argentina

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Many species of primates are reported to be territorial, with group members engaging in collective resource, mate, and range defense. We describe what we term coordinated same sex aggression (CSSA) in wild male and female Alouatta caraya. When engaged in CSSA behavior, resident male or female group members collectively and silently chased and attacked lone conspecific intruders of the same sex. CSSA was recorded at two sites: Isla Brasilera (27º 20' S-58º 38' W) and San Cayetano (27° 30’ S-58° 41’ W) in northern Argentina. We report 36 cases of CCSA during 1188 day-group follows (12 groups). In 30 instances at least two same-sex adult group members were observed to coordinate a sustained attack to drive the intruder from the group. In 3 cases, a resident adult acted alone in aggressively attacking the intruder (in the remaining 3 cases there were no available adult residents present for a coalition to form). Groups with more adult and subadult males were less likely to be invaded by intruders (Spearman correlation test, r=-0.74, p<0.05), and groups with more adult and subadult males had greater success in repelling intruders (G test, p>0.05). CCSA occurred at a greater frequency during the breeding season than at other times of the year (Chi Square test, p<0.05) and was a highly effective behavioral tactic used by resident male and female howlers to collectively evict intruders. Additional relationships between CCSA, male takeovers, extragroup copulations, and female breeding competition are discussed.

Funded by The Leakey Foundation, The Wenner-Gren Foundation, American Association of Primatologists, University of Illinois, Sigma Xi, American Society of Mammalogists, Zoo of Barcelona, Ideawild Inc. and Consejo Nacional de Investigaciones Científicas y Técnicas de Argentina (CONICET)

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