Aggressive interactions can serve to secure resources. These interactions determine female dominance relationships, which have been related to the monopolizability of food patches. Patches of medium size, relative to group size, cause within-group contest competition which is hypothesized to produce linear, nepotistic and formalized dominance relationships. Small dispersed or very large and abundant patches lead to reduced within-group contest competition which should lead to egalitarian and individualistic dominance relationships without a formal hierarchy. This relation was investigated in two sympatric primate species at Ketambe, Northern Sumatra: the Thomas langur (Presbytis thomasi) and the long-tailed macaque (Macaca fascicularis). The female dominance relationships of the two species differed as predicted. Both species engaged in competitive interactions for food. The Thomas langurs competed in small patches, but not in large patches. These large patches could be considered abundant. About two-thirds of their food patches incited contest competition. Long-tailed macaques were aggressive in fruit patches, irrespective of size. Most food patches incited contest competition. Contest competition was probably more important for macaque females than for langur females. Outside food patches macaque females were more aggressive than langurs, whereas inside food patches aggression rates were similar.