Differences in party size and cohesiveness among females have been primary topics in socio-ecological comparisons of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). This paper aims to review previous studies that attempted to explain these differences and propose some hypotheses to be tested in future studies. Comparisons of recent data show that relative party size (expressed as a percentage of total group size) is significantly larger for bonobos than chimpanzees. Although the prolonged estrus of females, close association between mother and adult sons, female social relationships including unique homosexual behavior, and high female social status might be related to the increased party size and female cohesiveness of bonobos, these social and behavioral factors alone do not appear to explain the differences between the two species. Differences in ecological factors, including fruit-patch size, density of terrestrial herbs, and the availability of scattered foods that animals forage as they travel between large fruit patches could also contribute to the differences between chimpanzees and bonobos. However, these factors cannot fully account for the increased party size and female cohesiveness of bonobos. The higher female cohesiveness in bonobos may be explained by socio-ecological systems that reduce the cost in feeding efficiency incurred by attending mixed-sex parties. These systems may include female initiatives for party ranging movements as well as the factors mentioned above. Because of their geographical isolation, the two species probably evolved different social systems. Chimpanzees, whose habitats became very dry during some periods in the Pleistocene, likely evolved more flexible fission-fusion social systems to cope with seasonal and annual variation in food availability. On the other hand, bonobos had a large refugia forest in the middle of their range even during the driest periods in the Pleistocene. Therefore bonobos, whose habitats had more abundant food and smaller variation in food availability, probably evolved systems that help females stay in mixed parties without incurring large costs from contest and scramble competition.