Factors underlying party size differences between chimpanzees and bonobos: a review and hypotheses for future study

Abstract

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.

Bonobos apparently search for a lost member injured by a snare

“…Otherwise, there have been no other reports of attempts by one animal to remove snares from another animal's hands or fingers, despite the large number of cases of animals caught in snares and experiencing snare injury that have been reported at many study sites (Hashimoto 1999;Boesch and Boesch-Achermann 2000;Waller and Reynolds 2001;Quiatt et al 2002;Reynolds 2005;Ohashi and Matsuzawa 2011). One reason for the lack of such reports is that chimpanzees that tend to range alone or in small parties (Furuichi 2009) may be less likely to be observed when caught in snares.…”

Seed predation by bonobos (Pan paniscus) at Kokolopori, Democratic Republic of the Congo

“…Bonobo females tend to be more gregarious than chimpanzee females (Furuichi 2009). One of the explanations proposed for this species difference is that bonobos have more seasonally stable and abundant fruit supplies than chimpanzees do (White and Wrangham 1988;Chapman et al 1994;Hohmann et al 2006).…”

The Conditions Favoring Between-Community Raiding in Chimpanzees, Bonobos, and Human Foragers

“…van Schaik et al 2006). As argued above, despite the limited data on skew in paternity, the extended sexual activity of bonobo females (leading to thousands of matings per conception) and their dominance over males make it very likely that β is lower for bonobos (Furuichi 2009;Hohmann and Fruth 2002). This would suggest that the values of σ are higher in chimpanzees than in bonobos.…”

“…Bonobos do not engage in patrols and raids and tend to have higher home range overlap (Furuichi 2011;Hohmann and Fruth 2002). The imbalance of power hypothesis suggests that the absence of lethal attacks in bonobos is due to their larger mean party sizes (Furuichi 2009;Wrangham 1986), presumably because of their greater vulnerability to scramble competition (Wrangham 2002). This means that major asymmetries that enable risk-free attacks are rare.…”

“…The latter may be a prerequisite for low-risk killing of opponents by providing practice for effective collaboration in high-risk situations. Second, females are dominant to males and have extended periods of sexual activity (Furuichi 2009;Hohmann and Fruth 2002;Surbeck et al 2011), which is expected to result in generally lower male reproductive skew (although the actual data on this are limited; Gerloff et al 1999). When skew becomes too low, the males may face a collective action problem and thus refrain from effective between-community contest in the form of raids.…”