Discussion

Figure 7.7 is a summary of our results. The monthly average of the party size of E1 significantly increased with the abundance of ripe food fruit. The daily mean ranging rate, and hence the daily ranging distance, significantly increased with that of party size. The monthly average of the daily mean ranging rate also increased with the abundance of ripe food fruit, but the correlation was not significant. Many studies on chimpanzees have suggested positive relationships among fruit abundance, party size, and daily mean ranging rate or daily ranging distance (e.g., Chapman et al. 1994, Janson and Goldsmith 1995, Wrangham 2000, Williams et al. 2002), with which the results of our study are generally compatible.

Party size may increase with fruit abundance because favorite fruits may attract many chimpanzees, and fruiting trees may serve as large food patches in which a large number of chimpanzees feed together. Although there is some variability among studies, our results generally agree with those of previous studies of wild bonobos (Kuroda 1979, White 1998, Hohmann and Fruth 2002, Mulavwa et al. 2008). The ranging rate or distance may increase with party size, because parties including many individuals may consume fruits in a patch more quickly and, therefore, may shift between food patches more frequently. On the other hand, the correlation of ranging rate/distance with fruit abundance is unclear, because the ranging rate/distance may be influenced not only by abundance of fruit, but also by its distribution. If fruit abundance increases with the fruiting of favorite species that are distributed evenly or randomly, then ranging rate or distance will increase as the apes seek out their preferred fruits. In contrast, if fruit abundance increases with an increase in the fruiting of large trees or trees that show a clumped distribution, then ranging rate/distance may decrease.

Preceding studies of chimpanzees have suggested that the cost of ranging in large parties would be greater for females, which may explain the tendency of female chimpanzees to range alone or in smaller parties (Wrangham 2000, Williams et al. 2002, Pontzer and Wrangham 2006). Females may suffer from harassment and contest competition when they feed with dominant males in large mixed parties. Females may also suffer more from scramble competition than males would. Females, particularly those with dependent offspring, may move more slowly and

Fruit abundance

Fruit abundance

Ranging rate (ranging distance)

Higher attendance ratio than males irrespective of changes in party size or ranging speed

Females

Fig. 7.7 Relationships among fruit abundance, party size, and ranging rate, and attendance of females to parties with different size and ranging rate/distance.

therefore need to spend more time traveling between food patches, and may arrive at a new patch later than fast-moving males. However, in bonobos, females are more likely than males to join mixed parties (Kano 1982, 1992, Furuichi 1987, 1989, White 1988, White and Wrangham 1988, Mulavwa et al. 2008). Our study showed that females did not avoid joining larger or fast-moving parties. Why is there such a difference between chimpanzees and bonobos? Is the theory developed to explain female dispersion of chimpanzees not a valid explanation for variations found in Pan species? Or are there bonobo-specific factors that affect the ranging pattern of females?

Earlier studies suggested that larger fruit patches, a higher density of fruit patches, the existence of feed-as-you-go foods such as terrestrial herbaceous vegetation, and less marked seasonality of food abundance and distribution in the bonobo habitat may prevent an increase in ranging rate and allow the formation of large stable parties (White and Wrangham 1988, Malenky and Stiles 1991, Kano 1992, Chapman et al. 1994, Malenkey and Wrangham 1994, White 1998, Wrangham 2000, Furuichi and Hashimoto 2002). Our results seem to agree with this hypothesis. Although party size of bonobos changes with the seasonal change in fruit abundance, the expected party size in the highest fruiting season increased only 37% relative to that of the lowest fruiting season. Similarly, although the ranging rate changed with the change in party size, it increased by only 10.6 m/hr with the addition of each new individual. Thus in bonobos, the change in party size or ranging rate may be too small to suppress female attendance in mixed-sex parties.

However, there is a problem in the hypothesis that attributes the difference in female ranging pattern between chimpanzees and bonobos to the differences in food availability and scramble competition. The abundance, distribution, and sea-sonality of food resources in bonobo habitats may not be different enough from those of chimpanzee habitats to explain the differences in female ranging patterns. For example, Chapman et al. (1994) reported that the density and diameter at breast height (DBH) of food trees are not different between Lomako, D.R. Congo, for bonobos and Kibale, Uganda, for chimpanzees. Chimpanzees inhabit a wide variety of habitats that vary from rain forest in Tai', Cote d'lvoire (Boesch and Boesch-Achermann 2000), to semi-arid areas in Senegal and Tanzania (Moore 1996, Pruetz et al. 2002), and the habitats of bonobos may be included in the range of this variation.

Another factor that may explain the difference in female ranging patterns between chimpanzees and bonobos is the high social status of female bonobos. Many studies of captive bonobos indicate that female bonobos have higher social status than males (Parish 1996, Paoli and Palagi 2008, Stevens et al. 2008). Studies of wild bonobos at Wamba indicated that males and females had almost equal status, and that females had priority for feeding (Kano 1992, Furuichi 1997). The high social status substantially reduces the cost of contest competition for females that join mixed-sex parties.

Moreover, the high social status may reduce the cost of scramble competition for females. Ranging of mixed parties of E1 is usually controlled by females. Because they ranged in one area for up to several weeks, the daily ranging distance is not very long. Even when males want to travel a longer distance to shift the foraging area, they often give up the attempts if females do not want to do so, and return to the females by evening. If females can control the ranging pattern, then the ranging rate may not exceed the acceptable level for females. Therefore, females can attend mixed-sex parties without incurring the additional cost of scramble competition that comes from their lower mobility.

If the preceding hypotheses do not sufficiently explain the differences in female ranging patterns between the two species, we may need to further investigate the influence of the high social status of female bonobos on the cost of both contest and scramble competition. For that purpose, we may need to carry out ecological studies at more sites of chimpanzees and bonobos, using common methodologies for quantitative comparisons.

Acknowledgements Dr. Takayoshi Kano and Dr. Toshisada Nishida have provided continuous support and advice for the study at Wamba. Dr. Tetsuro Matsuzawa offered invaluable support for the resumption of the study after the civil wars. Discussions with Dr. Juichi Yamagiwa, Dr. Shigeru Suzuki, Dr. Yuji Takenoshita, and researchers in the Laboratory of Human Evolution and Primate Research Institute, Kyoto University, were most helpful. Staff of the Research Center for Ecology and Forestry (CREF) and the Luo Scientific Reserve in the D.R. Congo helped with various aspects of our study. We sincerely thank them all. This study was financially supported by the National Geographic Fund for Research and Exploration (#7511-03 to Furuichi), the Japan

Society for the Promotion of Science (JSPS) core-to-core program HOPE (#15001 to Matsuzawa), and JSPS Grants-in-aid for Scientific Research (#12575017, #17255005 to Furuichi).

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