Chimpanzees and Bonobos: More Similar Than We Thought?

Review of Behavioural Diversity in Chimpanzees and Bonobos edited by
Christophe Boesch, Gottfried Hohmann, and Linda F. Marchant. Cambridge,
Cambridge University Press, 2002

Rybak, J. 2004. American Journal of Primatology 63:245–249

This comprehensive work on the study of behavioral diversity in wild populations of chimpanzees and bonobos (Pan) is based largely on the proceedings of a conference entitled “Behavioural Diversity in Chimpanzees and Bonobos,” which was held in June 2000 in Seeon, Germany. The conference was organized and hosted by the editors of the book, and included presentations by 30 invited international scientists studying 13 different wild populations of Pan paniscus (bonobos) and Pan troglodytes (chimpanzees). Although it is based heavily on contributions from the conference, the book also contains chapters from other researchers in the field. Taken together, these studies address a broad range of behavioral diversity both within and between wild populations of Pan, and do justice to the book’s seemingly ambitious title.

The overall goal of the book is to document and assess the most current knowledge regarding behavioral diversity in wild populations of Pan by focusing on both species-typical and population-typical behavior patterns. A large body of literature already exists on the topics discussed in the book. Nevertheless, this book provides new and exciting information that supplements the current data. Additional data from ongoing longitudinal studies and from lesser-studied populations of both species are examined. Areas where further research is required are also highlighted, indicating promising new avenues of study. The book includes 19 articles, which are presented in five main sections: Behavioural Diversity, Social Relations, Female Strategies, Hunting and Food Sharing, and Genetic Diversity.

In the first section, Chapter 1 (by Doran et al.) encompasses what I find to be the most interesting part of the book. In the process of identifying the variability among chimpanzee and bonobo populations, it becomes clear that the distinction between the two species is not as clear-cut as previously thought. While certain species distinctions still hold true, such as female- vs. male-dominated societies, and differences in tool use, other distinctions are brought into question. This is largely because the taxonomic grouping of these two species has been based on incomplete behavioral data. However, the data presented in this section illustrate that as we learn more about population-specific differences within each species, the distinction between the species becomes more blurred. For instance, while Mahale and Gombe chimpanzees are taxonomically similar, and Wamba and Lomako bonobos are similar, the chimpanzee populations of Bossou and Tai forest vary in their placement along the taxonomic spread. As an additional example of these behavioral distinctions exemplified throughout the book, Matsumoto-Oda (Chapter 12) suggests that “the gregariousness seen at Mahale might not be characteristic of chimpanzees in general” (p. 177). Since a great deal of our understanding of these species is based on a few specific groups, it seems that the full breadth of Pan behavior is not yet well understood. Thus, it becomes obvious to the reader that it is difficult to say whether or not chimpanzees and/or bonobos have specific behavioral repertoires, a theme present throughout the remainder of the book.

In Chapter 2, Hunt and McGrew provide further evidence for this theme by drawing on observations of wild savannah chimpanzee populations at Assirik, Senegal, and Semliki Wildlife Reserve, Uganda. The habitats of these populations are quite different from those of forest-dwelling groups, such as the Gombe and Mahale groups in Tanzania. Ecological differences between the savannah sites and Tanzania, including forest structure and predation pressure, result in a range of different behaviors. The savannah chimpanzees nest in larger groups, possibly due to predation pressure, and range farther than other populations of chimpanzees, possibly due to the limited forest canopy. In addition, preferences for prey items and hunting styles are different between forest- and savannah-dwelling chimpanzees. In contrast to forest-dwelling chimpanzees, the Semliki population rarely hunts, and no carnivory has been observed. Chimpanzees at Assirik consume two species of nocturnal prosimian (Galago senegalensis and Perodicticus potto) instead of the preferred red colobus (Procolobus badius) hunted by chimpanzees in Tanzania. Rather than actively pursuing prey, as east African chimpanzees do (see Boesch et al., Chapter 16), the chimpanzees of Assirik take prosimians from nesting areas and sleeping holes during the day.

Throughout the book are examples of behaviors seen only in certain populations of chimpanzees and bonobos, such as tuber-eating by chimpanzees in Tongo (Chapter 3), where water is sometimes scarce and the animals have adapted accordingly, and the grooming hand-clasp that is observed in only a few populations of chimpanzees (Chapter 5). Examples such as these, initially presented as outliers of ‘‘typical’’ chimpanzee behavior, are now being used to broaden our understanding of the diversity of the species. Rather than simply describing these differences among populations, the authors have attempted to explain them in terms of ecological and evolutionary variations (e.g., Chapter 5 by Nakamura).

The second section of the book investigates social relations within various populations of chimpanzees and bonobos. Party composition in chimpanzees is addressed by the first two chapters of the section (Chapter 6 by Anderson et al., and Chapter 7 by Mitani et al.), which are somewhat confusing. In reading Chapter 7, it seemed to me that the information presented was slightly redundant, since it covered many of the same points discussed in the previous chapter. Indeed, both chapters investigate essentially the same topic in two different populations of chimpanzees, focusing on party size, estrous females, and food abundance. However, neither provided the cohesiveness that might have been better accomplished by combining efforts. In addition, the work of Wallis in Chapter 13 provides further data related to this topic, which suggest that seasonality and food abundance influence the estrous cycle of females. Since Chapters 6 and 7 addressed estrous females and food abundance as factors affecting party size, the addition of data from Wallis’s study might have provided a more complete understanding of chimpanzee social dynamics while combining studies from four chimpanzee sites (Tai (Chapter 6), Ngogo (Chapter 7), and Gombe and Budongo (Chapter 13)).

Male chimpanzee relationships in Budongo Forest are examined by Newton-Fisher in Chapter 9. Data were collected regarding the association and proximity of male chimpanzees during a period when their social relations were unstable and a change in alpha status occurred. While the results did not indicate that changes in social status had an effect on the grooming, association, and proximity of the males, the author postulates that the differences were not seen because the shift in status was beginning to take place prior to the onset of the study. Therefore, collecting data over a longer time period may help complete the picture of adult male chimpanzee associations throughout the course of status changes. However, the results did indicate that “for individual males, the size of the party may be less important than the number of other males, and the number of males may be less important than the identities of those other males’’ (p. 135).

Studies such as these may also provide useful information in terms of the welfare and management of captive populations, some of which include bachelor groups. The social organization of bonobos as presented by Hohmann and Fruth (Chapter 10) provides not only an interesting comparison with chimpanzee society, but also new information about bonobos. This study investigates the effect of estrous females and food abundance on the social organization of bonobos (similarly to the first two chapters of the second section), in addition to exploring intercommunity contacts. This chapter also provides support for the overall theme of the book, in that bonobos are similar to the chimpanzees of Tai forest in terms of party size and patterns of association, and share a similar habitat. In fact, the authors make the bold statement that in their opinion, “the behavioral diversity of bonobos resembles that of chimpanzees” (p. 147), a statement that is well supported by their research.

The third section of the book discusses female social and sexual strategies. In this section the commonly-held view that bonobos are the more sexually active of the two species is challenged. Furuichi and Hashimoto (Chapter 11) found that the rate of copulation for estrous females was actually lower in bonobos than in chimpanzees. The authors do a superb job of examining various hypotheses to explain these findings, and discuss many of the implications and strategies that are associated with differences in sexual behaviors. Nevertheless, the notion that bonobos are more sexually active than chimpanzees may still be valid if noncopulatory sexual behaviors or those occurring outside the estrous period are included. Furthermore, since this chapter compares the bonobo data primarily to previous data on eastern chimpanzees, the inclusion of data from western chimpanzee populations may reduce the clear distinctions seen in this chapter.

Wrangham continues the underlying theme of the book by examining the sexual attractiveness of chimpanzees and bonobos in Chapter 15. By including new data from east African chimpanzees, Wrangham applies his cost-of-sexual-attraction hypothesis to numerous populations of Pan in order to illustrate differences in such factors as the copulation rate, number of mating days per conception, and male coercion and mate guarding. These differences are evaluated with ecological and demographic factors in mind.

Hunting and food-sharing are the common themes in the fourth section. This section is especially interesting because Fruth and Hohmann test several hypotheses to explain food-sharing in bonobos (Chapter 17). Similar hypotheses could be inferred in the next chapter from the data presented by Watts and Mitani on chimpanzee hunting and food-sharing. While Fruth and Hohmann found that bonobos share food based on mutualism and buy-off strategies, Watts and Mitani report that chimpanzees share meat based on what appears to be reciprocity. One of the most interesting disparities between chimpanzee and bonobo hunting is that bonobos successfully hunt alone. This may be because bonobos prefer to hunt species such as duikers (Cephalophus spp.), a solitary, ground-dwelling ungulate, or forage for large fruits such as Treculia africana and Anonidium mannii, while chimpanzees prefer to hunt red colobus (Procolobus badius), a highly active, group-living, arboreal species, which requires a certain level of cooperation. A combined study on food-sharing and hunting behaviors among various Pan populations, or a chimpanzee study designed in a manner similar to that used by Fruth and Hohmann could potentially clarify the behavioral differences between the species. As an interesting aside, it was found that female bonobos do not share food with males who are being aggressive (Chapter 17), whereas aggression appears to be an important part of chimpanzee life (although it was not considered in Chapter 18 as a variable in whether or not one received food from a possessor). Since bonobos are commonly thought to be the more peaceful of the Pan species, is it possible that the females are in a sense training the males to be less aggressive by not reinforcing their behaviors?

While I read these chapters, the thought of a child throwing a tantrum in a candy store and being reinforced by an exasperated mother came to mind. The child, being reinforced by getting what he wanted, would likely continue to throw tantrums in the future in order to get candy from his mother. This basic training concept of extinction (bonobos) vs. variable reinforcement (chimpanzees) may be a feasible explanation for some of the differences in aggressive behaviors observed in Pan.

In Chapter 16, Boesch picks up the common thread of behavioral diversity in terms of predator–prey relationships among different populations of chimpanzees. The finding of stark contrasts in hunting styles (for example, quietly assessing the situation vs. actively hunting) exemplify the behavioral diversity found among populations of chimpanzees. These differences are again explained in terms of ecological and demographic differences.

The final section of the book, Genetic Diversity, is covered exclusively by Bradley and Vigilant in the final chapter. The authors provide a thorough review of the current knowledge regarding chimpanzee and bonobo genetics. They discuss the general methods of molecular analysis, and approach genetic diversity and similarities through phylogeny, systematics, and socioecology. While the chapter is heavy with terminology, the authors provide a detailed summary of the current understanding in this emerging field, supplementing the behavioral research presented in the preceding chapters. The addition of genetic information to behavioral and morphological data assists in the completion of a comprehensive understanding of the diversity of these species.

Overall, I found this book to be a useful supplement to my own knowledge and experience gained from working with chimpanzees. I believe this book makes an important contribution to the field of primatology, and will be of great interest to both students and researchers from a wide range of backgrounds, such as primatology, ecology, anthropology, ethology, and comparative psychology. In addition, those who care for these animals in sanctuaries and captivity will benefit from a more complete picture of the natural behavioral diversity of the animals in their charge. The work presented in this book is also essential to the conservation of these species. The data presented clearly indicate that we still have much to learn about the behavioral diversity among the populations and between the species of Pan, and that this diversity exceeds previous estimates that were based on just a few groups. I look forward to continued information generated from each of the populations being studied, and highly recommend this book to anyone interested in chimpanzee and bonobo behavior.

Jennifer L. Rybak
Yerkes National Primate Research Center
Emory University
Lawrenceville, Georgia