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Chimpanzees and bonobos More similar than we thought.

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American Journal of Primatology 63:245–249 (2004)
MEDIA REVIEW
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, x + 285 pp, 50 tables, 6 photos, 75 figures, $40
paperback.
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,
DOI 10.1002/ajp.20055
Published online in Wiley InterScience (www.interscience.wiley.com).
r
2004 Wiley-Liss, Inc.
246 / Rybak
the distinction between the species becomes more blurred (Figs. 1.2–1.5, p. 23–
24). For instance, while Mahale and Gombe chimpanzees are taxonomically
similar, and Wamba and Lomako bonobos are similar, the chimpanzee
populations of Bossou and Taı̈ 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 ‘‘ythe
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 savannahdwelling 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
Media Review / 247
a more complete understanding of chimpanzee social dynamics while combining
studies from four chimpanzee sites (Taı̈ (Chapter 6), Ngogo (Chapter 7), and
Gombe and Budongo (Chapter 13)).
Male chimpanzee relationships in Budongo Forest are examined by NewtonFisher 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 ‘‘yfor 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 Taı̈ 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, ‘‘ythe
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-sexualattraction 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
248 / Rybak
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
Media Review / 249
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
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