Foreword to Behavioral Study Section

Frans B.M. de Waal1

In the minds of some, bonobos compete with chimpanzees as the best nonhuman primate model of our species. Given that there can only be one best model, and that chimpanzees were known first, bonobos are at a disadvantage. Moreover, anthropologists seem heavily invested in the chimpanzee as a model for human social evolution. Chimpanzees show male bonding, intergroup "warfare," proficient tool-use, power "politics," hunting, and meat-eating. There is no shortage of similarities, albeit mostly on the male side, with our own species. Consequently, a coherent picture of human social evolution has arisen around the chimpanzee as close relative, one emphasizing meat, violence, and male superiority. This picture fit well with post World War II developments, led by Konrad Lorenz in Europe and Robert Ardrey in the United States (Lorenz 1963, Ardrey 1963). Understandably perhaps, emphasis was put on Homo sapiens as a "mentally unbalanced predator" endowed with vigorous aggressive instincts (Cartmill 1993, p. 14).

Then the bonobo came along. It is good to realize, though, that long before this happened, Robert Yerkes wrote an entire book about an ape named "Prince Chim." This ape was notably more sensitive, altruistic and intelligent than other apes familiar to Yerkes. The great psychologist did not know it at the time, but Prince Chim was a bonobo. Yerkes was so struck by this individual's behavior that he entitled his book Almost Human (Yerkes 1925). Anatomically as well, bonobos were found to be strikingly human-like. Harold Coolidge, the anatomist who gave Pan paniscus its eventual taxonomic status (and who performed the post-mortem on Prince Chim), concluded that this ape "may approach more closely to the common ancestor of chimpanzees and man than does any living chimpanzee" (Coolidge 1933, p. 56).

We had to wait until the 1980s, however, for detailed studies of bonobo social behavior. We know now that, compared with chimpanzee society, bonobo society is less violent (i.e. lethal aggression has thus far never been observed, neither in captivity nor in the field, quite in contrast to the chimpanzee), bonobos seem sexrather than power-oriented, and adult males tend to defer to adult females (de Waal 1997). Obviously, the virtual absence of hunting and "warfare" in this ape, combined with its relative peacefulness and female dominance, should raise

'Living Links, Yerkes National Primate Research Center, Emory University, 954 N. Gatewood Road, Atlanta, Georgia, 30322 United States questions about earlier scenarios of human evolution built around themes of violence and predation. However, what we have seen thus far instead is a concerted effort to marginalize bonobos and keep them out of evolutionary scenarios. This is despite the fact that the split between bonobo and chimpanzee occurred well after their lineage split off from ours, meaning that both species are phylogenetically equidistant to us.

Thus, Wrangham and Peterson (1996) treated bonobos as an afterthought, a nice respite of the blood-soaked image of our primate heritage drawn in Demonic Males. Whereas these authors assigned bonobos secondary importance, Stanford (1998) simply declared them less special than often assumed, downplaying differences between both Pan species even though study after study has shown them to be quite different (Parish and de Waal 2000, Doran et al. 2002). Stanford's (1998) argument that reliance on captive data explains why bonobo sexual behavior is said to be so highly developed ignores the fact that chimpanzees in captivity do not show anything close to such behavior. If the same conditions affect two species so differently, the logical conclusion is that the difference is due to the species, not the conditions (de Waal 1988, 1998). The high-point of the sidelining attempt with regards to bonobos came with Konner's (2002, p. 199) suggestion that science can safely ignore them, because "chimps have done far better than bonobos, which are very close to extinction."

The earlier citation from Coolidge was not offered to make the point that the bonobo is a better model of human ancestors than the chimpanzee, even though one could argue this point based on a) the bonobo's body proportions (Zihlman et al. 1978), b) the probability that this forest ape has encountered fewer evolutionary pressures to change than the ancestors of both humans and chimpanzees (Kano 1992), and c) the recent discovery that bonobos and humans share genetic code in relation to affiliative behavior that is absent in the chimpanzee (Hammock and Young 2005). Despite these arguments, there is no urgent need to choose between bonobo and chimpanzee with regards to our ancestry. We can be sure that more discoveries are on their way, and they may again change the picture. What we need most at this point is behavioral and ecological data so as to develop better models of how and why the bonobo evolved into what it is today. In the last couple of years, scientists have been actively collecting new data in both captivity and the field. This volume's first section features some of the best zoo studies. By themselves, such studies cannot provide the full comparison, but they do permit behavioral records of greater detail than possible in the field, and are capable of generating testable hypotheses about what makes bonobos different.

The first two chapters address the issue of agonistic dominance. Female dominance is rare in primates, which explains the initial skepticism towards claims that female bonobos dominate males. Stevens et al. find that all captive bonobo groups are dominated by an alpha female rather than alpha male, but also note that not all females necessarily dominate all males. Males and females rather seem to have overlapping ranks, with females being disproportionally represented near the top. Paoli and Palagi do not reach an equally clear conclusion on male vs. female dominance, but they do report that an individual's sex hardly predicts its rank, which is by itself already a huge contrast with chimpanzees. Perhaps the best way to look at the dominance relation between the sexes is not so much in terms of individual ranks, but on the basis of the outcome of multi-party encounters. As shown by Stevens et al., bonobos seem characterized by collective female dominance, as expressed in alliances among females against males, the effectiveness of which is facilitated by the virtual absence of bonding and alliances among adult males (see also de Waal 1997).

In discussing the adaptive potential of bonobos, Stevens et al. stress the preponderance of alliances directed down the hierarchy. Such alliances are bound to reinforce the hierarchy, hence create a more rigid structure than found in chimpanzees, which do show frequent coalitions from below (de Waal and Luttrell 1988). This may explain the relatively stable hierarchies of bonobos, which apparently change only when one of the older matriarchs falls ill or dies (Kano 1992). Here we recognize two major differences with chimpanzee society as observed in captivity. The first is that all healthy adult male chimpanzees dominate all females, which clearly does not apply to bonobos. The second is that flexible male alliances in chimpanzees create a rather unstable hierarchy, hence a volatile social environment compared to the more predictable social structure of bonobos.

The latter may explain the relative peacefulness of bonobos society, but in addition, bonobos employ effective conflict prevention and resolution strategies as described by Palagi and Paoli. Not only do they counter conflict by sexual means (cf. de Waal 1987), they also have playful ways of diffusing tensions. This is entirely consistent with the reputation of bonobos as neotenous animals, that is, animals that (like humans) have evolved by retaining juvenile characteristics into adulthood.

Pollick et al., finally, explore the gestural communication of bonobos, comparing and contrasting it with their facial and vocal communication. Bonobos use many different gestures, and it was confirmed that this mode of communication is used more flexibly than facial and vocal displays, most of which are closely tied to specific contexts. The meaning of gestures likely depends on how they combine with the social context and other forms of communication, whereas the meaning of facial and vocal displays is relatively invariant, not only within each species, but even between species. Comparing communication across both species and groups of each species, Pollick et al. found that by knowing the usage of a facial expression or vocalization in one species, one also knows its usage in the other species. With gestures, on the other hand, not only are there few similarities between the species, but even within each species gesture usage varies greatly from group to group. This makes gesture the better candidate for early language evolution. If our ancestors used gestures in the same flexible manner, this may have provided a stepping stone for the evolution of symbolic communication, which may have originated in the gestural rather than vocal domain (e.g. Corballis 2002).

Even though more detailed studies are urgently needed, especially studies that compare the social behavior and cognition of the bonobo with those of the other apes, we have come a long way since the first zoo research on their behavior (Tratz and Heck 1954). Given that the bonobo's behavior is in some respects rather unique, it has the potential of greatly enriching scenarios of human evolution.

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