THE BEHAVIOR OF BONOBOS AND COMMON CHIMPS

THE BEHAVIOR OF BONOBOS AND COMMON CHIMPS

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THE BEHAVIOR OF BONOBOS AND COMMON CHIMPS

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For papers with more than two authors, the names of all authors must be given in the references cited section. For example: The ear morphology of Ignacius graybullianus is supportive of a close relationship to euprimates, rather than dermopterans (Bloch and Silcox, 2001). Chronolestes simul is best considered a basal plesiadapoid, not a crown group carpolestid (Silcox et al., 2001). In the case of a quotation, the reference should be placed at the end of the quotation, with the addition of the relevant page numbers. For example: “The simple presence of an entotympanic is also unlikely to be a synapomorphy linking dermopterans and paromomyids to the exclusion of other archontans.” (Bloch and Silcox, 2001, p. 191) 1 Web pages should be referenced similarly, but including the author of the webpage (if provided), the date the page was most recently updated (if provided), and the name of the webpage. For example: All mammals have hair at some point in their development (Myers, Class Mammalia, Animal Diversity Web). DO NOT USE FOOTNOTES OR ENDNOTES—USE OF THESE REFERENCING METHODS WILL BE PENALIZED. Your paper must include a section entitled “References Cited”, which should include all the sources referenced in your essay, listed in alphabetical order by the first letter of the first authors` last name. Do not number the references listed in this section. Do not include papers unless they are explicitly referenced in the text. Follow these examples as to how references should be formatted in your References Cited section. Marks will be deducted for deviations from these styles. An article from a journal: Asfaw, B. 1987. The Belohdelie frontal: new evidence of early hominid cranial morphology from the Afar of Ethiopia. Journal of Human Evolution 16: 611-624. A book: Leakey, M.G. and Harris, J.M. 2002. Lothagam: The Dawn of Humanity. New York: Columbia University Press. An article in an edited volume: Oates, J.F. 1994. The natural history of African colobines. Pp. 75-128 in A.G. Davis and J.F. Oates, Eds. Colobine Monkeys: Their Ecology, Behaviour and Evolution. Cambridge: Cambridge University Press. An abstract in an abstract volume: Baker, E.W., Malyango, A.A. and Harrison, T. 1998. Phylogenetic relationships and functional morphology of the distal humerus from Kanapoi, Kenya. American Journal of Physical Anthropology 26 (Suppl.): 66. An unpublished PhD dissertation: Kirkpatrick, R.C. 1996. Ecology and behavior of the Yunnan snub-nosed langur (Rhinocolobus bieti, Colobinae). Ph.D. Dissertation, University of California. 2 A web page: Wund, M. and Myers, P. 2005. Mammalia, Animal Diversity Web. Accessed August 10, 2010 http://animaldiversity.ummz.umich.edu/site/accounts/information/Mammalia.html. If no author is provided for a web-page, substitute the name of the webpage for the author in your in-text referencing, and alphabetize with respect to that name in your references cited list. Suggested sources: The following journals can be good starting points for researching your presentation and position paper topics. All are available through the University Library Portal (http://www.library.utoronto.ca/home/). Evolutionary Anthropology Journal of Human Evolution American Journal of Physical Anthropology American Journal of Human Biology Current Anthropology American Anthropologist Nature Science Proceedings of the National Academy of Sciences of the United States of America It is recommended that you use peer reviewed journal articles as your main sources of information for your essay. Secondary and tertiary sources such as newspapers, encyclopaedias, and popular science magazines (e.g., Scientific American, National Geographic, New Scientist) tend to be less up to date, and less reliable. Journal articles are also useful for their reference lists, which can provide follow-up resources and new “leads” on relevant information. Alternatively, if you have an article that is particularly relevant to your topic, you can do a “cited reference search” in Web of Knowledge (http://simplelink.library.utoronto.ca/url.cfm/54558) to see which other papers have cited your source. This can lead you to more recent perspectives on the topic in question. Notes on using the Internet to research your paper: The Internet can be a tremendous resource for the easy location of information on a huge range of topics. However, it is very important to remember that there is no quality control whatsoever about what is made into a web page. This contrasts with the many levels of evaluation that occur before any book or article is published. When using the Internet, it is important to be very critical of the source of particular assertions. Who is the author of the web page you are using? Is he or she an academic, a student, or a crank? Is the information properly cited? Are the citations to reputable sources (e.g., refereed publications), or questionable sources (e.g., other web pages)? In other words BUYER BEWARE. If you rely on a website as a key reference to your paper, and the information it contains is incorrect or out of date, it will hurt your essay grade. In general I would recommend that you use web pages only as starting points to provide you with places to look for more conventional reference sources. 3 Other formatting issues: Please note that there are sections of the Essay Evaluation Guide dedicated to style, spelling, and grammar—in other words, all of these count towards your grade. A particular formatting issue becomes apparent when working with taxonomic names. Please follow these conventions whenever taxonomic names are used: Genus names are both capitalized and italicized or underlined (e.g., Homo) The trivial or species name is italicized or underlined but not capitalized (e.g., sapiens) Other formal taxonomic groupings are capitalized but not italicized or underlined (e.g., Hominidae) Informal derivations of taxonomic group names are neither capitalized nor italicized or underlined (e.g., hominids) Why is this important? When you get this kind of thing wrong, it communicates to your reader that you are not well versed in the topic you are writing about, hurting your credibility. It is important to get in the habit of doing this correctly. Students formatting taxonomic names incorrectly can expect to receive a penalty. essay topic topic : 2. Compare and contrast the behaviour of bonobos and common chimps. What do the similarities and differences suggest about the reconstruction of behaviour in our ancestors?Reference: Stanford, C.B. 1998. The social behavior of chimpanzees and bonobos. Current Anthropology 39: 399-420. essay The Social Behavior of Chimpanzees and Bonobos Empirical Evidence and Shifting Assumptions1 by Craig B. Stanford As our closest living relatives, chimpanzees and bonobos have been widely used as models of the behavior of early hominids. In recent years, as information on the social behavior and ecology of bonobos has come to light, many interspecific comparisons have been made. Chimpanzees have been characterized in terms of their intercommunity warfare, meat eating, infanticide, canni- balism, male status-striving, and dominance over females. Bo- nobos, meanwhile, have been portrayed as the ‘‘Make love, not war`` ape, characterized by female power-sharing, a lack of aggres- sion between either individuals or groups, richly elaborated sex- ual behavior that occurs without the constraint of a narrow win- dow of fertility, and the use of sex for communicative purposes. This paper evaluates the evidence for this dichotomy and consid- ers the reasons that contrasting portrayals of the two great apes have developed. While there are marked differences in social be- havior between these two species, I argue that they are more sim- ilar behaviorally than most accounts have suggested. I discuss several reasons that current views of bonobo and chimpanzee so- cieties may not accord well with field data. Among these are a bias toward captive data on bonobos, the tendency to see bo- nobos as derived because their behavior has been described more recently than that of chimpanzees, and the possibility that inter- pretations of bonobo-chimpanzee differences are reflections of human male-female differences. craig b. stanford is Associate Professor and Co-Director of the Jane Goodall Research Center, Department of Anthropology, University of Southern California (Los Angeles, Calif. 90089- 0032, U.S.A. []). He was educated at Drew University, Rutgers University, and the University of Cali- fornia, Berkeley (Ph.D., 1990) and has taught at the University of Michigan (1989–91) as well as conducting research on nonhuman primates in Uganda, Tanzania, Peru, and Bangladesh. His publica- tions include ‘‘Chimpanzee Hunting Behavior and Human Evolu- tion`` (American Scientist 83:256–61), ‘‘Predation and Male Bonds in Primate Societies`` (Behaviour, in press), and Chimpan- zee and Red Colobus: The Ecology of Predator and Prey (Cam- bridge: Harvard University Press, 1998). The present paper was submitted 12 vi 97 and accepted 11 ix 97. 1. This paper was originally prepared for the Wenner-Gren Founda- tion Conference ‘‘Changing Images of Primate Societies,`` June 15– 22, 1996, in Terresopolis, Brazil. I am grateful to the Foundation and to the organizers, Shirley Strum and Linda Fedigan, for their Molecular studies indicate that humans, chimpanzees (Pan troglodytes), and bonobos (P. paniscus) are very closely related in a lineage that split into hominid and Pan lines approximately 6–7 million years ago, possibly following a divergence from the gorilla lineage about 1– 2 million years earlier (Caccone and Powell 1989, Ru- volo et al. 1991). Chimpanzees and bonobos have a more recent common ancestry only some 2–2.5 million years ago (Caccone and Powell 1989). Although it is now an endangered species, the chimpanzee is an ex- tremely successful species ecologically, occurring in a wide range of habitat types across the equatorial portion of the African continent. The bonobo, by contrast, is found in a much more geographically and ecologically restricted region of lowland rain forest in central Zaı ̈re. Until the 1980s, so little was known about the behavior of wild bonobos that detailed comparisons between the two Pan species were not possible. The number of field observation hours on bonobos is today still a small frac- tion of the database of chimpanzee behavior and ecol- ogy (White 1996a), but cross-species comparisons are nevertheless commonplace. These two African apes have been reported to differ dramatically in patterns of sexuality, dominance, same- sex social bonds, and the frequency and intensity of both intragroup and intergroup aggression. Chimpan- zees have long been described in terms of male dom- inance over females, hunting and meat eating, and intercommunity warfare. According to Wrangham and Peterson (1996:191), ‘‘What most male chimpanzees strive for is being on top, the one position where they will never have to grovel. It is the difficulty of getting there that induces aggression.`` Bonobos have been seen as sharply contrasting with chimpanzees, displaying fe- male dominance over males, richly elaborated sexual behavior that often occurs in a nonconceptive context, and a general lack of aggressiveness. In de Waal`s (1997: 22) description, ‘‘Bonobo society, unlike that of chim- panzees, is best characterized as female centered and egalitarian, with sex substituting for aggression. Fe- males occupy prominent, often ruling positions in soci- ety, and the high points of bonobo intellectual life are found not in cooperative hunting or strategies to achieve dominance but in conflict resolution and sensi- tivity to others.`` The importance of these closely re- lated apes in the ontogeny of theories about the origins of human behavior cannot be overstated. Our under- standing of the biology of extinct forms represented invitation to participate. I thank the other participants for their constructive comments on an earlier version of the manuscript, es- pecially Richard Byrne, Sarah Hrdy, Alison Jolly, Karen Strier, Rob- ert Sussman, and Hiroyuki Takasaki. Christopher Boehm, William McGrew, Jim Moore, and two anonymous reviewers also read and improved the paper. Research on Gombe chimpanzees was sup- ported from 1990 to 1995 by the National Geographic Society, the Fulbright Foundation, the L. S. B. Leakey Foundation, and the Uni- versity of Southern California. Research clearance was granted by the Tanzanian Commission for Science and Technology, Tanzania National Parks, and the Serengeti Wildlife Research Institute. 399 400 current anthropology Volume 39, Number 4, August–October 1998 only by fossilized skeletal remains would be quite dif- ferent if we lacked living individuals of Pan for compar- ison. Chimpanzees and bonobos provide us with exam- ples of the range of possible adaptations for feeding, ranging, territoriality, mating, offspring rearing, and a variety of other behaviors without which there would be no starting point for reconstructing hominid soci- eties. They are, because of their kinship with humans, their similar morphology, and their cognitive abilities, the main referential models for early hominids (Tooby and DeVore 1987). Wrangham and Peterson (1996) have recently argued for an evolutionary continuity of male violence that extends from our close ancestry with chimpanzees. They argue on behavioral and morpholog- ical grounds that humans have a greater phylogenetic affinity to chimpanzees than to bonobos. In this paper I examine behavioral differences be- tween chimpanzees and bonobos and argue that the so- cial behavior of these two great apes, while distinct in some respects, is more similar than is often claimed. I use data from field studies of the two species to address interspecific differences in female dominance, sexual behavior, and male aggression. Chimpanzee Social Behavior The difficulty in generalizing about the natural history of Pan troglodytes is illustrated by the fact that the fol- lowing two statements, either of which might be found in a textbook description of chimpanzee behavior, are equally accurate: A. Chimpanzee society is characterized by male con- trol and dominance over females and by male aggres- sion and sexual coercion directed at females. Male terri- toriality and patrolling exclude extracommunity males and acquire new females for male reproductive benefits. Females are essentially reproductive commodities over which males compete. B. Chimpanzee society is characterized by actively mate-soliciting females that incite male competition during their periovulatory period and that with their in- fants form the nuclear units of the social system. Fe- males forage solitarily to optimize food intake in fruit patches and become more social when it suits their re- productive tactics. Males may appear to dictate mating efforts, but the promiscuous, mate-soliciting female is the driving force in the mating system of the species. Female chimpanzees are indeed active mate solici- tors, play important dominance roles in chimpanzee so- ciety, and strongly influence the shape of the social sys- tem through their frequent sociality and periodic multiple matings. Few observers have observed wild chimpanzees, however, without concluding that fe- males live in a largely male-dominated and male- controlled social environment. Chimpanzees have been more intensively studied than bonobos, with several long-term (ten-plus years) and numerous shorter field studies. The study sites from which data in this paper are drawn are Gombe Na- tional Park, Tanzania (Goodall 1986), Mahale National Park, also in Tanzania (Nishida 1990), Taı ̈ National Park in Coˆ te d`Ivoire (Boesch and Boesch 1989, Boesch 1994), and Kibale National Park, Uganda (Wrangham, Clark, and Isabirye-Basuta 1992, Chapman, White, and Wrangham 1994). Our current view of chimpanzee society has emerged slowly, mainly because of the difficulty of obtaining a clear portrait of their fission-fusion form of polygyny. Increasing knowledge and changing attitudes about chimpanzee society can be divided into three stages, each corresponding to a decade of field research. During the first stage of modern primate research in the 1960s, chimpanzee behavior first became a subject of system- atic field study. After several years of observation in the wild, Goodall (1968) had made the landmark discoveries of meat eating and tool use. Throughout the 1960s Goo- dall believed that chimpanzee society, unlike that of other group-living primates, had no group structure whatever. Relationships among individuals appeared to be in constant flux. Nishida (1968) was the first to put forward a model of chimpanzee society based on the ‘‘unit-group`` (later called the ‘‘community`` by Western primatologists). This large-scale structure has a stable membership but no stable grouping patterns other than mothers and their dependent offspring. Other members come together and depart unpredictably, giving rise to the label ‘‘fission-fusion society`` (taken from Kum- mer`s [1968] study of Papio hamadryas). Males tend to be social with each other, and male alliances play a cru- cial role in the maintenance of territorial borders and in attempts to control females. In the second stage of research, in the 1970s, there were two major advances. The ecological influences on chimpanzee behavior became clear, and key aspects of their behavior that had previously been unsuspected came to light. In both Mahale National Park and Gombe National Park, the negative effects of artificial provisioning, which had led to heightened intracom- munity aggression, were recognized and curtailed, and the collection of data became more systematic and in- cluded more ecological information. At Gombe, obser- vation of the animals during long follows through the forest began to replace data collection in the feeding sta- tion. Wrangham (1979) conducted the first thorough study of chimpanzee behavioral ecology, focusing pri- marily on the males. It was this study that developed Nishida`s idea of the community as a male-defended structure within which less sociable females traveled alone to optimize their use of food patches. Later obser- vation of intercommunity lethal territoriality rein- forced the view that chimpanzee society was male-con- trolled from both within and without. This represented a fundamental change in thinking about chimpanzees and also a revelation for its similarity to the homicidal aggression that is a regular feature of many human soci- eties. Meanwhile, life histories of females showed that they typically emigrate from their natal communities at or after puberty (Goodall 1986). Female reproductive strategies involve multiple matings with males of their own and other communities. Despite Goodall`s early observations of intensely aggressive competition among males for estrous females, chimpanzee social systems came to be described in terms of casual promiscuity. In the third stage of chimpanzee field research, from the early 1980s through the 1990s, the diversity of chimpanzee behavior became clear as studies from dif- ferent regions of Africa began comparing research find- ings on tool use (McGrew 1992), hunting styles (Boesch 1994, Stanford et al. 1994b), and feeding ecology (Chap- man, White, and Wrangham 1994). Two further long- term field studies produced new perspectives on tool use, hunting, and social cognition (Boesch and Boesch 1983, 1989; Chapman, White, and Wrangham 1994). Ecological data on food patches and their utilization led to predictions that explained the variation in party size and cohesiveness between the major study sites. Chim- panzee populations exhibit considerable cultural varia- tion (McGrew 1992), with learned group traditions that parallel traditional human societies technologically. This emerging realization has made it clear that the ex- termination of a chimpanzee population represents the permanent loss of any traditions that were unique to those animals. Party size in chimpanzees appears to be a function of both food-patch size and distribution and the presence of sexually receptive females (Goodall 1986, Wran- gham, Clark, and Isabirye-Basuta 1994). At Gombe, Goodall (1986) considered females with sexual swell- ings to be the primary attractant influencing the forma- tion of large mixed-sex parties. She noted that years in which many females were cycling were also the years of largest mean party size. Stanford et al. (1994b) also found a significant positive correlation among Gombe chimpanzee party size, the presence and number of swollen females, and the tendency to hunt. Most pub- lished papers suggest that party size is primarily a func- tion of food availability (White and Wrangham 1988, Chapman, White and Wrangham 1994). Data on female cycles in relation to fluctuation in party size from vari- ous chimpanzee study sites are critical to testing this hypothesis. Social relationships between the sexes vary among the best-studied chimpanzee populations, and there has been some disagreement about the nature of chimpan- zee community structure. Gombe foraging parties are small (fig. 1) relative to those at other sites. At Taı ̈, Boesch (1991) argued that large party sizes were a re- sponse to the threat of predation by leopards. He also argued that the social system of Taı ̈ chimpanzees was more of a bisexually bonded community than other chimpanzee populations (Boesch 1996). Recently, Doran (1997) has disputed both of these claims, using Taı ̈ data from a different period that showed commu- nity structure and party sizes essentially the same as for other well-studied chimpanzee populations. At Gombe, Wrangham (1977) considered individual female home ranges as somewhat independent of the male commu- Fig. 1. Mean number of adult males to adult females in foraging parties for chimpanzees and bonobos. P. paniscus: W, Wamba; L, Lomako. P. troglodytes: G, Gombe; K, Kibale; M, Mahale. nity structure; centrally located females were thought to be core members of the Kasakela community, while more peripheral-ranging females might have member- ship in the neighboring community as well. Goodall (1983) pointed out that females are wary of the males of neighboring communities, suggesting that they cannot travel with impunity between adjacent male kin groups and may suffer severe, even lethal attacks from stranger males when ambushed in territorial overlap zones. Male chimpanzees remain in their natal community and join groups of males as they approach maturity (Goodall 1986). These male groups tend to be highly re- lated (Morin et al. 1993). They patrol territorial borders and attack all extracommunity individuals encountered except for reproductively fertile females, whom they at- tempt to recruit into their own community (Goodall 1986, Nishida 1990). Females, meanwhile, transfer be- tween and may even belong to multiple communities, being allowed to do so when they possess sexual swell- ings. Recent data from Taı ̈ National Park show that fe- males may reside in one community but mate and con- ceive offspring with males from another community (Gagneux, Woodruff, and Boesch 1997). This suggests that chimpanzee mating systems and social systems are not necessarily the same. Bonobos For many years bonobos occupied a dimly understood place in the biology of the great apes because of the lack of captive or field studies, and they were necessarily ig- nored in reviews of great-ape behavior. Even a recent re- view of great-ape reproductive behavior (Nadler 1995) omitted bonobos, referring repeatedly to the gorilla, orangutan, and chimpanzee as ‘‘the three great ape spe- stanford Chimpanzees and Bonobos 401 402 current anthropology Volume 39, Number 4, August–October 1998 cies.`` Bonobos were known as pygmy chimpanzees in earlier descriptions, and morphological studies that compared them with chimpanzees were undertaken de- cades before any behavioral information became avail- able. In the 1970s these morphological studies began to focus on shared traits of the bonobo, chimpanzee, and humans. Zihlmann and Cramer (1978) described mor- phological distinctions between bonobos and chimpan- zees, and Zihlmann et al. (1978) put forward the bonobo as the best model for earliest hominid functional mor- phology and behavior because of evidence of retention of early-hominid-like traits. Zihlmann`s bonobo model was met with skepticism on the grounds that the bo- nobo might be an ecologically and morphologically di- vergent chimpanzee rather than having traits homolo- gous with earliest hominids (Latimer et al. 1981, Johnson 1981). Bonobo behavior is well-known only from two field sites in central Congo, Lomako (Badrian and Badrian 1984; White 1988, 1996a, b; Hohmann and Fruth 1993, 1994) and Wamba (Kano 1983, 1992). In the early 1970s, researchers conducted surveys of bonobo populations in Congo that led to the establishment of these two sites. These sites continue to produce the bulk of field data, with Wamba, under the direction of Takayoshi Kano, producing especially detailed observations of sociosex- ual behavior and intracommunity social dynamics (Kano 1983, 1992; Furuichi 1987, 1989; Idani 1991; Hashimoto and Furuichi 1994). Lomako has produced detailed studies of social behavior and behavioral ecol- ogy, with an emphasis on the latter (Thompson-Han- dler, Malenky, and Badrian 1984; White 1988; White and Burgman 1990; Malenky and Stiles 1991). The dif- ference in research foci may be due to differences in ha- bituation. At Wamba, bonobos have long been habitu- ated to observation in an artificial sugarcane plantation, while at Lomako no provisioning was used and the ani- mals were less observable for many years. Less inten- sively worked bonobo research sites have been estab- lished at Yalosidi (Kano 1983) and Lilungu (Sabater Pi et al. 1993). Like chimpanzees, bonobos eat mainly ripe fruit, sup- plemented with herbaceous terrestrial plants. Wran- gham (1986) has suggested that bonobos, lacking food competition with gorillas, have adopted a diet high in widely available pithy foods to mitigate the risk of fruit patchiness. Bonobos live in fission-fusion polygynous societies, the territories of which are defended by strongly male-philopatric kin groups. Bonobo foraging parties form for the apparent purpose of providing fe- males with optimal fruit-foraging opportunities (Kano 1992). Bonobo foraging parties tend to be substantially larger than those of chimpanzees, though there is con- siderable size overlap between the species among differ- ent study sites. The smaller party sizes seen in chim- panzees appear to be related at least partially to differences in food-patch size and distribution and to bonobo use of herbaceous groundcover plant foods in addition to fruit. Whether the food-patch sizes that allow bonobo parties to be larger is a spatial difference (Malenky and Stiles 1991) or a temporal one (Chapman, White, and Wrangham 1994) is unclear. Female bonobos emigrate from their natal commu- nity at or near sexual maturity and establish themselves in neighboring-community ranges (Idani 1991). Furui- chi (1989) found that immigrant female bonobos estab- lished bonds with one female at a time and slowly be- come central in their new communities. He found that the oldest females tended to be the highest-ranking. Idani (1991) and Kano (1992) reported that most trans- ferring females were nulliparous and that they estab- lished bonds with females that were unrelated to them. Female-female bonds are thus based on patterns of af- filiation but not necessarily on kinship. Male bonobos are strongly philopatric, but this philo- patry is not accompanied by territorial aggression as consistently intense as in chimpanzees. Both hostile and peaceful intercommunity encounters are seen, and copulation between females and extracommunity males has been reported (Kano 1992). It would be wrong, however, to characterize bonobo communities as coex- isting peaceably, since half of encounters do involve aggression of some sort (Kano 1992). Chimpanzees were observed for more than 15 years and thousands of observer-hours, including many intercommunity encounters, before lethal aggression was seen. We should therefore not assume that lethal or injurious intercommunity aggression never occurs among bo- nobos. Lethal aggression during chimpanzee inter- community encounters has been reported from Gombe (Goodall 1986), Mahale (Nishida 1990), and Kibale (Wrangham, personal communication). The all-male pa- trols that characterize chimpanzees are rarer among bo- nobos (Kano 1992), though as bonobo party sizes in- crease the percentage of the party that is male also increases (White 1988). Bonobo-Chimpanzee Comparisons A number of stark differences in social behavior be- tween bonobos and chimpanzees have been reported. I examine the evidence for these differences below. reproductive ecology Female-female relationships and sexual behavior are perhaps the two most-discussed differences between the societies of bonobos and chimpanzees. Adult and adolescent females of the genus Pan are characterized by the vivid advertisement of their sexual receptivity with perineal swellings for a portion of their menstrual cycles. Ovulation in both species occurs at the end of the period of maximal swelling, just before the swelling begins to detumesce (Wallis 1992). Male chimpanzees are attracted to females with swellings during the entire duration of their maximal swelling, though there is great individual and age-related variation among fe- males in the level of interest they receive from males (Goodall 1986). When female chimpanzees are swollen they become more sociable, attract males, and join mixed-sex parties that may comprise a majority of the community. These aggregations often stay together for days, the males dispersing once the female detumesces (Goodall 1986). Swellings are thus an important influ- ence on chimpanzee grouping patterns. While the length of the cycle varies among different populations of the two species (table 1), chimpanzees and bonobos are similar in the percentage of the men- strual cycle during which maximal swelling occurs. Wild female bonobos experience a period of about 14 days of a 42-day cycle during which their anogenital re- gion is tumescent, pink, and highly visible to others in the community (Furuichi 1987). Longer swelling dura- tions (up to 23 days) have been reported from captivity (Dahl 1986, Dahl, Nadler, and Collins 1991). Female chimpanzees are maximally swollen for slightly fewer days (mean 􏰂 13 at Gombe) of a shorter (36-day) men- strual cycle (Wallis 1997). At Gombe, although births are aseasonal (Goodall 1986), swelling cycles are sea- sonal, peaking in the dry season and influencing party aggregations (Wallis 1995). Whether olfactory cues ac- company the visual stimulus of the swelling is un- known but strongly suspected. Female chimpanzees and bonobos also swell while they are pregnant or lac- tating, though these anovulatory swellings may be less regular in frequency and duration (Wallis 1992). Males are nevertheless attracted to nonovulating swollen fe- males. Swellings during pregnancy and in adolescent fe- males thus play a role in nonreproductive sex. bonobo sexuality It has become a fundamental premise of bonobo sexual- ity and of the bonobo`s link to human behavior that among primates only bonobo and human females are sexually active outside the periovulatory period. In the wild, nearly all female chimpanzee sexual behavior is observed during maximal or near-maximal swelling table 1 Anogenital Swelling Durations in Chimpanzees and Bonobos (Goodall 1986). This has been considered in sharp con- trast to the behavior of bonobo females, which are said to have been released from the constraints of reproduc- tive sex and remain sexually receptive throughout the menstrual cycle (Nadler et al. 1991; Thompson- Handler, Malenky, and Badrian 1984). This nonconcep- tive sex has been considered an evolved mode of social communication (de Waal 1987, Wrangham 1993, Parish 1994). The most detailed study of sexual receptivity in wild female bonobos, however, does not support this claim. Furuichi (1987) found that although female bo- nobos at Wamba do copulate when not maximally swol- len, more than 95% of matings were observed during periods of maximal or near-maximal swelling. This is approximately the same as for chimpanzees (97% [Goo- dall 1986]). Female bonobos are thus somewhat more flexible than other apes in the timing of sexual receptiv- ity, but they are not released from the constraints of sexual swelling cycles. Female bonobos are frequently portrayed as hyper- sexual, but mating frequencies in the wild are actually quite comparable for the two species of Pan. De Waal (1987) reported that bonobos in the San Diego Zoo copu- lated five times as frequently as chimpanzees, although he acknowledged that this rate might be an artifact of captive confinement. Comparisons between a wild and a captive population may not reflect naturalistic mating patterns. Swollen female chimpanzees copulate with multiple males during the early stages of their swelling cycle. As many as 50 copulation bouts with eight males in a day have been recorded, and swollen females have copulated with up to eight adult males in several min- utes (Goodall 1986). At Wamba, bonobo females ac- tively solicit sex from a range of males and may copu- late multiple times per hour while swollen (Kano 1992). However, female chimpanzees at Mahale have higher reported mean copulation rates than Wamba bonobos (Takahata, Ihobe, and Idani 1996). Moreover, male cop- ulation rates in the wild are higher among chimpanzees than they are among bonobos. Mahale male chimpan- zees copulated at a higher mean rate than male bonobos at Wamba (P. troglodytes 0.20–0.29/hr., P. paniscus 0.10–0.21/hr.). These rates are for adults, but adoles- cent male chimpanzees also have higher copulation rates than adolescent male bonobos. Bonobo mating rates are therefore not higher than those among chim- panzees if the swelling duration and not the entire in- terbirth period is used as the time frame. If the entire interbirth period is included, then bonobos do show higher copulation rates, because the period of maximal swelling occupies a slightly larger percentage of the menstrual cycle in bonobos than it does in chimpanzees (Kano 1996). Female bonobos do not mate more frequently or sig- nificantly less cyclically than chimpanzees, but there are some fundamental differences between the two spe- cies. One difference relates to the presence of swollen females in foraging parties. At Gombe, at least one es- trous female chimpanzee is present in the community about half of all days (Tutin 1979). However, only a mi- Mean Maximal Swelling Duration (days) 13 (36.1% of 36- day cycle) 12.5 (39.7% of 31.5-day cycle) 14.6 (34.8% of 42- day cycle) 12.9 (39.3% of 32.8-day cycle) 23.5 (47.9% of 49- day cycle) Species/Site P. troglodytes/Gombe P. troglodytes/Mahale P. paniscus/Wamba P. paniscus/Wamba P. paniscus/captive Source Wallis (1997) Hasegawa and Hirai-Hasegawa (1983) Furuichi (1987) Kano (1996) Dahl (1986) stanford Chimpanzees and Bonobos 403 404 current anthropology Volume 39, Number 4, August–October 1998 Fig. 2. Percentage of foraging parties containing one or more estrous females, Wamba (P. paniscus [Kano 1992]), Gombe (P. troglodytes [unpublished data]), Mahale (P. troglodytes [Nishida 1968]). nority of mixed-sex foraging parties contain a swollen female (fig. 2; 20–30% for Gombe and Mahale). More- over, the data presented for Gombe are somewhat in- flated because the sample of parties used is drawn from hunting parties, which tend to be substantially larger than foraging parties on the whole (Stanford, unpub- lished). By contrast, nearly every mixed-sex party of bo- nobos has at least one sexually swollen female (98%, Wamba [Kano 1992]). This is because of the larger mean size of bonobo parties and the greater number of days on which each female is maximally swollen. The im- portance of this difference is that access to reproduc- tively active females is much greater for bonobo males than for chimpanzee males, perhaps accounting for the lower levels of intermale aggression that are reported for bonobos. bonobo female dominance Another reported difference between bonobos and chimpanzees involves the web of relationships among adult females and males. Chimpanzee society is male- dominated; adult males are typically dominant over all adult females, and adolescent males rise in rank by do- minating each adult female before reaching the bottom of the male hierarchy (Goodall 1986, Nishida 1990). The overall level of affiliative behaviors such as grooming or support in conflicts is lower among chimpanzees than among bonobos, even among females whose infants may form play groups in their presence (Goodall 1986). It may be that female chimpanzees are affiliative with each other but that their bonds are less apparent than female-male and male-male ones. Eventually, female chimpanzees establish themselves as members of a new community, though many spend the majority of their time alone. Dominance relationships among female chimpanzees are often not obvious to an observer and may be nonlinear (Goodall 1986). Female bonobos also transfer to new communities at adolescence and also may receive aggression from the females in the new community (Idani, cited in Furuichi 1989). Bonobo females are by contrast often dominant to males (Kano 1992) and form close relationships with males and especially with other females (Furuichi 1989, Idani 1991). The power base, which in chimpanzee soci- ety rests solidly with adult males, is therefore more female-centered in bonobos, perhaps because of greater bonobo female sociality that enables female coalitions to form and to dominate males (Furuichi 1989; Parish 1994, 1996). However, Wood and White (1996) have shown that at Lomako females are dominant only in the arena of feeding priority. Most sex-for-food exchanges among bonobos at Lomako occur just before males allow females priority of access to feeding patches, and this may be part of a larger pattern of strategic male def- erence in this species. If social dominance is considered separately from priority of feeding access, the pattern of dominance in bonobos more strongly resembles that of chimpanzees. Genital-genital rubbing is a nonreproduc- tive sexual behavior of female bonobos and is a further example of affiliation among females. The primary goal of genital rubbing appears to be easing intracommunity tensions, particularly when competition over food threatens to disrupt social harmony (White 1988). Such affiliative encounters between females occur frequently in captivity as well (de Waal 1987; Parish 1994, 1996). It is probably not true that male bonobos are not af- filiative with each other; rather, their bonds may be less apparent and perhaps less strong than female-female and female-male bonds tend to be. Male bonobos engage in territorial defense (Kano 1992), and bonobo society is strongly male-philopatric. The emphasis on female sex- uality and female power is the result of studies showing that of female-female, male-female, and male-male af- filiation the last is least frequent. meat eating Meat eating by chimpanzees is well documented (Teleki 1973; Takahata, Hasegawa, and Nishida 1984; Uehara et al. 1992; Boesch and Boesch 1989; Stanford et al. 1994a; Stanford 1998) and is a systematic aspect of chimpanzee behavior across their geographic range. Chimpanzees incorporate the meat of hunted mammals in their diet, and at some sites the biomass of the meat captured by a community may approach 1,000 kg per year (Stanford 1996). Red colobus monkeys (Colobus badius) are the main prey item at Gombe, Mahale, Ki- bale, and Taı ̈. The impact that chimpanzees have on red colobus populations at these and other sites may be an important population regulator (Stanford 1995) as well as an influence on the structure of the red colobus social system (Stanford 1998). Captured meat is often shared among the hunting party and may be shared nepotisti- cally and strategically. Most kills (approximately 92% at Gombe [Stanford et al. 1994a]) are made by males. Chimpanzees probably hunt for both nutritional and political reasons in that alliances are cemented by the giving of meat. They also appear to obtain meat for so- ciosexual benefits in that males sometimes offer meat to females and receive matings in the process (Nishida et al. 1992, Stanford et al. 1994b). Hunting at Gombe, Mahale, and Taı ̈ is seasonal, and at Gombe this season- ality corresponds with periods of the availability of swollen females that are a robust predictor of party size (Stanford et al. 1994b). At both Gombe and Taı ̈, hunting success increases with increasing party size. Among bonobos, meat eating is rare. Indeed, forest monkeys (Cercopithecus ssp. and Colobus ssp.), which would be relished prey for chimpanzees at Gombe, Ma- hale, Kibale, and Taı ̈, have been used as playthings rather than as food items by bonobos at Lilungu (Sabater Pi et al. 1993). Wamba bonobos rarely hunt even though their mean party size is larger than that found in any chimpanzee population (Kano 1992). If male chimpan- zees hunt primarily for political and sociosexual rather than nutritional reasons, then one might expect male bonobos to be less interested in hunting, for two rea- sons. First, female bonobos do not need to engage in po- litical or sexual behavior to obtain a share of the meat; they may simply take it away. Hohmann and Fruth (1993) reported instances of females` taking fresh kills away from male captors, which occurs very rarely in chimpanzees. The manipulative use of meat seen in male chimpanzees would not be effective for male bo- nobos, since they typically defer to females in feeding situations. These observations point also to a social rather than nutritional basis for hunting in chimpan- zees. Given that there is a range of potential prey, in- cluding monkeys, at both bonobo study sites, it is hard to understand why bonobos would show little interest in hunting if meat were the prized nutritional resource it has been thought to be in chimpanzees. sexual swellings The sexual swelling is an evolved feature which, be- cause it is visually obvious and connected to reproduc- tion, has received much attention from primatologists. Although Lovejoy (1981) assumed that humans had evolved concealed ovulation as a reproductive adapta- tion by females, it is also possible that the Pan lineage evolved swellings to advertise ovulation from a con- cealed-ovulator ancestor. Although it exerts a major in- fluence on chimpanzee and bonobo society, its function is not clear. Vividly advertised ovulation has arisen sev- eral times in the primate order, including among the an- cestors of some cercopithecines, some colobines, and the Hominoidea (Sille ́ n-Tullberg and Møller 1993). Hrdy (1981) and Harcourt (1981) have argued that sexual swellings may serve to confuse paternity and thereby increase a male`s parental investment. The likelihood of aggression toward that female or toward her infant might also be reduced as a result (Takahata 1985). Since swollen female chimpanzees and bonobos transfer be- tween communities and female chimpanzees that are encountered by stranger males are attacked and even killed if they are not swollen, the sexual swelling ap- pears to allow female chimpanzees mobility between communities. This hypothesis also addresses the object of male- bonded territorial defense in chimpanzees. The number of males in a community is positively correlated with the size of the territory that is defended against other communities (Stanford 1998). This male-bonded behav- ior may be related to either female defense or food de- fense; data on territoriality and female transfer are seen as key in modeling the behavior of the common homi- noid ancestor (Ghiglieri 1987, Wrangham 1987). If fe- male swellings are related to intense male territoriality because they grant the females safe passage between communities, then territoriality may be food-resource- based. This is because, from a male`s perspective, hav- ing as many females as possible in the community should be a reproductive benefit, and so any immigrat- ing female should be welcomed whether currently cy- cling or not. To resident females new immigrants repre- sent both food and mating competition. Swellings may therefore allow female chimpanzees to enter communi- ties against the wishes of previous immigrants because they do so at a time when their swellings make them highly attractive to males. The same may apply to fe- male bonobos, which also face aggression from resident females when immigrating to a new community. Conversely, Hamilton (1984) and Hrdy and Whitten (1987) have hypothesized that by signaling ovulation, genital swellings might increase paternity certainty and therefore paternal investment by males. Clutton-Brock and Harvey (1976) argued that because a swollen, ovu- lating female becomes a focus of excitement and com- petition among males, the swelling`s function is to in- cite male competitive behavior, allowing her to choose the most fit mate or mates. Harcourt (1981) has sug- gested that the swellings may function simply to extend the time period during which females can locate and stay in proximity to potential male mates. Tutin (1979) reported, however, that most conceptions during her study of Gombe chimpanzee reproductive behavior probably occurred during consortships, outside of the polygynous setting for which Clutton-Brock and Har- vey`s ‘‘best-male`` model predicts that swellings evolved. She also pointed out that the ‘‘best-male`` hy- pothesis does not fit chimpanzee society well, since fe- males choose mates on the basis of prior affiliative pat- terns with males who offered grooming and food- sharing opportunities. Reproductive benefits may ac- crue to them only indirectly through benefits to their offspring. More recently, Wallis (1997) has reviewed two decades of Gombe data on conceptions and found that the majority occurred in polygynous settings, not in consortships. Explanations of the nature and origin of chimpanzee and bonobo sexual swellings seem to be based primarily on the premise that of the two species it is the latter that is the derived one. This premise may stem from bo- nobos` more limited geographic range and their elabo- rated sexual behavior. In addition, bonobos may tend to be viewed as the divergent form because they have been stanford Chimpanzees and Bonobos 405 406 current anthropology Volume 39, Number 4, August–October 1998 described and studied more recently. This premise may not, however, be warranted. For instance, implicit in the literature about bonobos is that females have ex- tended the duration of the sexual swelling, the result of which is thought to be increased female cooperation and increased affiliation between males and females. Highly visible swellings are unique traits in this homi- noid clade. However, since the ancestral behavioral pat- terns of chimpanzees/bonobos are unknown, it is also unknown whether swellings in bonobos have been elab- orated and extended in duration through natural selec- tion. Alternatively, female chimpanzees may have shortened swelling cycles as an adaptation to attract multiple mates while minimizing feeding competition. The latter is a reasonable speculation given the typi- cally asocial foraging strategy of female chimpanzees and the number of food competitors in large mixed-sex parties alongside whom the swollen females must forage. Goodall (1983) argued that females in each chimpan- zee community belonged to one community only; they were wary of other community territories and might be ambushed while foraging in a territorial overlap zone. Wrangham (1979) argued that female home ranges were relatively fixed and that they might be part of more than one community over which the males ranged; the idea that males attempt to maximize territory size as a way to encompass more females` ranges for male reproduc- tive advantages follows from this point of view. Data from some study sites tend to support Goodall`s view; Hasegawa (1990) has shown that the concept of females and males occupying separate ranges is untenable for Mahale chimpanzees. Influences on Our View of Great-Ape Societies It is clear that much of the research on these two inten- sively studied apes remains fraught with untested as- sumptions. In the search for behavioral patterns that may be adaptations shared by humans and chimpan- zees, the behavioral traits of chimpanzees and bonobos discussed here—hunting, reproductive behavior, mat- ing systems—have been the most often mentioned. In this final section I consider some underlying influences that may help to account for the ways in which both primatologists and the public understand the societies of these two apes. The primary reason for change in our ideas about great-ape societies over time is the accumulation of new data. Research findings and their interpretations are strongly influenced by the simple accumulation of knowledge about a species until a particular behavioral trend becomes evident or it becomes possible to recon- cile a set of seemingly unrelated behavioral facts using a single paradigm. The paradigm formation may itself be subject to social influences, because research biases lead one to collect some types of data rather than others at different stages of the research history of a topic. However, it is especially the case in field primatology that the logistics of research, the longevity and ex- tended ontogeny of the subjects, and the importance of documenting the life histories of many individuals of all age- and sex-classes contribute substantially to a growth of ideas in the discipline through cumulative normal science. Considering chimpanzees to be entirely promiscuous without any definable social system was a result of the difficulty in documenting rare events: fe- male migrations, intercommunity conflicts, hunting and meat sharing, male dominance upheavals. Chang- ing this view took more than a decade; as data accumu- lated so did the depth of the portrait of chimpanzee soci- ety. The number of observer-hours spent with bonobos in the wild is still comparable to the time spent watch- ing chimpanzees in the 1960s. Comparisons between chimpanzees and bonobos therefore suffer from the scantiness of the data on bonobo behavior, allowing in- terpretations based more on perceived contrasts with chimpanzees than on observational evidence from bo- nobos. While the evidence for chimpanzee behavior may be subject to interpretation, there can be no ques- tioning the fact that hunting is a primarily male activ- ity, that lethal aggression occurs between communities, and that these are characteristic of chimpanzees in the wild across the African continent. The history of prima- tology has been composed of a series of new paradigms to explain accumulating new data. These often involve dichotomies that are shown later to be false; for exam- ple, the male-philopatric versus female-philopatric di- chotomy has been called into question (Moore 1984a, Strier 1994). Such models tend to be predictive for a time, but as new, contradictory data accumulate they become obsolete. The dichotomy currently drawn be- tween the social systems of chimpanzees and bonobos may not accord well with field data. Second, although many of the most detailed studies of intragroup social dynamics among living primates come from captive colonies, captive behavior fre- quently differs from behavior in the wild. In the case of bonobos, early captive studies demonstrated behaviors that became the focus of much public and scientific at- tention (e.g., de Waal 1987). Bonobos engage in a rich array of sociosexual behaviors in the wild, but the fre- quency of the behaviors is much lower than in captiv- ity. Captivity produces heightened frequencies of many behaviors for a variety of reasons, among them release from the need to forage, greater opportunities for social interaction, and enforced proximity and boredom. Orangutans are, for example, highly sociable in captiv- ity relative to their solitary wild counterparts. While there is much to be gained from the captive study of be- haviors that are impossible to record well in the field, biases are introduced into our interpretation of species- typical behavior because of the way in which captivity influences behavior. Third, contextual biases may emerge from the cir- cumstances in which the research is done. They repre- sent the situating of ideas and interpretations of evi- dence in terms of the perspective the researcher brings to the research. These influences are pervasive (though whether they fundamentally change the doing of sci- ence is open to debate). The behaviors at the heart of the chimpanzee-bonobo interspecific variation — sexu- ality, power and dominance, aggression—are those that also lie at the center of the debate about human gender issues and what molds our own behavior. Ortner`s (1974) argument that men are to women as culture is to nature may be reflected in a more recent version that is evident in popular-scientific portrayals of these two apes: chimpanzees are to bonobos as men are to women. A recent account of the evolutionary continuity of hu- man violence from an ape ancestor makes a strong case for chimpanzees as the referent models for the behavior of modern human males (Wrangham and Peterson 1996). Murder, sexual coercion, hierarchy, and striving for status are all traits that Wrangham and Peterson as- cribe to chimpanzees as well as to human males. De Waal`s recent (1997) account of bonobos ascribes to them characteristics often used to describe women: nonaggressive, sensual, power-sharing, strong through alliances rather than individually. He writes, ‘‘The chimpanzee resolves sexual issues with power; the bo- nobo resolves power issues with sex`` (p. 32). While these characterizations are based on observational data, they may also be influenced by views of the two apes that accord with human male and female gender stereo- types. These stereotypes are influenced by the public`s desire for explanations of the roots of human behavior. This issue has risen to enough prominence that the newsletter of the American Anthropological Associa- tion recently devoted a series of columns to the treat- ment of biological anthropology by the media. The por- trayal of humans as at an evolutionary crossroads, able to choose the bonobo`s sensuous ‘‘Make love, not war`` nature or the chimpanzees warring, status-striving na- ture, finds an eager audience. Whether current images of chimpanzee and bonobo societies and the dichotomy between them accord well with field data remains for future fieldwork to determine. Comments frans b. m. de waal Living Links Center and Psychology Department, Emory University, Atlanta, Ga. 30329, U.S.A. (). 24 iii 98 In the same way that paleontologists prefer their fossil finds to belong to a human ancestor rather than to an extinct side-branch, experts on ape behavior like to claim that their subjects are the only or best model of the last common ancestor, Chimpanzee researchers are used to this situation, routinely describing the chim- panzee as humanity`s closest living relative. There ex- ists another relative, however, that is equally alive, equally close, and equally relevant to an understanding of human evolution. While this has delighted some, it obviously disturbs others. Two strategies have emerged to keep bonobos at a dis- tance so as to preserve chimpanzee-based scenarios of human evolution, which traditionally emphasize war- fare, hunting, tool use, and male dominance. The first strategy is to describe the bonobo as an interesting but specialized anomaly that can be safely ignored as a pos- sible model of the last common ancestor (see Wrang- ham and Peterson 1996). The second strategy, adopted by Stanford, is to minimize the differences between the two Pan species: if bonobos behave, by and large, like chimpanzees, there is no reason to question the latter species`s prominence as a model. Let me review the fac- tual basis of some of Stanford`s similarity claims: Rate of sexual behavior. In arguing that the rates of sexual behavior in bonobos and chimpanzees are simi- lar, Stanford counts only copulations between males and females. This is indeed the bulk of sexual activity in the chimpanzee, but the bonobo has sex in virtually all possible partner combinations: male-female sex is not even the most common pattern. Genital rubbing among females, the most typical pattern of the species, is absent in chimpanzees. This pattern is conveniently ignored in the calculations. Including all sexual encoun- ters, I have reported a much higher interaction rate among bonobos than chimpanzees (de Waal 1995 and fig. 1). Effects of captivity. The argument that the high rate of sociosexual activity in captive bonobos might be at- tributable to confinement fails to consider that the above comparison is not with wild chimpanzees but with captive ones. Why are chimpanzees not similarly affected by confinement? Only captive studies control for environmental conditions and thereby provide con- clusive data on interspecific differences; field studies usually concern different species under different ecolog- ical conditions. Peacefulness. Given our current knowledge, it is en- tirely correct to describe bonobos as relatively peaceful. In captivity, bonobos show less violence and consider- ably higher rates of reconciliation following fights than chimpanzees. Bonobos are by no means lacking in ag- gression, however, and despite their reputation they are actually less tolerant in relation to food than chimpan- zees (de Waal 1992). In the field, lethal intercommunity aggression, forced copulation, and infanticide have never been observed. Stanford is not the first, however, to warn that such behavior may yet be discovered (de Waal 1989:221). Female dominance. If a male chimpanzee chases a fe- male away from his food, we generally attribute this to his dominance. This rule has been followed by etholo- gists for every species on the planet, but now we learn that female bonobos ‘‘are dominant only in the arena of feeding priority.`` It is claimed that if we look beyond feeding priority the relations between the sexes in bo- nobos and chimpanzees are more similar. This is based on a study by Wood and White (1996) that failed to iden- stanford Chimpanzees and Bonobos 407 408 current anthropology Volume 39, Number 4, August–October 1998 Fig. 1. Mean (􏰁 SEM) number of sociosexual acts initiated per hour per individual for the San Diego bonobos and an outdoor colony of chimpanzees at the Yerkes Primate Center`s field station, separately for adults and adolescents (black) and younger individuals (hatched). The adult rate was significantly higher in the bonobo group despite a reduced number of available partners compared with the chimpanzee group. The juvenile rates of the two species did not differ. From de Waal (1995). tify male and female bonobos individually; possibly some males in this community were able to chase some females and did so frequently, but a pooled analysis would show the class of males dominating the class of females. Other investigators of the same community did achieve individual recognition and claim obvious fe- male dominance (Fruth and Hohmann, cited in de Waal 1997:79 – 80). Similarly, at another bonobo field site Furuichi (1997) noted that the alpha female could chase high-ranking males and that the alpha male sometimes retreated for low-ranking females. Furthermore, in all captive groups that I know female bonobos dominate males — an enormous contrast with chimpanzees (e.g., Parish 1994). Finally, when Stanford speculates about the sociocul- tural context of the current fascination with bonobos, it would be good to include an analysis of why it has taken so long for scientists to discuss the matrifocal na- ture of bonobo society and the species`s rich sexuality. It is no accident that the first time Frans Lanting and I worked together on an illustrated account of bonobo society we did so for GEO Magazine; U.S. publishers panicked at the thought of a full story. Rather than con- cluding that the bonobo seems a species made for the media, the question is really what has hampered its unique social features from surfacing and why attempts are still being made to push it to the sidelines. Anyone interested in the reconstruction of our evolutionary past will need to face the implications of having a sexy, female-centered close relative. barbara fruth Max-Planck-Institut fu ̈ r Verhaltensphysiologie, 82319 Seewiesen, Germany. 20 iii 98 Stanford is to be congratulated for his attempt to focus on the similarities rather than on the differences be- tween chimpanzees and bonobos. I agree that the em- phasis so far put on the behavioral dichotomy between the two Pan species may be due to the relatively briefer study of bonobos from only two major sites compared with the longer-term investigations of many study sites of chimpanzees. Nevertheless, the combined data set of only these few studies, mostly at Wamba and Lomako, allows Stanford to place the bonobo within the behav- ioral range of chimpanzees. He excuses the remaining behavioral gaps between the Pan species by a lack of published data from the wild and ends his comparison with an appeal for future field research to seek data that will eventually allow systematic treatment of the ap- parent dichotomy. However, published literature is al- ready available that would allow some of the gaps to be filled and would make his point even stronger. Discussing the differences in meat eating between the two Pan species, Stanford argues chimpocentrically, as if blinded by the quantity and quality of Pan troglo- dytes`s favourite prey, red colobus monkeys. This is al- most as if a Texan consuming a T-bone steak daily were to consider a Bavarian savoring a pork roast on Sundays a vegetarian because it was pork instead of beef and be- cause it was consumed only occasionally. At Lomako, Pan paniscus regularly kills and eats adult duikers, Cephalophus spp. (Hohmann and Fruth 1993, 1996). Admittedly, the amount of animal prey killed and eaten by bonobos at Lomako is not comparable to that re- corded for chimpanz
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