Recent Developments in the Study of Wild Chimpanzee Behavior

Evolutionary Anthropology 9 ARTICLES Recent Developments in the Study of Wild Chimpanzee Behavior JOHN C. MITANI, DAVID P. WATTS, AND MARTIN N. MULLER Chimpanzees have always been of special interest to anthropologists. As our organization, genetics and behavior, closest living relatives,1–3 they provide the standard against which to assess hunting and meat-eating, inter-group human uniqueness and information regarding the changes that must have oc- relationships, and behavioral endocri- curred during the course of human evolution. Given these circumstances, it is not nology. Our treatment is selective, and surprising that chimpanzees have been studied intensively in the wild. Jane Good- we explicitly avoid comment on inter- all4,5 initiated the first long-term field study of chimpanzee behavior at the Gombe population variation in behavior as it National Park, Tanzania. Her observations of tool manufacture and use, hunting, relates to the question of chimpanzee and meat-eating forever changed the way we define humans. Field research on cultures. Excellent reviews of this chimpanzee behavior by Toshisada Nishida and colleagues6 at the nearby Mahale topic, of central concern to anthropol- Mountains National Park has had an equally significant impact. It was Nishida7,8 ogists, can be found elsewhere.12–14 who first provided a comprehensive picture of the chimpanzee social system, including group structure and dispersal. SOCIAL ORGANIZATION No single issue in the study of wild Two generations of researchers erything about the behavior of these chimpanzee behavior has seen more have followed Goodall and Nishida apes in nature. But this is not the case. debate than the nature of their social into the field. As a result, chimpanzees In fact, we are entering a new and system. We now know that chimpan- are now one of the best and most extremely exciting era in the study of zees live in a “fission-fusion” society. widely studied of nonhuman pri- wild chimpanzee behavior. The pur- Individuals form socially and geo- mates. Long-term field research has pose of this review is to highlight graphically circumscribed “unit- been conducted at six sites by several some intriguing findings that have groups” or “communities,” within researchers spanning 42 years (Fig. 1). emerged through recent study. We fo- which they associate in temporary Shorter field studies have also been cus specifically on results from our subgroups or “parties” that vary in carried out in some areas.9–11 With own field research conducted in the size, composition, and duration. this extensive body of research, one Kibale National Park, Uganda, giving Males are philopatric, whereas fe- might think that we have learned ev- special emphasis to five areas: social males typically disperse. This seem- ingly clear-cut picture of chimpanzee society did not emerge easily. Given John C. Mitani is in the Department of Anthropology at the University of Michigan. His research involves studies of primate behavioral ecology and vocal communication. He has the fluid nature of chimpanzee soci- conducted fieldwork on all five species of living apes, including gibbons and orangutans in ety, it took exceedingly long for field Indonesia, gorillas in Rwanda, bonobos in the Democratic Republic of Congo, and chimpan- observers to discern regularities in zees in Tanzania and Uganda. Along with David Watts, he currently co-directs the Ngogo chimpanzee project, Kibale National Park, Uganda. grouping, dispersal, associations, and range use. David Watts received his BA from the University of Pennsylvania and his MA and PhD in 15 Anthropology from the University of Chicago. He is currently a Professor of Anthropology at Kortlandt was the first to report Yale. He has done fieldwork on white-faced capuchin monkeys in Panama and, for many temporary associations among wild years, on the social behavior and ecology of mountain gorillas in Rwanda. He also served as chimpanzees based on early observa- Director of the Karisoke Research Centre in Rwanda for two years. Since 1993, he has been conducting research on the behavior and ecology of chimpanzees at Ngogo, in Kibale tions in the Belgian Congo. Here he National Park, Uganda, in collaboration with Dr. John Mitani and Dr. Jeremiah Lwanga. described groups of 1–30 individuals Martin N. Muller has studied chimpanzees in Tanzania and Uganda. He recently completed that either contained members of his dissertation, “Endocrine Aspects of Aggression and Dominance in Chimpanzees of the both sexes or consisted of “nursery” Kibale Forest,” for the Department of Anthropology, University of Southern California. He is currently a postdoctoral fellow at Harvard University, where he continues his research on the groups of females and their young. behavior of wild chimpanzees. Kortlandt was prescient in his de- scriptions of the temporary and fluid nature of chimpanzee aggregations Key words: chimpanzees, behavioral ecology, Pan troglodytes and laid the groundwork for further Evolutionary Anthropology 11:9–25 (2002) study. Subsequent reports by Good- 10 Evolutionary Anthropology ARTICLES the choice of companions. Males are more gregarious than females and prefer each other’s company, except when females are in oestrus. Females are less sociable and spend most of their time with their own offspring— except when cycling, at which time they become very sociable.” Consideration of sex differences in reproductive strategies and the costs of feeding competition provide a theoretical rationale to explain observed sex differences in chimpanzee association patterns. Females forage alone because the potential reproductive costs of scramble and contest feeding competition are higher for them than they are for males.21,26,27 Males may be more willing to assume the costs of feeding competition to gain mating opportunities and to derive social benefits from associating with other males.21,26,27 Part of the power of this model lies in its ability to accommo- date intraspecific variation in associa- Figure 1. Chimpanzee study sites. tion patterns. The theoretical predic- tion is that such variation will occur between study sites because for fe- all16 and the Reynolds17 tended to blur ties are not closed. While males typi- males the costs of grouping are ex- distinctions between communities. cally spend their entire lives in their pected to vary with differences in local After five years of field observations at natal communities, females com- resource abundance and distribu- Gombe, Gooodall16 wrote: “Since monly transfer to neighboring ones tion.28,29 chimpanzee groups in the reserve during adolescence.8,18,19 In keeping with this model, consid- freely unite from time to time without While effectively laying to rest per- erable intraspecific variation exists in signs of aggression, they cannot be sistent questions regarding the exis- chimpanzee association patterns. As divided into separate communities. It tence of chimpanzee communities, far back as 1979, Sugiyama and seems likely that only a geographic Nishida7 simultaneously described Koman30 reported that females were barrier would constitute a limiting sex differences in association. He as sociable as males in a small, iso- factor on the size of a community, noted that males associated more fre- lated community at Bossou, Guinea. although individuals living at opposite quently with each other than females Ghiglieri31 found no evidence of sex ends of the range might never come did with other females. From this he bias in grouping tendencies in the un- into contact.” concluded that strong bonds form be- provisioned and largely unhabituated Nishida7 altered this picture by de- tween males and that males compose Ngogo community at Kibale. More re- fining the social group of wild chim- the core of chimpanzee society. Sub- cently, Boesch32 and Boesch-Acher- panzees at Mahale. In contrast to sequent field research at Gombe,20,21 mann,33 working in the Taı¨ National Goodall and the Reynolds, Nishida Mahale,22,23 and the Kanyawara study Park, Ivory Coast, observed that most emphasized the stable nature of chim- area of Kibale National Park, Ugan- parties contained adults of both sexes, panzee communities. Although he da,24 validated and expanded Nishi- while parties consisting of only males never recorded all community mem- da’s picture of chimpanzee society. or only females were rare. This led bers together at a single place and Male chimpanzees in these popula- them to conclude that members of time, his longitudinal observations of tions are more gregarious and distrib- this population are “bisexually bond- association between individuals re- ute their activities more widely and ed.” Given a lack of information re- vealed an unambiguous social net- evenly over their territories than do garding feeding behavior and food work and structure (Fig. 2). From females. Goodall25 aptly summarized availability across study sites, we are these observations, Nishida7 con- the standard picture of chimpanzee not presently capable of evaluating cluded that, “The chimpanzees live a society to emerge from these studies: whether sex differences in the costs of clear-cut social unit which consists of “The most deep-seated principles un- competition account for these re- adult males, adult females, and imma- derlying chimpanzee community ported intraspecific differences. Tem- ture animals.” Subsequent field work structure are those concerned with poral variation in association patterns indicated that chimpanzee communi- sex differences in sociability and in at Taı¨ appears

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