Journal of Comparative Psychology Copyright 1997 by the American Psychological Association, Inc. 1997, Vol. 111, No. 2, 159-173 0735-7036/97/$3.00 Development of Stone Tool Use by Wild Chimpanzees (Pan troglodytes) Noriko Inoue-Nakamura Tetsuro Matsuzawa Kwansei Gakuin University Kyoto University At the age of 3.5 years, wild chimpanzees at Bossou, Guinea, begin to use hammer and anvil stones to crack oil-palm nuts to get the kernels. To clarify the developmental processes, the authors did a field experiment in which stones and oil-palm nuts were provided. Infant chimpanzees' stone-nut manipulation was observed and video recorded. Data were collected from 3 infants younger than 4 years old from 1992 to 1995. The authors analyzed 692 episodes of infants' stone-nut manipulation and 150 episodes of infants' observation of nut cracking performed by adults. Infants observed other chimpanzees' nut cracking and got the kernels from them. The stone-nut manipulation developed from a single action on a single object to multiple actions on multiple objects. Although infant chimpanzees at the age of 2.5 years already acquired basic actions necessary for nut cracking, they did not combine the actions in an appropriate sequence to perform actual nut cracking. The acquisition of tool-use behavior is a sort of problem- tool-using skills indicated the cognitive capacity of the solving task and inevitably reflects cognitive development. children and the emergence of strategies to solve a pm-tic- Connolly and Dalgleish (1989) examined the developmental ular problem. Thus, it is important to investigate tool-using change of a spoon-using skill by human children younger skills for understanding the cognitive development of hu- than 2 years old. They suggested that the acquisition of man children. In an evolutionary context, it is also important to study tool-use behaviors in chimpanzees (Pan troglodytes), the Noriko Inoue-Nakamura, Department of Psychology, Kwansei species genetically closest to humans. So far there have Gakuin University, Nishinomiya, Japan; Tetsuro Matsuzawa, De- been a number of observations about the various kinds of partment of Behavioral and Brain Sciences, Primate Research Institute, Kyoto University, Inuyama, Japan. tool use by chimpanzees in the wild (e.g., Goodall, 1970, The present research was financed by Grant 01041058 from the 1986; McGrew, 1992; Nishida & Hiraiwa, 1982). However, International Research Program of the Ministry of Education, as far as we know, there have been few studies in the wild Science, Sports, and Culture, Japan. Preparation of the article was on the acquisition processes of tool-using skills (Boesch, supported by the Fellowship of the Japan Society for the Promo- 1991a; Matsuzawa, 1994). Although there were a couple of tion of Science for Japanese Junior Scientists (No. 0801). The field studies in groups of captive chimpanzees (Hannah & study was carried out with the collaboration of the Direction McGrew, 1987; Sumita, Kitahara-Frisch, & Norikoshi, Nationale de la Recherche Scientifique et Technique, R6publique 1985), no previous studies in the wild have reported the de Guin6e, and the villagers at Bossou. We wish to thank Yukimaru Sugiyama, who has conducted field quantitative analysis of behavioral change from an ontoge- research at Bossou since 1976 and who gave us the opportunity to netic perspective through direct observation and video do this research. Special thanks are due to the following colleagues recording. during the research periods: Rikako Tonooka, Gen Yamakoshi, Among the tool-use behaviors in the wild, nut-cracking Jeremy Koman, Guano Goumy, and Tino Camara. A part of the behavior is of special interest because it is the most "com- video-recorded data (February-March 1994) was collected by R. plicated" tool use chimpanzees perform. It basically con- Tonooka. We also express our thanks to members of the Japanese sists of the following actions: (a) picking up a nut, (b) Embassy in the R6publique de Guin6e and C6te d'Ivoire and the putting it on an anvil stone, (c) holding a hammer stone, (d) Japan International Cooperation Agency of C6te d'Ivoire. The preparation of the manuscript was done during the period when hitting the nut on the anvil stone with the hammer stone, and Noriko Inoue-Nakamura stayed at the Department of Ecology and (e) picking up the kernel in the cracked hard shell and eating Evolutionary Biology, Princeton University. We wish to express it. This behavior is characterized by the collaboration of our thanks for the helpful comments on an earlier draft made by bimanual and asymmetric manipulation. The chimpanzees Alison Jolly. Noriko Inoue-Nakamura is grateful to Hiroshi Imada at Bossou, Guinea, use a pair of stones as a hammer and an for his generous guidance throughout the present study and also anvil to crack open oil-palm nuts to get the kernels (Sug- thanks Katsuki Nakamura for invaluable suggestions on data anal- iyama & Koman, 1979b). This tool-use behavior has been yses and constant encouragement. Correspondence concerning this article should be addressed to reported only among chimpanzees of a limited area of West Noriko Inoue-Nakamura, who is now at the Department of Behav- Africa (Boesch, Marchesi, Fruth, & Joulian, 1994; Kort- ioral and Brain Sciences, Primate Research Institute, Kyoto Uni- landt, 1986; McGrew, 1992; Sugiyama, 1993). No chim- versity, Inuyama, Aichi, 484 Japan. Electronic mail may be sent panzees in East Africa have been observed to crack nuts via Internet to [email protected]. with stones although stones and nuts are available (Goodall, 159 160 1NOUE-NAKAMURA AND MATSUZAWA 1986; Nishida, 1990). Nut cracking with stones is a strong folding for drinking water (Tonooka, Inoue, & Matsuzawa, 1995), example of the diversity of material culture among and pestle pounding for extracting sap from oil-palm trees (Sug- chimpanzees. iyama, 1994b; Yamakoshi & Sugiyama, 1995). In the field, however, it is difficult to directly observe nut This study focused on 3 infant chimpanzees younger than 4 years old in January 1995: Yolo, Fotayu, and Vuavua. The subjects cracking of chimpanzees because the bush beneath the palm were observed for 4 years. The longitudinal data of the 3 infant trees is thick, and also because the chimpanzees are too chimpanzees were divided into four age groups: 0.5 (0-1 year timid to be observed when they are in the secondary forest old), 1.5 (1-2 years old), 2.5 (2-3 years old), and 3.5 (3-4 years near the village where palm trees are. Therefore, field old). Table 1 shows the information about each age"group. Chim- experiments have been carded out at Bossou since 1987 to panzees younger than 4 years old are usually classified as "in- study chimpanzees' tool-use behavior in detail. An outdoor fants." Weaning occurs around age 2.5 to 3 years. The interbirth laboratory was established in the central part of chimpan- interval is about 4 to 6 years. Length of life is estimated as 40 to zees' free-ranging area where stones and oil-palm nuts were 50 years (Goodall, 1986). provided by the experimenters (Fushimi, Sakura, Matsu- zawa, Ono, & Sugiyama, 1991; Matsuzawa, 1991, 1994; Procedure Sakura & Matsuzawa, 1991; Sugiyama, Fushimi, Sakura, & Matsuzawa, 1993). Observational periods. The present study reports the field The present study aims to investigate the developmental experiments and the behavioral observation done during four dry process of the tool-use skill from the point of ontogenetic seasons: (a) January 1992 (18 days), (b) December 1992 to January view. The field experiments have revealed that chimpanzees 1993 (20 days), (c) January 1994 (5 days) and February to March at Bossou younger than 3.5 years old cannot perform nut 1994 (30 days), and (d) January 1995 (21 days). Table 1 also provides general information about the records of observation: the cracking (Inoue, Tonooka, & Matsuzawa, 1996; Matsu- days we observed, the total time of observation, and the number of zawa, 1994). The purpose of the present study was to parties that visited the outdoor laboratory. answer the following questions by means of behavioral Setting. In general, about 50 stones and approximately 2-5 kg observations and an analysis of video-recorded data in the of oil-palm nuts (Elaeis guineensis) were provided at an outdoor outdoor laboratory: How do infant chimpanzees younger laboratory sized 10 m wide by 5 m deep. The observer hid behind than 3.5 years old acquire nut-cracking skill? What kind of a screen made of grass about 4 m long and 2 m high. The distance social influences are involved in the acquisition process? between the observer and the cracking site was about 20 m. The observer stayed there continuously from 7 a.m. till 6 p.m. All behaviors were directly observed and videotaped (Sony, CCD- Method TR3) for further analysis. Figure 1 shows a representative scene of nut cracking by chimpanzees in the outdoor laboratory. Subjects The subjects of this study were wild chimpanzees (Pan trogl- Data Analysis odytes) at Bossou, on the southeastern edge of the Republic of Guinea, West Africa. There was a group of 18-19 chimpanzees at Definition of behavioral episodes and observational episodes. Bossou during the present study of 1992-1995. Focal animal sampling was performed on all of the video-recorded The chimpanzees at Bossou have been investigated without data of the 4 years (4,674 min total). The target behaviors were (a) supplemental feeding since 1976 by Sugiyama and his colleagues manipulation of stones and/or nuts and (b) observation of the other (Sugiyama, 1984, 1988, 1994a; Sugiyama & Koman, 1979a, chimpanzees that performed nut cracking. 1987). Each member of this group has been identified. Chimpan- We defined a behavioral episode as infants' manipulating stones zees at Bossou show various tool-use behaviors, such as nut and/or nuts by themselves.