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BRIEF REPORT

Incident of Intense Aggression by Chimpanzees Against

an Infant From Another Group in Mahale Mountains

National Park, Tanzania

NOBUYUKI KUTSUKAKE1nand TAKAHISA MATSUSAKA2

1Department of Cognitive and Behavioral Science, Graduate School of Arts and Sciences, University of Tokyo, Tokyo, Japan

2Department of Zoology, Graduate School of Science, Kyoto University, Kyoto, Japan

We document here an unusual case of intense aggression against an infant male from another group by chimpanzees (Pan troglodytes schweinfurthii) of the Mahale Mountains National Park, in Tanzania. Adult males of the study group collectively attacked an unknown male infant. Although an unknown female, probably the mother, tried to retrieve him, the infant was seriously injured and most likely died. During this incident, the unknown female attacked and injured two researchers. After the aggressive encounter, it was found that six of the nine adult males in the study group were wounded. Attacking the extragroup male infant may have the effect of weakening the future power of the neighboring group, leading to better access to resources and enhanced safety in the future for the study group. Am. J. Primatol. 58:175–180, 2002. r2002 Wiley-Liss, Inc.

INTRODUCTION

In wild chimpanzees, intergroup (or community) relationships are antag- onistic, and many cases of intergroup aggressive conflict have been reported [reviewed in Wrangham, 1999]. In some cases, aggressive encounters involve physical contact [e.g., Goodall, 1986; Boesch & Boesch-Achermann, 2000]. In some extreme cases, lethal aggression toward vulnerable individuals (e.g., a mother–infant pair or a male that had become separated from the main party) occurred [reviewed in Wrangham, 1999]. This level of aggression can lead to the extinction of a neighboring group [Goodall, 1986; Nishida et al., 1985].

During our fieldwork on wild chimpanzees in the Mahale Mountains National Park in Tanzania, we observed aggression against an infant from another group by individuals of our study group. In this case, the males of our

Contract grant sponsor: Monbusho Scientific Research Fund; Contract grant number: 12375003; Contract grant sponsor: Japan Society for the Promotion of Science.

nCorrespondence to: Nobuyuki Kutsukake, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan. E-mail: kutsu@darwin.c.u-tokyo.ac.jp

Received 17 May 2002; revision accepted 21 October 2002 DOI: 10.1002/ajp.10058

Published online in Wiley InterScience (www.interscience.wiley.com).

r2002 Wiley-Liss, Inc.

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study group collectively attacked a male infant from a neighboring group, and we strongly suspect that intergroup infanticide occurred. We report the details of this incident in this work.

METHODS

The observations reported here occurred during our ongoing study of M-group chimpanzees in the Mahale Mountains National Park, on the eastern shore of Lake Tanganyika in western Tanzania. A long-term study of the wild chimpanzees in Mahale has been in progress since 1965 [Nishida, 1990]. M-group chimpanzees encountered individuals from a neighboring group in the northern part of their range on 12 December 2000, at the beginning of the rainy season. It is known that there is at least one neighboring group (B-group) to the north of M- group [Nishida, 1990]. In addition, there have been sporadic observations of survivors from K-group, a group that used to occupy an area to the north of M- group [Nishida et al., 1985; Uehara et al., 1994]. We could not judge, therefore, whether the unknown individuals belonged to K- or B-group.

At the time of the intergroup encounter, M-group consisted of 52 individuals, including eight adult males (>15 years old) and 20 adult females. One adolescent male (PM, 12 years old) was also classified as an adult, as he had reached adult body size and had received pant-grunts from adult females.

During the season in which the incident occurred, the main food item was the fruit of Saba comorensis (S. florida). When the fruits of S. comorensis are abundant, M-group chimpanzees typically move back and forth from the northern to the southern ends of their range, over several days. In the year of this study, the fruits ofS. comorensis were far less abundant than usual and were found only patchily in the study group’s range in December. Presumably because of this, the M-group chimpanzees split into two large parties the day before the incident; the main party of M-group moved from the southern to the northern part of their range, and 12 females and their offspring remained in the south. All nine adult males, three out of four adolescent and juvenile males, eight adult females (one with full swelling (TZ), one with semi-swelling (ZL), and six with one or two offspring), and one adolescent female had moved to the north, where the incident occurred. We confirmed the presence of one adult male (PM) and one adult female (EF) only after the incident, and not before or during the encounter on that day. The results of this study are based on ad-lib observations of known individuals.

RESULTS

At 07:52, we located the chimpanzees of M-group in the northern part of their territory by their calls. At 08:13, we met with a mixed-sex party, consisting of three adult males (DG, HB, and BB), one adolescent male (DW), and one adult nonestrous female with an infant (PI). They were moving north. It seemed that they had heard the calls of stranger(s) from the north and were going to investigate, since they showed the typical tense responses to strange voices: 1) they ignored a food patch that had plenty of S. comorensis fruit, and 2) they moved rapidly and in one direction, making little sound. We did not follow them, since our observation target individual was not included in that party. At 09:04, a chorus of pant-hoots, pant-barks, and waa-barks began from the north. As soon as they heard this chorus, the chimpanzees we were observing ceased feeding, descended from the trees to the ground, and ran straight toward the north, in the direction of the chorus. While they were moving, the chimpanzees frequently

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touched and hugged each other. They moved very rapidly, so we lost sight of them. As we approached the location of the chorus we met the alpha male of M- group (FN), who was also traveling north, which suggested that FN had not instigated the incident.

At 09:12 it began to rain heavily. At 09:35 we caught up with the chimpanzees at the north end of the Mpila Valley, at the northern periphery of their ranging area. We found that four adult males (FN, DE, AL, and MA) had surrounded and were collectively attacking an unknown male infant. All four males were very excited, and their hair was erect. This infant was estimated, based on body size, to be 2.5 years old. We observed two deep gashes (each about 7 cm long) on the head of the infant, and his face was covered with blood, suggesting that he had already been severely attacked before we arrived. The observation was made from a distance of about 6 m. Fifteen meters from where the four males were attacking the infant, more waa-barks were heard, and many chimpanzees (estimated as more than six chimpanzees from the calls, but we could not confirm whether they were M-group chimpanzees) appeared to have gathered there. One adult male (BB) passed the four males that were attacking the infant and ran toward the origin of the waa-barks. We were unable to investigate that location, both because of the dense brush and because we wanted to concentrate on recording the aggression toward the infant. DE was the most aggressive of the males. He grasped the infant by the leg, hurled him to the ground, stamped on him, and repeatedly tossed him. The infant attempted to escape, but was soon caught by the surrounding males. When AL and MA captured the infant, they also tossed him. One unknown nonestrous adult female (probably the mother of the infant) repeatedly tried to approach and retrieve the infant from the M-group males with a ‘‘full closed grin’’ expression on her face [Goodall, 1986]; however, she was attacked by the M-group males and could not approach the infant. We did not directly observe unknown chimpanzees other than the female and infant. We saw no aggression toward the M-group males by the unknown female, and no aggression among the M-group males. Two very excited M-group adult females (TZ and FT) barked and walked about nearby, but they were not observed to attack or threaten the unknown female and infant. Three other M-group females (OP, RB, and XT) remained in a tree and did not participate in the aggressive interaction. During the encounter, M-group males moved continually toward the southwest. After escaping from DE, the infant approached and hugged FN, who sat nearby; this action resembled reassurance-seeking behavior. However, FN grasped the infant and bit him on the head. This was the last we saw of this aggressive encounter (at 09:40).

During our observation of the attack on the infant, the first author and one Tanzanian assistant were suddenly attacked and wounded by the unknown female chimpanzee. M-group individuals ignored the female’s attack on the researchers. After the attack on the researchers, she fled to the east, and subsequently neither chased us nor followed the individuals that had attacked the infant. Owing to the danger of further attacks and heavy rain, we abandoned the observation (09:42). The chorus that had begun at 9:04 continued, and judging by the movement of the vocalizations, the M-group chimpanzees (presumably with the unknown infant) had continued to move to the southwest.

At 10:38, after the rain had lessened, the second author resumed observations. M-group chimpanzees were found in parties consisting of one or two individuals about 200 m southeast of the spot of the intergroup encounter. They were headed straight to the south. The body of the infant was not found on

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the ground or observed being carried off by members of M-group. After the incident, we confirmed that six of the nine adult males had wounds about 4–5 cm in length (DG: back of left hand; AL and MA: upper side of mouth; DE: back; CT: the side; and HB: back). Of the six males, at least three (DG, AL, and HB) had bled, and one (MA) was suspected to have bled. Three of these six males were observed to attack the infant, but the other three males were absent during our observation of the encounter. On that day, we did not see the one adult female (PI), the one adolescent male (PR, son of PI), and the one adolescent female (PP) who had all been in the party before the encounter occurred, but we confirmed their presence a few days later (none of them had been injured). Except for the injuries suffered by the males, the behavior of the M-group chimpanzees (e.g., group cohesiveness and party size) did not change after the encounter relative to that before the encounter.

DISCUSSION

Previous cases of intergroup attacks on an unknown female with an infant (usually a mother–infant pair) from a neighboring group may be classified into two types, according to whether the main target of the aggression was the adult female or the infant [Goodall, 1986; Wrangham, 1999]. Intergroup aggression toward the mother of nulliparous females from a neighboring group has sometimes been followed by immigration of the young females to the aggressor’s group [Goodall, 1986], suggesting that the function of this type of aggression is to recruit mates and to increase mating opportunities for males. This hypothesis does not seem to fit the present case, since the target of the intergroup attack was clearly the male infant.

It has been hypothesized that attacking and killing a male infant reduces the number of future adult males in a neighboring group, and that this may have the effect, in the long-term, of weakening the power of a neighboring group; it may also lead to better access to resources, and to expansion of the group range, thus ensuring their future safety [Takahata, 1985]. This hypothesis is supported by the fact that the targets of intergroup infanticide previously reported from four study sites of wild chimpanzees apparently were mostly male (4: male, 1: female, 5: unknown [Bygott, 1972; Goodall, 1977, 1986; Nishida et al., 1979; Newton-Fisher, 1999; Watts & Mitani, 2000]). In the present case, we do not know the fate of the infant male victim because we did not observe the end of the attack, nor did we find the body. However, since 1) the infant was severely injured and lost a lot of blood for his body size, 2) the unknown female was unable to protect him, and 3) he was completely under the control of males from M- group, we infer that he was killed. Therefore, the present case appears to fit this hypothesis.

During the incident, the female chimpanzee injured two humans. Although chimpanzee attacks on human infants have occasionally been reported [e.g., Goodall, 1986], this may be the first reported case of researchers being attacked by an unhabituated chimpanzee. In our study, the observation was made from about 6 m, a distance that ensured detailed observations of the fast-paced agonistic interactions. Other studies have used similar or shorter distances to observe intergroup aggression and infanticide [e.g., Nishida & Hiraiwa- Hasegawa, 1985; Arcadi & Wrangham, 1999; Watts & Mitani, 2000]. Thus, it would seem that the researchers in those investigations faced a similar risk of being attacked by the excited chimpanzees. The best way to prevent this kind of accident from happening is to maintain a safe distance from the animals; this is

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also critical for minimizing other risks, such as disease transmission between humans and wild chimpanzees [reviewed in Wallis and Lee, 1999; Butynski, 2001].

In this observation, it was puzzling that most of the M-group males were wounded. It is possible that the unknown female of the neighboring group injured the males; however, we suspect that this is unlikely, because during our observation she seemed powerless. One possibility is that another intergroup battle occurred in the place where the many waa-barks were heard, about 15 m from the attack on the infant, and the males may have been involved in that incident. If so, it would be surprising because, in general, only a few vulnerable individuals have been injured during previously reported intergroup aggression that involved physical contact [Goodall, 1986; Wrangham, 1999]. Alternatively, the M-group males may have fought each other for the body of the infant. Unfortunately, we do not have sufficient data to determine which of these possibilities was the case.

ACKNOWLEDGMENTS

We thank the Tanzanian Commission for Science and Technology, Serengeti Wildlife Research Institute, Mahale Mountains Wildlife Research Center, and Tanzania National Parks for permitting our research and for their support while we were in Tanzania. We also thank Toshisada Nishida, Kenji Kawanaka, Shigeo Uehara, Kazuhiko Hosaka, Michio Nakamura, Shiho Fujita, James Wakibara, and Watongwe research assistants, as well as other colleagues of the research team, including Noriko Itoh and Chisa Tokimatsu, for their support in various ways. N.K. thanks Toshikazu Hasegawa and Duncan Castles for their supervision and support over the course of this study, Toshimichi Nemoto and his family for their help, and of course the medical staffs in Dar es Salaam, Rashidi Hawadi, and Abudala Ramadhani Ismaili, for saving his life. Toshisada Nishida, Shigeo Uehara, Kazuhiko Hosaka, Toshikazu Hasegawa, and four reviewers of the American Journal of Primatology provided critical comments on the manuscript, for which we are grateful. This study was financially supported by the Monbusho Scientific Research Fund (basic research A1, #12375003 to T. Nishida) and Japan Society for the Promotion of Science Research Fellowships for Young Scientists (to N.K.).

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