AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 78547-573 (1989)

Hunting Behavior of Wild Chimpanzees in the Tai’ National Park

CHRISTOPHE BOESCH AND HEDWIGE BOESCH Department of Ethology and Wildlife Research, University of Zurich, CH-8057 Zurich, Switzerland KEY WORDS Cooperation, Sharing, Traditions

ABSTRACT Hunting is often considered one of the major behaviors that shaped early hominids’ evolution, along with the shift toward a drier and more open habitat. We suggest that a precise comparison of the hunting behavior of a species closely related to man might help us understand which aspects of hunting could be affected by environmental conditions. The hunting behavior of wild chimpanzees is discussed, and new observations on a population living in the tropical rain forest of the TaY National Park, Ivory Coast, are presented. Some of the forest chimpanzees’ hunting performances are similar to those of savanna-woodlands populations; others are different. Forest chimpanzees have a more specialized prey image, intentionally search for more adult prey, and hunt in larger groups and with a more elaborate cooperative level than sa- vanna-woodlands chimpanzees. In addition, forest chimpanzees tend to share meat more actively and more frequently. These findings are related to some theories on aspects of hunting behavior in early hominids and discussed in order to understand some factors influencing the hunting behavior of wild chimpanzees. Finally, the hunting behavior of primates is compared with that of social carnivores.

Hunting is generally described by paleoan- Goodall, 1986; Kudo and Mitani, 19851, and thropologists as a central behavior in homi- the increase in complexity and frequency of nid evolution (Hill, 1982; Isaac, 1978, 1983; hunting behavior among primates (compari- Isaac and Crader, 1981; Johanson and Edey, son of baboon hunts with chimpanzee hunts- 1981, Leakey, 1981; Leaky and Lewin, 1978; Strum, 1981; Goodall, 1986),cause us wonder Washburn, 1978), even though opinions still whether hunting is really a characteristic of diverge about which adaptation was crucial man and whether the shift toward an omni- for the making of mankind, i.e., tool making vorous diet did not occur early in primate (Washburn, 1960, 1978), food sharing (Isaac, evolution. The actual field data tend to show 1978, 1983), seed eating (Jolly, 19701, or divi- how hunting evolved, both in Carnivora sion of labor (Hill, 1982; Zihlmann, 1981). (Ewer, 1973; Estes and Goddard, 1967; Recently, Shipman (1986) cast doubt on the Kruuk, 1972, 1975; Mech, 1970, 1975; Peters hunting proficiency of early hominids based and Mech, 1975; Schaller, 1972) and in on findings from bed I in Olduvai Gorge (2- Primates, among the latter mainly in the 1.7 million years old). Analyzing cut marks superfamilies of Cercopithecoidea and Hom- on bones, she concludes that hominids in that inoidea. This evidence suggests that it may period relied on scavenging for their major be too simple to propose a unique evolution meat procurement rather than on hunting, of hunting in early man; indeed, anthropolo- thus confirming the increasing importance gists favor the idea that a radical change in according to scavenging in descriptions of the habitat (a shift from the forest to a more early hominids’ diet (Isaac and Crader, 1981; open savanndwoodland environment east of Potts, 1984). Similarly, new evidence coming the Rift Valley in East Africa) of early homi- from the field of primatology has cast doubt nids forced them into a series of adaptations, on the uniqueness of hunting in man. Recent of which the hunting of mammals would be observations on hunting of mammalian prey in nonhuman primates (for example, Bad- rian and Malenky, 1984; Butynski, 1982; Received August 10, 1987; accepted May 2,1988.

0 1989 ALAN R. LISS, INC. 548 C. BOESCH AND H. BOESCH a major one (Coppens, 1983; Johansen and gest that it is important to analyze differ- Edey, 1981; Leakey, 1981 Leakey and Lewin, ences in hunting behavior (and factors 1978; Pilbeam, 1980). influencing them) between different primate Data on predation on mammals in pri- species, with a view toward comparing such mates have been collected from a wide range traits and abilities on the level of zoological of species, for example, blue monkey (Butyn- orders. ski, 1982)) vervet monkey (Galat and Galat- The present paper has four aims: Luong, 1977), macaque (Estrada and Es- trada, 1977, 1978), mandrill (Kudo and Mi- 1. To present the hunting data of a wild tani, 1985), most savanna-dwelling baboons chimpanzee population living in a dense (Altmann and Altmann, 1970; Hamilton and tropical rain forest, the Tai National Forest, Busse, 1982; Harding, 1975; Hausfater, 1976; Ivory Coast, an environment not studied so Goodall, 1986; Kummer, 1968; Strum, 1981; far for the hunting behavior of chimpanzees. Rhine et al., 1986), orangutan (Sugardjito and 2. To compare the forest study with two Nuhuda, 1981), pygmy chimpanzee (Badrian studies made on wild chimpanzee popula- and Malenky, 19841, and the common chim- tions living in a more open environment (sa- panzee (see references below). However, ob- vanndwoodland) in the Gombe Stream servations on all these species vary in length, National Park and in the MahaIe Mountains living conditions of the animals, and record- National Park, both in Tanzania, in order to ing procedure, so that comparisons done in acquire an impression of the variability order to understand the factors influencing chimpanzees can display within the same be- hunting in primates are difficult. For such a havior, i .e., hunting. purpose, studies on one behavior (hunting) 3. To understand how some of the differ- and one species (chimpanzee), living in a nat- ences between these three populations ap- ural environment, may permit a reliable peared and how the environmental comparison of such a complex behavior. differences affected them. The Chimpanzees are predators of at least 4. To compare some aspects of the hunting 19 different mammalian species (Goodall, behaviors of chimpanzees and the social 1986; Nishida and al., 1983; Teleki, 1981). carnivors. They tend to reach kill rates of the prey species comparable with those of some social carnivores, total meat intake apart, which is METHODS quite different. In Gombe Stream, chimpan- The wild chimpanzees (Pan troglodytes) of zees kill an estimated 8-13% of the red colo- the Tai’ National Park, Ivory Coast, have bus population (Busse, 1977). In the been studied since September, 1979. At the Ngorongoro crater, spotted hyenas kill 11% time of writing, the community included 79 of the wildebeest population and 9% of the individuals, with 9 adult males and 26 adult zebra population (Kruuk, 1972). In the Ser- females, living in a 27-km2 home range in engeti, lions kill 6% of the zebra population the western part of the park, about 20km (Schaller, 1972). One community of chimpan- from the village of Tai (for more details on zees in Gombe Stream was observed to hunt the environment, see Boesch and Boesch, 33 prey per year during a 10-year period 1983). The closest traditional plantations are (Goodall, 1986). A comparison between chim- located about 6 km from their home range panzee populations should allow us to under- limits. The native tribes, the Oubi and the stand how hunting in a primate species may Guere, do not hunt them for meat or totemic be influenced by different factors, such as the reasons. The forest is the evergreen rain for- environment, prey availability, food avail- est type, with an average rainfall of 1,800 ability and social aspects. mm per year and an average temperature of In addition, with the accumulation of ob- 24°C. Among the larger mammals present, servations on chimpanzees’ hunting be- many of them potential prey, we note: the havior, a behavioral comparison between forest elephant (Loxodonta cyclotis), the Carnivores and Primates becomes feasible, pygmy hippopotamus (Cheoropsis liberien- allowing us to test the hypothesis proposed sis), the bongo (Tragelaphus euryceros), the by some authors (Schaller and Lowther, 1969; bushpig (Potamochoerus porcus), the giant Thompson, 1975,1976) that carnivores’ hunt- forest hog (Hylocheorus rneinertzhageni), six ing behavior is closest to that of humans and species of duikers (Cephalophus jentink6 C. that it should be studied as a possible model sylvicultur, C. ogilbyi C. dorsalis, C. zebra for human evolution. However, we also sug- and D. monticola), the royal antelope (Nee HUNTING BEHAVIOR OF CHIMPANZEES 549 tragus pygmaeus), one species of otter (Lutra meat-eating episodes were recorded mostly maculicollis), the honey badger (Mellivora ca- by the two of us, regularly controlling agree- pensis), the long-nosed mongoose (Mungos ment of observations. In Gombe Stream and obscurus) the African brush-tailed porcupine Mahale Mountains, hunting and meat eating (Panthera pardus), and some carnivores-the were recorded by a number of observers, who leopard (Panthera pardus), the golden cat changed regularly over the years. Intercom- Profelis aurata), the pardine genet (Genetta patibility of the observations may, therefore, pardina), and the civet cat (Viverra civetta). be lower in these two sites than in Tai’, but Besides the chimpanzees, ten species of pri- the quantity of observations is probably more mates live in the area: three colobus (Colobus important in those sites. We shall base our badius, C. polykomos, and C. verus), four cer- comparisons on published materials. For copithecoids (Cercopithecus diana, C. petaur- Gombe, we shall use Busse (1977, 19781, ista, C. campbell6 and C. nictitans), the sooty Goodall (1968,1975, and especially 19861, Te- mangabey (Cercocebus atys), the dwarf gal- leki (1973, 1975, 19811, and Wrangham ago (Galago dernidovii), and the Bosman’s (1975). Whenever possible, we shall give pref- potto (Perodicticuspotto). erence to the data of the 10-year span from We habituated the chimpanzees to our 1972 to 1981 (Goodall, 19861, when banana presence without artificial provisioning, and provisioning was kept very low. For Mahale, we followed them only by their drummings, the three published papers on hunting be- and vocalizations, and, more recently, by havior will be used (Kawanaka [1982], Nish- sight. Due to the very limited visibility in ida and al. [1983], and Takahata et al. [1984]). this dense forest (at most 20 m on the ground) Comparing behaviors of different animal and the shy character of the chimpanzees, populations living in different sites and stud- habituation was a slow process; only after 2 ied by different observers may be, in the 112 years couId we observe some chimpan- strictest sense, impossible, as one can hardly zees that were aware of our presence. From ever control all variables. We shall, never- March, 1984 onwards, we could follow all the theless, attempt such a comparison, and we males by sight and remain within their group consider that small differences should be at- while they were hunting. For this reason, we tributed to the different observational condi- considered only the huntings observed during tions and methods, whereas large, and clear- the last 2 years for the detailed analysis (be- cut differences should reflect a reality. When tween March, 1984 to May, 1986). The main methods differ strikingly, we shall make no focus of our study was the nut-cracking be- comparison. havior of the chimpanzees (Boesch and Boesch, l981,1983,1984a,b), and our interest in hunting grew slowly with the increasing DEFINITIONS OF HUNTING AND FOOD SHARING evidence of differences from the known be- Another aspect that makes comparisons havior of other chimpanzee populations. Dur- difficult is the many different meanings of ing the nut seasons we mainly followed the certain terms. In an attempt to clarify the females, who hunt less frequently than males issue, we shall present our definitions of com- (see below), and a certain amount of hunts mon terms that cannot be avoided when were, therefore, missed. However, we gained speaking of hunting behavior. They prove to the impression that hunting mostly occurs be useful for describing the Tai‘ chimpanzees’ when Coula nuts are out of season and that hunting behavior but may be more difficult we missed few hunts performed by the cen- to use with predators living in very different tral animals. In order to keep the human environments or using different hunting disturbance at the lowest level, not only for techniques. the hunters but also for the prey, usually monkeys, we trained ourselves to move on Bystander the ground under groups of arboreal mon- Any animal present in the group in which keys without being noticed by them, partic- a hunt is taking place, not playing any active ularly not until the chimpanzees were role. It may look intently at what is happen- noticed. ing and even follow the movement of the Comparisons of hunting behavior at differ- hunt. During the same hunt, an individual ent study sites are difficult, and data-record- can change from the role of bystander to that ing procedures have to be compared. In TaY, of hunter, according to its actions. For exam- all hunting behavior data were recorded by ple, a bystander, interested in participating one observer (C.B.), and all aspects of the actively in a hunt, can do so when the situa- 550 C. BOESCH AND H. BOESCH tion stimulates it, such as when a prey falls a minimal approach toward the prey of 60 m to the ground. to classify a hunt. In Gombe, it seems that it is often difficult to distinguish between by- Hunter standers and hunters. Busse (1978) describes Any animal that shows by its physical “group hunts” in which only one hunter acts movements its intention to play an active at a time but within a group of bystanders. part in a hunt: In Tai’, the distinction be- Similarly, Teleki (1973) describes a “cooper- tween bystanders and hunters is simple since ative hunt” (May 14, p. 68), in which only the hunts take place mostly up in the trees. one individual acts; the second one is sitting Once a bystander climbs up to the height in and merely watching the action. the canopy where the prey live, looking and orienting itself toward them, it becomes a Cooperation hunter. For hunts taking place on the ground, This word has acquired two different mean- the rush toward the prey is used to differen- ings, which need to be clarified. In the hunt- tiate between the two states. ing context, cooperation has generally been attributed to the behavior of two or more Hunt individuals acting together to achieve a com- Any situation during which at least one mon goal (see, for example, definitions in animal, due to its behavior, can be classified Busse, 1978; Goodall, 1986; Kruuk, 1972; as a hunter. Schaller, 1972). In the analysis of the individ- ual behavior, Hamilton (1964) has defined Group hunt cooperation more strictly: an animal acts co- Any hunt in which more than one hunter operatively only if it does things that benefit acts at the same time against the same prey the fitness (in terms of survival and repro- or group of prey. If only one hunter is hunt- duction) of both itself and the recipient of its ing at a time, within or outside a group of actions (Axelrod and Hamilton, 1981; Wran- bystanders, or if many hunters are acting gham, 1982), even at the expense of its own each against another group of prey (with dif- fitness (Krebs and Davies, 1987; Maynard ferent hunting directions), we classify such Smith, 1982, 1983). However, this more strict hunts as solitary. definition of cooperation can lead to the par- Some hunters may test the physical abili- adox that the same group behavior could be ties of a prey by rushing toward it for only considered cooperative or not only according some meters (hyenas, lions or wolves). Clas- to its outcome. For example, an elaborate sifying such a rush as a hunt can be question- collaborative group hunt performed by one able. Schaller (1972) uses a clear criterion of population could increase its hunting success

TABLE 1. Operational definition of cooperation for group hunts: four levels of growing complexity of organization between hunters Category Definition Variation Similarity All hunters concentrate similar Similar actions are actions on the same prey, but varying elements of without any spatial or time pursuing a prey, i.e., relation between them; however, stalk, chase . . . at least two hunters always act simultaneously Synchrony Each hunter concentrates similar Hunters may begin at actions on the same prey and the same time or tries to relate in time to each adjust their speed to other’s actions remain in time Coordination Each hunter concentrates similar Hunters may begin actions on the same prey and from different tries to relate in time and space to directions or adjust each other’s actions their position and speed to remain coordinated Collaboration Hunters perform different Examples are driving, complementary actions, all blocking escape way, directed toward the same prey and encirclement HUNTING BEHAVIOR OF CHIMPANZEES 551 and would, accordingly, be classified as coop- crease of success, compared with that of lone erative, whereas the same collaborative individuals, is subsequently proved. group hunt in another population could fail Food transfer between two individuals can to increase their hunting success and would range from a mere stealing to the most giv- then not be considered cooperative (see also ing. In order to keep track of this variability, Busse, 1978). To avoid such a paradox, we we adopt an operational definition of food proposed to use cooperation in the hunting sharing, using six levels of increasing shar- context according to the first definition and ing intention of the food owner (Table 2). refer to “cooperation sensu Hamilton” when it fulfils the additional criteria of increasing RESULTS the outcome, compared with similar actions Prey species performed by a single individual. Table 3 presents the data on Tai’ chimpan- In the literature on hunting, the meaning zee hunts including all observations during of the word cooperation can change according the 7 years of the ongoing study. We consid- to author (see, for example, Busse, 1978; ered only the observed hunts, including at- Goodall, 1986; Kruuk, 1972; Schaller, 1972). tempts and successes; our experience of In our opinion, this change of meaning oc- collecting feces during 2 years showed that curs partly because groups of individuals may such a method is not reliable as it does not perform a large variety of actions represent- match with the visual observations. It is still ing different levels of cooperation, with in- worth mentioning that we could identify in 1 creasing organization. In order to reflect this out of 381 feces the bones of a Latham’s fran- complexity within group-hunting tech- colin (Francolinus lathami), confirming our niques, we adopt an operational definition of impression that we are still unaware of some cooperation based on the level of organiza- rarely hunted prey. tion in the actions of the hunters (Table 1). The main finding seen in Table 3 is that We want to emphasize that individual be- Tai‘ chimpanzees chose monkeys, mostly col- havior within such a cooperative group may obus, for all but one hunt during this 7-year be classified by the four categories proposed period. This specialization in Colobus mon- by Hamilton (1964); altruistic, spiteful, sel- keys is extreme: they are the prey in 95% of fish, and cooperative. If feasible, we shall the hunts, and 81% of the hunted monkeys proceed to such an analysis of hunting at the are red colobus. Three species of prey are individual level. The definitions given in Ta- new for chimpanzees, i.e., Cercopithecus ble 1 represent different levels of organiza- diana, Colobus uerus, and Cercocebus atys. tion of cooperative groups. They will be called Table 4 presents a comparative list of the “cooperative sensu Hamilton” only if an in- major mammal prey of the three chimpanzee

TABLE 2. Operational definition of sharing: six levels of increasing sharing intention of the food owner Category Definition Interpretation S1-Theft B forcefully takes part or all Sharing takes place against of A’s food A’S will: A shows opposition S2-Recovery B takes part of the food that A Sharing without intention of has dropped on the ground A and merely tolerance of or placed there B’s proximity if close S3-Passive B takes part of the food that A A passively tolerates B’s sharing is holding. A makes no action but may accept that movement to facilitate nor to B holds the food pooled in avoid B’s action common for some time S4-Active-passive B takes part of the food that A A actively tolerates B’s action sharing is holding. A makes a and expresses it by movement to facilitate B’s withdrawing its hand or by action bringing the food toward B SS-Active A gives part of its food to B A makes an explicit action sharing either cutting off a piece or showing its intention to B by holding a piece toward B S6-Gift A gives a part at least three A’s generosity seems to show times larger than its own a greater sharing intention remaining piece to B than the category above 552 C. BOESCH AND H. BOESCH

TABLE 3. All observations of hunting attempts and successes observed during a 7-year period from September, 1979 to May, 1986 in Tac, classified according to prey species hunted and captured Hunts Captures No. % No. % ~ Colobus badius 110 81 63 77 Colobus polykomos 16 12 11 14 Cercoprthecus diana 3 2 3 4 Colobus uerus 2 1.7 2 2.5 Cercocebus atys 2 1.7 1 1.2 Perodicticus potto 1 0.7 1 1.2 Rat (sp.?) 1 0.7 0 Total 135 81

TABLE 4. Major mammal prey species observed to be eaten by chimpanzees in Tai; Gombe, and Mahale' Tai Gombe Mahale No. % No. % NO. % Colobus badius 63 77 203 64 9 13 Colobus polykomos 11 14 - - - - Papio anubis - - 83t- Cercopithecus - - 5 2 11 16 ascanius Pan troglodytes t- 522 3 Other primates 79- - 46 Bushpig t- 51 16 6 8 Bushbuck - - 39 12 13 19 Blue duiker t- - - 19 27 Others - - 41 57 'Tai' and Mahale data include all observed hunts, whereas Gonibe data include only the 10-year period from 1972 to 1981 (Goodall, 1986). Prey species hunted by chimpanzees in one site that are present but not hunted in another site are marked for that site with a single dagger (t). populations and the frequency with which al., 1979; Nishida and Hiraiwa-Hasegawa, they were observed to be caught andor ea- 1985; Nishida and al., 1983). Cases of canni- ten. Tai chimpanzees are the most special- balism within a community are rarer; an ized hunters among chimpanzees; only six adult female killed and ate new-born infants mammalian species have yet been seen to be of her own community (Goodall, 1986), and eaten out of 27 available prey species (we twice males ate new-born infants of females counted as available species all mammals belonging to their community (Norikoshi, present in an area except the Carnivora, the 1982; Takahata, 1985). In Tai, we never saw Tubulidentata and the Proboscidea), whereas anything of that nature, and, since March, Gombe chimpanzees eat 9 species out of 16 1984, the period in which we could more con- available, and Mahale chimpanzees eat 13 stantly follow the males, we never saw them out of 23 available (no data are available on encounter any lone stranger chimpanzee. All the presence of small mammals). encounters with strangers were made be- As can be seen from Table 4,the proportion tween groups, the males of both communities of monkeys in mammal prey decreases from facing each other and the females with in- Tai' to Mahale, as does the proportion of red fants remaining at the rear. The possible lack colobus which remains, nevertheless, the of cannibalism in Tai might be explained by major prey in Gombe. Infant chimpanzees this seemingly lower rate of encounters with have been observed to be killed and eaten by lone female strangers. adult chimpanzees in Gombe and Mahale when males of a community encountered a Availability ofpotential prey lone stranger female with an infant. In the consecutive fights, the infants were often The Tai' chimpanzees' specialization in snatched away from their mothers, killed, monkeys may reflect a simple difference in and in part eaten (Goodall, 1986; Goodall and the availability of potential ungulate prey. HUNTING BEHAVIOR OF CHIMPANZEES 553 The major difference in Table 4 comes from She smelled it, threw it 3 m aside, and left. the absence of hunting of bushpigs and duik- The examination of the body revealed no ers in the Tai’ forest, which are, however, wound; only some fur was taken away on its fairly common there. We encounter small side, probably by the infant biting it duikers (Blue, Bay, Zebra, and Ogilby) 2 to 3 playfully. times per day in the forest. Many close en- counters between chimpanzees and duikers This observation tends to indicate that Tai’ were witnessed (12 observations). The chim- chimpanzees have a highly specialized ‘>rey panzees either ignored them or seemed image”. When they have the opportunity to scared when duikers rushed through one of capture an animal not belonging to this im- their groups, and the duikers avoided them age, they tend to ignore it (see also the next without giving any alarm calls. Descriptions section). Subadults were interested in the from Gombe (Goodall, 1986 and personal duiker but handled it as a toy rather than as communication) and from Mahale (Takahata an edible animal. and al., 1984) show that chimpanzees concen- Close encounters between chimpanzees and trate on small infants of blue duikers or bushpigs were witnessed (four observations); bushbucks. In Tai’, we have not even seen a chimpanzees tended to ignore them, but they small infant duiker in 7 years, which may would flee or bark aggressively when an mean that, in the forest, these animals hide adult male bushpig tried to keep them at their babies so well that they are difficult for distance during food competition for Coula the chimpanzees to find by chance. This fac- nuts (two cases). For bushpigs, another factor tor may explain in part the lack of hunting has to be taken into account. In Gombe, they for duikers. However, on the January 10, live in small social groups; 93% include five 1987, we made a most intriguing observation or fever adult members and they seem to be concerning the duiker-chimpanzee relation mainly nocturnal (Goodall, personal commu- and generally perception of prey by them: nication). Tai bushpigs were always seen in groups larger than five individuals, and they The core group was cracking Coula nuts seem to be very active during the daytime. and moving in a hilly region. While watch- The active defense of adults living in large ing some males, including the most keen and groups could easily repel any chimpanzee; in experienced hunter, we heard a blue duiker Gombe, they defend themselves efficiently in (Cephulophus mnticolu) screaming some 60 small groups (Goodall, 1986). Interestingly, m away. They looked in the direction, then the giant forest hogs of the Tai’ forest live in ignored it. I went there; the screams came much smaller groups then bushpigs, and from a thicket where infant chimpanzees twice we observed clear signs of hunting in- were playing. When they moved out, I saw a terest on the part of the chimpanzees who 9-year-old male juvenile (Fitz) dragging along made detours toward the hogs after having by one foot a very young blue duiker still heard them scream. Thus differences in alive, followed by three infants and their group size and in breadth of prey image and mothers, who seemed less concerned. The difficulty in finding prey in the forest may four youngsters played with the duiker for partly explain the apparent disinterest of the 15 minutes, Fitz repeatedly trying to place it Tai chimpanzees toward species that are on his back or his shoulder while walking, as commonly preyed upon in other regions. if it was a young chimpanzee. For each rough Other abundant species have been noted as movement, the duiker screamed loudly. By prey in East Africa and seem to be neglected then they had reached another thicket where in Tai, i.e., squirrel and mongoose. Thus ad- they were joined by more females and two ditional reasons must exist to account for the adult males. The screaming stopped, but a 4- differences in prey selection; we shall discuss year-old infant came out with the duiker, some of them later. followed by two other infants. They played In Gombe, some authors tried to estimate for 8 minutes with the duiker, meanwhile the frequency of encounters between the prey dead. An adult female joined the play ses- and the chimpanzees. Busse (19771, using as sion, kicking the duiker with one hand and a criterion for an encounter a distance of one foot. Several adult males passed nearby visible and/or auditory contact of 100 m be- without showing any interest. The dead body tween them, estimates that Gombe chimpan- was abandoned intact on the ground, 33 min- zees encounter red colobus once every 55 utes after the capture. The corpse was found daylight hours. Wrangham and Bergmann- 20 minutes later by an adolescent female. Riss (in press, cited in Goodall, 1986), using 554 C. BOESCH AND H.BOESCH a criterion of 200 m distance between both the trees, alert to the sound of monkeys. species, estimate the encounter rate to be one Many silent changes of direction may occur. every 12 hours. These estimations are sur- A chimpanzee might eat a fruit or leaf while prisingly low, when one considers that red passing by, but no general feeding of the colobus are as abundant in Gombe as in Tai, group occurs during a search. These searches with home ranges of 1 km2 for groups of last on the average 16 minutes, 27 seconds about 50 individuals (Gombe: Clutton-Brock, (Sample = 39, range = 5-50 minutes). Un- 1975; Tai’: Galat, 1978; Galat-Luong, 1983; clear cases were classified as opportunistic Galat and Galat-Luong, 1985). We estimate (prey overhead, or found in the way of the the encounter rate as three encounters every initial direction). 4 hours, using an auditory andlor visible dis- In Tai’, 31% (24 out of 78) of the hunts must tance of 200-300 m (sample size: 265 hours, be classified as opportunistic, whereas in half 45 minutes). Our different encounter rate of the hunts we could observe clear signs of could partly explain the difference in prey hunting intention before any prey was seen selection between the two populations. or heard (searches only). As the observation of January 10, 1987 stressed, Tai’ chimpan- Opportunism versus intentional hunting zees have a specialized prey image and there- Busse (1977) concludes that chimpanzee fore begin a hunt by selecting a prey. Since hunting in Gombe is mainly opportunistic; monkeys can be detected by the rustle of the during a 2-year period, he observed chimpan- foliage in which they jump, it has happened zees encountering red colobus 85 times with that searching hunters have arrived under hunts occurring 64 times (75%).All observers such a group before seeing them. In 13% of working in Gombe confirm the impression the searches, they then looked for another that hunting is, for the most part, opportun- prey. This always happened with C. diana, istic (Goodall, 1986; Teleki, 1981; Wrangham, which are very agile and much quicker than 1975). The same behavior was observed in colobus; they are hunted only when very low Mahale (Nishida et al., 1983; Takahata et al., in the canopy. Hence this image of the prey 1984). In Tai’, based only on our estimation of is also influenced by the context: duikers the encounter rate (7.5 encounters in a 10- would be rejected in any case, whereas diana hour day), 6.6% of encounters are followed by monkeys would be rejected except if they for- a hunt. age unalerted in the lower canopy. A more direct method of evaluating the We can conclude that Tai’ chimpanzees hunt opportunism of the chimpanzee hunts, how- more often by deliberately seeking out a prey ever, would be to record the behavior of the than do Gombe chimpanzees, but this behav- hunters before a hunt begins and to deter- ior does not exclude an opportunistic compo- mine the moment when a hunt is decided nent when the Tai’ chimpanzees have found and according to what signal. In 78 hunts a specific potential prey. out of the 100 meat-eating episodes we ob- served during the last 2 years, we were with Prey size hunters before a hunt began (minimum one There is a difficulty in determining the hour before). Each time we recorded all prey size because of low visibility, which changes of direction and any behavior indi- makes it impossible for us to identify the cating an incipient hunt, based on the follow- prey before the capture (and often the divi- ing behaviors: Tai’ chimpanzees tend to follow sion) is made. This is in sharp contrast with the same direction for hours; when changing Gombe, where the prey is normally identi- it, they communicate by means of drumming fied before its capture (Goodall, personal and pant-hooting to any group members communication). Therefore we shall limit within auditory distance. Hence we defined ourselves to comparing the capture fre- “detour for hunting” as a clear change of quency of adult versus infantljuvenile prey direction made without any auditory signal and not discuss the sex. Table 5 presents data directly following the sound produced by the from Tai’, Gombe, and Mahale on size of prey. possible prey. Visibility being at most 20 m Tar chimpanzees tend to hunt proportion- in the forest, we have the impression that ally more adult prey than chimpanzees in the detection of prey is mostly made by ear. Gombe and Mahale, but the prey species may We have defined “search” for prey as follows: affect the prey size, as can be seen from the the chimpanzees become totally silent, re- Gombe and Mahale data when comparing main very close together, move one behind primate with ungulate prey. If we compare the other, and stop regularly to look up into only red colobus data, Tai’ chimpanzees hunt HUNTING BEHAVIOR OF CHIMPANZEES 555

TABLE 5. Age ofprey captured by chimpanzees from Tai; Gombe, and Mahale, classified as adults (A) and infantouuenile (I+J)' Tai Gombe Mahale Prey A I+J A I+J A I+J Colobusbadius 27 31 29 101 2 4 Other primates 6 2 - 50 2 6 Ungulates - - 0 90 5 23 All prey 33 33 29 241 9 33 'For Gombe no data is published for ungulates, but Goodall (1986) says no adult bushpigs nor busbbucks were killed.

TABLE 6. Duration of all Tar hunts (in minutes) in which we could see the beginning and the capture observed during the 2-year period March, 1984 to May, 1986 < 10 10-20 20-30 > 30 Total Number 27 30 11 14 82 % 33 37 13 17

'Mean time of all hunts: 18.19 minutes:, range: 1-120 minutes. more adult monkeys than do those from the M group during a 33-month period (Tak- Gombe (X2 = 10.14, : P< 0.001). Qualitative ahata et al., 1984), giving a hunting fre- observations also confirm that this difference quency of 1.63 hunts per month. This figure is a major one. Gombe hunters aim their represents a third of estimation 1 for Tai', actions toward infants and, frequently snatch and, despite differences in recording methods them away from their mothers' bellies, leav- (i.e., in Mahale six observers made the sam- ing the mothers unharmed (Goodall, 1986). ple-no indication of number of days spent in This behavior was never observed in Tai', the field could be found), it tends to indicate where small infants seemed to be caught in- a smaller hunting frequency in Mahale than cidentally, the objective being the mother. In in Tai'. addition, the capture of only a large infant In Gombe, during the last 10 years (1972- seemed to be a disappointment, and the hunt 19811, 315 hunts were observed by the Tan- usually continued afterward (13 observa- zanian field assistants, who are permanently tions out of 19 single infant captures). in the field. This figures gives a hunting fre- quency of 2.62 hunts per month and means Hunting frequency that chimpanzees in Gombe hunt more than During the 22 months we were in the field in Mahale but less than in Tai' (estimation from March, 1984 to May, 1986, we observed 1).New estimations in Gombe, including sol- 100 hunts (attempts or successes). From these itary hunts, give a rate of 150 red colobus data,we calculated two hunting frequencies: killed in 5 years (Wrangham and Bergmann, in press, cited in Goodall, 19861, which gives, 1. One hundred hunts observed in 22 when corrected by taking into account hunt- months = 4.5 hunts per month. We should ing success and the proportion of other prey correct estimation 1 by the number of days hunted (Goodall, 19861, an estimation of 103 we effectively followed (minimum auditory hunts per year. This figure is slightly smaller contact) the core male group for a minimum than our estimation 2 (P > 0.051, which does of 1 hour per day. The other days we either not include solitary hunts (see Methods). We saw no chimpanzees at all or only a solitary may conclude that forest chimpanzees hunt individual for a short time. Therefore esti- at least as frequently as chimpanzees living mation 2 is still an underestimation unless in a more open environment. one excludes unobserved hunts. Hunting duration 2. One hundred hunts observed in 299 In 82 cases out of the 100 hunts or meat- group days = 10 hunts per month. eating episodes we saw during the last 2 years, we were within the group of hunters In Tai', according to estimation 2, chimpan- just before the onset of hunting and could zees hunt about 120 times per year, i.e., al- thus measures its duration, ending with the most once every 3 days. In Mahale, the capture of the prey. Table 6 presents these highest rate of hunting was 54 episodes for results. 556 C. BOESCH AND H. BOESCH

TABLE 7. Hunting success observed in TaL from March, 1984 to April, 1986 March 1984-1985 April 1985-1986 Total Success Success Success rate rate rate No. (%I No. (%) No. (%) Colobus badius 44 38 39 72 83 54 Colobus polykomos 5 20 7 86 12 58 Cercopithecus diana 2 100 1 100 3 100 Colobus uerus 0 - 2 100 2 100 Total 51 39 49 75 100 57

Published data on duration of colobus hunts turing several prey, either at the same time are available from Gombe (Busse, 19771, or one after the other. Goodall (1986) pre- where chimpanzees tend to be successful or sents a sample of 9 years for 116 colobus give up hunting within the first 7 minutes in hunts in which chimpanzees made 44 multi- 53% of observations (out of 64 hunts, 34 ple kills (mean number of kills = 2.29 colo- lasted less than 7-minutes). When compared bus). The proportion of multiple kills is the with the Tai’ colobus hunts using the same 7 same in Tai, i.e., 14 out of 55 successful hunts minutes criterion (out of 82 hunts, 23 lasted (mean number of kills = 2.14, X2 = 2.06, P less than 7 minutes), hunts in Tai’ lasted sig- > 0.05). nificantly longer than in Gombe (X2 = 8.47, We divided the Tai data into two periods, P < 0.01). the second one beginning in June 19, 1985, when suddenly the success rate increased to Hunting success 100% during 6 consecutive months. The An attempt to evaluate hunting success change between the two periods could not be should take into account all hunts that failed. attributed to any change in the observation, From Gombe we know that chimpanzees may procedure, which remained constant for the remain silent before and during a hunt, the 2-year period; the same observer was also screaming following only the capture, so that used. The only change we could find was that observers may easily miss a hunt when fol- of the behavior of one of the hunters: Snoopy, lowing a chimpanzee not involved in it. Only who was always ready to participate in a an increase in the number of observers fol- hunt, became in this interval an adult male lowing different individuals increases the and had visibly gained in strength and con- probability of recording all attempts. This fidence when facing an adult colobus. He was done in Gombe and to some extend in developed an exceptional persistency, con- Mahale, whereas in Tai’, in order not to slow tinuing the hunt on and on until capture was down the dificult process of habituation we achieved. The other hunters were also aware did not use this method. Tai’ males hunt al- of his behavioral change and waited for most permanently in groups (see Table 9), Snoopy to reactivate the hunt. A subsequent and we were present before a hunt began 82 decline in success was because of the sudden times out of 100 predatory episodes; thus we noninvolvement of this young male during probably had a fair chance to witness almost the month of February, 1986. If we excluded all hunts while following the male core group. this month from the computations of the sec- Table 7 presents our data on hunts, both ond period, the rate of success would be 89% attempts and successes, classified according for 37 hunts. to the prey species. No important differences in hunting suc- Prey reactions cess for the two major prey, the red and black We considered four different types of prey and white colobus, are apparent. The overall reaction when faced with the hunters: 1) success rate in Tai is slightly larger than in freezing reaction; 2) flight reaction; 3) attack Gombe but not significantly (Gombe success of a single prey against the hunter($ and 4) rate for 10 years of red colobus hunts = mobbing reaction of many prey together 41.4%;X2 = 0.85, P > 0.05). No comparable against the hunters. data exist from Mahale. In colobus monkeys, the first two reactions Similar results between Tai and Gombe were always combined, depending on the were obtained for multiple kill rates; during hunter’s movements. As long as they were a hunt chimpanzees regularly succeed in cap- unnoticed by the hunters, they would freeze. HUNTING BEHAVIOR OF CHIMPANZEES 557

TABLE 8. Comparison of red colobus reactions when hunted by chimpanzees in Gombe and Tat' Attack by Success of Number of Flight + a single attack + hunts freeze colobus Mobbing mobbing Tai' 68 61 1 10 2 Gombe 64 - 15 3 13 'See text for further explanations. Success is defined as a resultant stop of the hunt. Pata on Gomhe come from Busse (1977).A group of prey may react in several ways, so the sum of reactions exceeds the number of hunts observed.

If the hunters approached, they would flee 5.96, P < 0.05). Goodall (1986), analyzing a farther away and freeze again. Thus we shall different period, reported an even higher rate analyze these two reactions together. Table 8 of success than does Busse: chimpanzees ran presents observable red colobus reactions away in 16 out of 19 colobus attacks. This when being hunted and compares responses comparison tends to indicate that Tai' chim- in Gombe and Tai. panzees are less impressed by the colobus We never observed a single adult colobus attacks than Gombe chimpanzees. Tai colo- chasing away an adult chimpanzee. (The only bus rely mainly on mobbing when they re- observation of this nature in Table 8 was spond aggressively to being hunted, although against a subadult female.) Even mobbing, their success rate is low. Descriptions pro- which includes at least four adult colobus at vided by Goodall (1986) of Gombe chimpan- one time, was never successful against an zees fleeing from colobus sound incredible to experienced hunter, who may even capture someone acquainted with the chimpanzee- one of the mobbers (two observations). Four colobus interactions in Tai. mobbings were successful inasmuch as they were directed toward young adults momen- tarily alone that retreated to the ground; the Size of the hunting groups hunt continue however. Two were successful According to the definitions we proposed at in that the hunt stopped__ as a result of the the beginning of this paper, we always differ- mobbing. entiate between bystanders, i.e., group mem- In Gombe and in Mahale, adult colobus bers watching the hunt passively, and monkeys can successfuIly chase away single hunters. Of the 100 predatory episodes ob- adult male chimpanzees and even groups of served during the 2-year period, we recorded them (see Goodall, 1986). Busse (1977) gives the number of hunters acting at the same some details on the prey's response when time during 80 of these hunts, using only hunted by chimpanzees. identified individuals. The number of hunt- Red colobus react aggressively in both Tai' ers varies during each hunt, and there is a and Gombe with the same frequency (X2 = tendency €or some hunters to wait and look 2.09, P > 0.05). In Tai', the mobbing reaction for the reaction of the prey when they are is more frequent than in Gombe (X2 = 12.36, driven by some of the chimpanzees before P < 0.001) but the success of the colobus taking part in the hunt. For Table 9, we reaction is lower in Tai' than in Gombe (X2 = considered only the maximum number of

TABLE 9. Size of hunting groups and other parameters among the Tai' Chimpanzees for the 2-year period (only for collaboration, the highest organizational level of group hunt, was the sample site large enough for each class) Duration of Number Number of Rate of hunts of hunts success > 7 minutes Collaboration hunters No. % No. % No. % No. % 1 6 7.5 1 16 1 16 0 0 2 11 13.7 1 9 7 63 3 27 3 14 17.5 6 42 9 64 6 42 4 15 18.7 10 66 11 73 10 71' 5 19 23.7 9 47 15 78 16 84 6 8 10.0 4 50 8 100 7 100' +G 7 8.7 6 85 7 100 7 100 'We could not determine if collaboration was used by the hunters for one of the hunts. 558 C. BOESCH AND H. BOESCH

TABLE 10. Comparison of group hunting tendencies in Tai, Gombe, and Mahale chimpanzees Solitary hunts Group hunts Nn. % No. % Total Tai 6 7 74 93 80 Gombe 55 64 31 36 86 Mahale 26 76 8 24 34 hunters in each of these hunts, recording only panzees (Tai’ versus Gombe: X2 = 54.42, P < adult animals. 0.001; Tai’ versus Mahale: X2 = 52.84, P < Table 9 shows that Tai’ chimpanzees hunt 0.001). The difference is more extreme for as a rule in groups: 9215% of all hunts include hunts involving only red colobus (Tai’ group at least two hunters acting together against hunting tendency = 94%, Gombe = 31.2%). the same prey. This result, combined with Furthermore, when hunting in groups, each the fact that the majority of the hunts seem Gombe chimpanzee often tends to hunt in a to be decided before any prey is visible to the different direction toward different prey hunters, gives us some clues as to how Tai’ (Goodall, personal communication), which chimpanzees hunt. The critical factor in ini- would be classified as solitary hunts in Tai’. tiating a hunt seems to be the presence of Some questions arise concerning the rea- other group members ready to hunt. The sons why chimpanzees show such a strong choice of a prey is restricted to the animals tendency to hunt in groups in Tai’. Is there a that can regularly be found within a time real need to hunt in groups? Is the hunting span of 10-20 minutes (for searching time success related to the hunting group size? see opportunism verses intentional hunting Table 9 presents Tai‘ data on these aspects as above) from the moment the chimpanzees be- well as on the duration of hunts and fre- gin to search for prey. In the Tai’ forest, diur- quency of collaboration. nal social monkeys have a density fluctuating Hunting success varies according to the from 15 to 66 individuals per km2 (Galat and number of hunters remaining very low for Galat-Luong, 1985) and are the most com- one (16%) or two (9%) hunters and increasing monly encountered animals. The red and the sharply for larger hunting groups (one and black and white colobus, with densities of 66 two hunters versus three or more hunters: and 23 ind/km2 respectively, and weights at X2 = 8.64, P < 0.01). Tai’ chimpanzees seem least double that of the Cercopithecus spe- to be aware of this difference, and the first cies, seem to be the optimal prey for hunters lone hunter behaves so as to attract more that can easily climb trees. Hence the spe- hunters rather than trying to catch the prey cialization in colobus monkeys of the Tai’ on his own. He will follow the prey slowly, chimpanzees can be understood as the result making them produce alarm calls. Some- of a hunting strategy at an early time of times his behavior may even look deceitful, decision-making for hunting. in that he gives “hunting barks,” although Published data on East African chimpan- he is never in the situation that normally zees allow us to compare solitary and group elicits such a call, i.e., rapidly pursuing or hunts (Table lo), including all observed hunts about to capture a prey. If no chimpanzees for Tai’ and Mahale. For Gombe, only data on join the hunt, he will normally stop (see be- baboon (Teleki, 1973) and red colobus (Busse, low for hunt durations in relation to group 1978) hunts are available. They account for size). The only capture by a lone hunter we 67% of all hunts (from 1972 to 1981 in Good- observed was made on an adult colobus that all, 1986); the rest often involve bushbuck. fell into the lower canopy as a result of strug- Typically, hunts of bushbuck infants in gles with other colobus. They seemed not to Gombe seem to be seizures of frozen prey by be aware of the chimpanzee’s presence un- solitary individuals, so Teleki’s and Busse’s derneath the tree. Groups of two hunters may data may somehow overestimate the group persevere for longer periods of time, but, in- hunting tendency. dependently of their skills, the presence of a The frequency of group hunts is similar in third hunter is necessary to make a capture Gombe and Mahale (X2 = 1.21, P > 0.05). more certain. Indeed, when a third joins the Gombe as well as Mahale chimpanzees hunt hunt, the speed of all movements increases in groups significantly less than do Tai’ chim- sharply. HUNTING BEHAVIOR OF CHIMPANZEES 559 TABLE 11. Group hunts for chimpanzees from Tail Gombe, and Mahale classified according to cooperation level' No. of group Similarity Synchrony Coordination Collaboration hunts No. % No. 9% No. % No. %

TaP I2 5 I 9 12 9 -12 49 68 Gombe 31 - 25 81 6 19 Mahale 8 - 8 100 - 0 0 'For each hunt, we recorded only the highest level of organization reached by the hunters. Tai' and Mahale data include all observed hunts. Gombe data include 2 years of red colobus hunts (Busse, 1978) and 1 year of baboon hunts (Teleki, 1973)

TABLE 12. Agelsex classes and roles of the Taf chimpanzees Age and sex Bystander Hunter Captor Adult males 132 221 21 Adolescent males 24 60 4 Adult females 81 33 I Adolescent females 31 11 - Total 214 331 38

A correlation between number of hunters 9). However, a comparison of the three fac- with hunting success is found (Spearman tors using a Kendall partial rank correlation rank correlation coefficient: rs = 0.86, P < coefficient (Siege], 1956) shows that the level 0.051, as well as with hunting duration of cooperation is related to the number of (Spearman rank correlation coefficient: rs = hunters, whereas hunting success depends 0.96, P < 0.01). Hunters persevere longer as on both the number of hunters and the coop- their number increases, and this may ex- erative level used (rxy.2 = 0.96). The data plain their greater success. Hunting in are not large enough to estimate the relative groups implies waiting for others before contribution to the hunting success of each hunting and accepting that a larger propor- cooperative level. tion of the capture will be shared (see Meat sharing, below). These costs seem to be com- Cooperation pensated for by an important increase in In Table 11, we present all the hunts in hunting success with three or more hunters. which we could determine the group hunting The decrease in success observed in groups techniques used, evaluated by recording each of four or five hunters may reflect difficulties individual's actions (see definitions in Table encountered by the hunters in organizing 1).As can be seen, TaY chimpanzes hunt as a themselves when hunting, but such difficul- rule by collaboration (68%). ties are seemingly resolved when six or more Table 11 also compares the data on collab- hunters participate (Table 9). orative hunting from Gombe and Mahale The available data from Gombe show no with those of Tai'. Tai' chimpanzees collabo- increase in hunting success according to the rate significantly more often than do Gombe number of hunters. Apparently solitary and Mahale chimpanzees (X2 = 18.74 and X2 hunts in Gombe are more successful than in = 11.32 respectively, both P < 0.001). If the Tai'. (Busse, 1978; Goodall, 1986; Teleki, same comparison is made between Tai' and 1973). The pressure existing in Tai' for group Gombe only for red colobus hunts, the differ- hunting may not exist in Gombe. ence is even larger (Tai' collaborative level = The hunting groups in Tai' must be consid- 69.8%, Gombe = 10%). The differences in ered as cooperative sensu Hamilton (1964) collaborative hunts between Gombe, Ma- since hunting success increases with the hale, and Tai' are clear-cut. number of hunters; such evidence is missing for hunting groups in Gombe and Mahale. Agelsex of the hunter The proportion of cooperation by collabora- Table 12 presents the agehex classes of all tions is directly related to the size of the observed hunters. Juveniles and even infants hunting groups (Spearman rank correlation show some interest in hunting but rarely go coefficient: rs = 0.99, P < 0.011, as well as to further than some intentional movements to- hunting success (rs = 0.83. P < 0.05) (Table ward the prey. One exception is a young ju- 560 C. BOESCH AND H. BOESCH venile male who permanently associates with kills, whereas in Tai' during a 2-year period the alpha male and who has developed an the proportion was 18%. Thus the hunting unusual interest in hunting. He even suc- participation of the females might be similar ceeded in capturing an infant red colobus at both sites. The female capture rate from when he was only 6 years old. Mahale seems to be comparable to that of Tai' To be confident of the identity of the captor, as well; Takahata et al. (1984) attribute 29% we have to observe the capture, as a prey can of the kills to females (TaT versus Mahale, X2 be stolen within the first few minutes. There- = 0.29, P > 0.05). fore captors were less often identified than other roles. Adolescents of both sexes ac- Killing techniques tively take an important part in hunts, and Because of poor visibility in the dense for- we observed that they willingly take up the est, we can rarely observe the killing of the driving role. Some differences appear when prey precisely. Some tendencies were, never- we compare the adults classes: theless, apparent:

1. Significantly fewer females are present 1. Infant/juvenile colobus are always im- at hunts than males (community composi- mediately bitten in the head and then trans- tion; 9 adult males and 26 adult females, ported in their captor's mouth. Death comparison between community composition immediately follows the first bite. versus bystanders + hunters: X2 = 35.72, P 2. Adult colobus are rarely killed by their < 0.001). Females tend to appear at the captor immediately. The chimpanzees begin hunting site once the capture has been made to eat them while they are still alive, usually but do not rush to join an audible hunt, a by biting open the belly and pulling out the behavior in strong opposition to that of the viscera. Death generally follows such a dis- males. embowelling, occurring - 2-4 minutes after 2. Females, when present during a hunt, the capture (11 observations). On three occa- are less active than males (comparison of by- sions an adult colobus was seen to be dead standers versus hunters: X2 = 42.99, P < while still intact, and large quarrels occurred 0.001). When present, females follow the hunt between males over ownership, indicating from the ground, remaining close to the ac- that the prey can be killed properly when tion so as to reach the capture site rapidly. strong competition over ownership occurs. On This is necessary if they are to claim a share one occasion the alpha male chimpanzee of the meat before division occurs, when own- killed an adult colobus by neck bite just after ership is not yet clearly marked and begging the capture. is not yet necessary. However, some females, Descriptions from Gombe are fairly similar with or without an infant to carry, do ac- for infant prey, but important differences are tively hunt (13% of the adult hunters are observed for larger prey. Adult colobus may females) (Table 12). be killed by methods never observed in Tai', 3. When hunting, females have a capture such as flailing the body so that the head is rate similar to males (X2 = 0.88, P > 0.05). smashed against the branches, rocks, or the Females are apparently physically capable of ground. Of 19 adult colobus captured in capturing a prey such as an adult colobus. Gombe (Goodall, 1986 p. 291), only 6 were The sexes differ in their general sociability quickly dispatched, partially by disembowel- inasmuch as females tend to forage away ling. All the others took more than 10 min- from the main core group more frequently utes to die (more than 40-minute struggles than males. Therefore the number of females are reported), and disembowelling, if at all, hunting, when no males are present, is happened only late in the consumption pro- underestimated, as we generally followed the cess. These difficulties in killing an adult main male core group. Recent observations prey were attributed by Teleki (1981) to the of groups of three females (three observa- poor biting capacities of Gombe chimpanzees, tions) indeed revealed that females may hunt but our observations differ. In Tai, division successfully even for heavy prey, such as of the prey is the rule for an adult prey, and adult black and white colobus. death always occurs within the first 4 For Gombe, Goodall (1986, p. 307) presents minutes. some data on the sexes involved in the hunts. When present during a hunt, females tend to Division of the prey hunt with the same frequency in Gombe as The capture is generally followed by an in Tai' (X2 = 0.54, P > 0.05). Over a 7-year outburst of screams that signals the success period, females in Gombe made 23% of the of the hunt and attracts chimpanzees within HUNTING BEHAVIOR OF CHIMPANZEES 561

TABLE 13. Outcome of contests for possession of the prey following capture, according to size ofprey Size of prey Respect Theft Transfer Division Total Infant/juvenile 22 3 1 2 28 Adult 3 1 4 25 33 Total 25 4 5 27 61 auditory distance. In Tai, some hunts are In Tai, small prey are usually kept wholly totally silent, and sometimes we knew that by their captor, while large prey are divided they had happened only from the sound of among two or three chimpanzees (Table 13) cracking bones. When an outburst of screams (X2 = 26.19, P < 0.001). occurs, a period of social excitement follows, The captor is one of the participants in the during which the captor of the prey is chal- division if he is a high-ranking male. The lenged by many adult males who are all in- second individual allowed by the group to terested in gaining access to a large portion take part in the division must also be high- of the meat. The outcome of these contests ranking. Analysis of who was allowed to take are not easy to follow, as many individuals part in such a division revealed a clear join in what may look like a “mad rush” to change in the dominance rank order of some get a piece of meat. Much aggression is also individuals. Dominance in wild chimpanzees seen between participating individuals. This is difficult to determine in a straightforward period of high excitement ends when owner- way, as an individual’s status depends in ship of meat portions is generally accepted part on the presence in his group of some and the meat consumption can begin. In Ta- potential allies. As the composition of such a ble 13, we present the general outcome of group fluctuates all day long, we may say these contests for ownership from the point that the individual’s status may to some ex- of view of the captor. We use the following tent vary accordingly. This applies to domi- four categories: respect indicates that the nance order in a social context as well as in owner was able to keep the whole prey; theft the context of prey division. In the last con- indicates that the owner lost the whole prey text, our sample size is too small to allow a to another chimpanzee; transfer occurred if precise analysis of the dominance changes the owner released the whole prey on his own for all individuals. However, some domi- initiative, normally during a social display nance order could be found in both contexts and usually to a third party; and division and was clear for the four highest ranks. Two occurred when the prey was cut in two pieces, spectacular changes were observed, as shown generally between the ribcage and the rump. in Table 14. Division, which normally occurs between Schubert, the beta male in social context, two individuals, does not exclude respect. Out dropped to the fourth rank in the division of the 19 divisions in which we knew captor, context, and he needed a complete 11 months he initiated 10 divisions and kept half of the to generalize his social rank to the division prey; in five situations he was allowed by the context. Ondine, the alpha female, rose to the more dominant animals to cut himself a piece third rank among the males in the division of meat before they proceeded to the division context, supplanting all males except Bru- among themselves; and in the last four cases, tus, the alpha male, and Falstaff, the oldest the captor was left empty-handed, although one. She was able to face the combined he later received some meat through shar- aggressions of up to five males to get her ing. In these four cases, division must also be place in the center of the division process. considered as theft and added to that cate- No such long-lasting changes in the hier- gory in Table 13. Theft is slightly higher for archy according to the context were de- large than for small prey (15% and lo%, re- scribed in Gombe. Females were regularly spectively). Captors of all prey were re- dominated by males, although they could spected in 86% of the captures. This partly sometimes manage to conserve their cap- reflects the fact that captors dominant indi- tures against adult males. Out of 19 captures viduals and consequently respected by the made by females in mixed groups, 12 were others. The Gombe stealing rate of whole kept by female captors (Goodall, 1986). In prey is similar to that found in TaY (Gombe; Tai’, we observed seven such situations. In 16 stealings out of 199 kills: X2 = 1.14, P > six of them the female succeeded in keeping 0.05; Goodall, 1986). their prey, and an adult prey (three cases) 562 C. BOESCH AND H. BOESCH

TABLE 14. Dominance order in 1984-1985 for all adult males and the dominant female of the Tat community in two different contexts, social and prey division Social rank Division rank Chimpanzee Sex order order Brutus M 1 1 Schubert M 2 4 Falstaff M 3 2 Macho M 4 5 Ulysse M 5l 6l Rousseau M 6l 7' Kendo M 7' 8' Darwin M 8l 9' Snoopy M g1 10' Ondine F 102 3 'The rank order hetween these young males was far from being so clear-cut, but we present this order based on the subsequent variation in their order observed in 1986. 'Ondine was probably the highest ranking female and as such could dominate young adult males in some social contests. In relaxed situations, she showed submissive behavior (pant-grunts) to some of them, i.e., Macho and Ulysse. was pooled between two or three of them; Some techniques of meat eating described in together they resisted the males' attacks, the Gombe have newer been observed thus far in ownership shifting between the three of Tal: first, twisting a limb round and round them. until it detached; second, biting into the prey's face in order to suck up and drink the Meat consumption and piece preferences blood; and third, consumption of the fecal Table 15 presents the sequential order in content of the large intestine. Goodall (1986) which prey are eaten by Tai' chimpanzees. and Teleki (1973) note that for Gombe chim- Infant colobus are eaten head first and then panzees the brain is a favorite item rarely downward through the vertical axis, the prey shared, and if division occurs the owner re- being held in the hands like an ice cream tains the heads end for himself. In Tai', as in cone. Adults are disembowelled first, and Gombe, if the sku11 is easy to open (infant then the limbs are eaten, with the head last. prey), the brain is never shared. For adult Tai' owners of large portions of meat (the prey, however, the skull was shared in 84% performers of division) normally keep either of the 25 cases in which we observed the the ribcage or the rump until last, nibbling consumption of the brain. Often the skull at them until they are clean. was shared many times, the brain being ea- Teleki (1973) found that in Gombe the prey ten only by the third or fourth owner. In the were in general consumed in the following only episode in which the chimpanzees did order; first the viscera, the chest, the ribcage, not eat all the prey, they left uneaten both and the limbs, and finally the head. This hindlegs, part of the ribcage, some bones, and sounds similar to Tai' adult prey consump- the complete skull. So it seems that in Gombe tion, except for the late limb consumption. the brain is much more prized than in Tai'.

TABLE 15. Sequential order of consumption of the different parts of a colobus prey according to its size Order Size eaten Bodv Dart

Infants (N = 23) 1st 87% bead, 8% limbs, 4% throat 2nd 82% ribcage, 8%viscera, 4% head or rump 3rd 82% viscera, 13% rump, 4% limbs 4th 74% limbs, 13%rump or ribcage 5th 61% rump, 39% limb Adults (N = 25) 1st 100% viscera 2nd 100% limbs 3rd 48% ribcage or rump, 4% head 4th 48% ribcage or rump, 4% head 5th 92% head, 4% ribcage or rump HUNTING BEHAVIOR OF CHIMPANZEES 563 Tai' chimpanzees always opened the skull ber of individuals involved in meat eating. of adult colobus by breaking the bones at the Tai' chimpanzees eat their prey more quickly back around the foramen magnum (ten ob- than Gombe chimpanzees (X2 = 39.44, P < servations). In Gombe, five out of seven skulls 0.001). The speed of consumption at Tai' is were broken through the top frontal part of related to both the size of the prey and the the cranium, and the brain was eaten from number of chimpanzees attending the meat there (Teleki, 1973); the others were opened eating. More chimpanzees seem to feed on a as in Tai'. Both in Tai' and in Gombe, meat is prey in Tai' than in Gombe; Gombe: eight commonly eaten by "wadging" with fresh chimpanzees eating, N = 12 kills (Teleki, green leaves (Goodall, 1986), but Gombe 1973); Tai': ten chimpanzees eating, N = 52 chimpanzees rarely swallow their wadges kills). The meat-sharing frequency (see be- (Teleki, 19731, whereas Tai' chimpanzees were low) may partly explain this difference in always seen to do so. meat-eating duration. On December 28, 1985, a small group of five chimpanzees, including four adult males, captured and ate an adult black and white Meat sharing colobus female as well as her infant (minimal Despite the capacity of chimpanzees to eat estimated weight of both individuals 10 kg). big portions of meat (up to 1.4 kg), they share This was the only case in which chimpanzees meat even from small prey, which could eas- were seen not to eat all the prey, probably ily be eaten by one individual. Table 16 pre- because of the small number of participants. sents the Tai' data on all episodes of meat The remains weighed 3 kg, so five chimpan- sharing. The frequency of meat sharing is zees were satiated with 7 kg of meat and 0.72 episodes per minutes in TaY (N = 3,542 bone, i.e., 1.4 kg per individual. Chimpan- minutes of observations), and 0.13 per min- zees rarely have the opportunity to eat such utes in Gombe (N = 2,580 minutes of obser- an amount of meat. In Gombe, out of 39 cap- vations; Teleki, 1973). In TaY, meat is shared tures, ten captors disappeared silently with 5.5 times more often than in Gombe, and their prey, thus avoiding having to share thus a larger amount of meat can be eaten meat and the risk of the prey being stolen by a larger number of animals in the same (Busse, 1978).In Tai', we observed this behav- amount of time, which may explain why prey ior only once out of 61 captures. Tai' chimpan- in Tai' are consumed more rapidly than in zees less often avoided conspecifics after the Gombe. capture (X2 = 11.65, P < 0.001). They seemed Sharing is influenced by the quantity of ready to share and did not avoid the risk of meat available. If we compare the sharing losing the prey to another individual (steal- frequency in Tai' for adult versus infantlju- ing rates are the same in both populations). venile prey, the difference is important: in- Tai' chimpanzees feed for an average of 103 fantijuvenile: 0.49 sharingiminute, N = 15, minutes on a prey (N = 39 eating episodes). total time of meat eating = 526 minutes; Infants are eaten more quickly than adult adults: 0.84 sharinglminutes, N = 22, total prey (infant/juvenile: N = 16, average = 48 time of meat eating = 2,266 minutes. Tai minutes, range = 17-140 minutes; adult: N chimpanzees share meat from small prey 1.7 = 23, average = 141 minutes, range = 80- times less often than from large ones. Com- 220 minutes). In Gombe, Teleki (1973) ob- parisons between Gombe and Tai', only €or served 12 eating episodes with an average small prey, show that in Tai', sharing is still duration of 215 minutes (total time = 43 3.8 times higher than in Gombe. hours, range = 90-540 minutes). He states In Tai', about 90% of the sharing implies that the size of the prey affects the duration close contact between the owner and the beg- of the meat consumption less than the num- gar, both touching the meat (Table 16). More

TABLE 16. Meat sharing occurrences observed during the 2-year period including 59 hours (3,542 minutes) of meat eating observed during 48 hunts (we differentiate, in addition, between recovery in the absence (S21) and presence (S22) of food owner) Recovery Recovery Active- with owner with owner Passive passive Active Theft absent present sharing sharing sharing Gift (S1) (521) 622) (53) (54) (S5) 66) Total Number 13 150 87 1,321 82 1 140 37 2,569 % 0.5 5.8 3.4 51.4 31.9 5.4 1.4 564 C. BOESCH AND H.BOESCH

TABLE 17. Meat sharing observed during the 2-year period including 48 hunts in which both the meat owner and the beneficiary of the sharing were identified Owner Beneficiary of meat of sharing s1 s21 s22 s3 54 s5 S6 Total Adult male Adult male 6 50 42 206 84 50 14 452 Adult male Adult female 3 30 10 543 142 63 12 803 Adult male Subadult - 7 4 222 26 5 - 264 Adult female Adult male 4 8 7 144 11 7 1 182 Adult female Adult female - 10 6 138 11 7 5 177 Adult female Subadult - 2 1 189 14 18 7 23 1 Total 13 107 70 1,442 288 150 39 2,109 than half is passively shared, and typically in an active-passive manner (S4) or in an the meat is put in common by the owner. active manner (S5 and 6) than do females. In Both individuals, the owner and the benefi- addition, males actively share significant1 ciary, hold and eat the same piece of meat for more with other males than with females (X3 periods of time regularly exceeding 15 min- = 94.98, df = 3, P < 0.001). This higher utes. In such situations, we were able to iden- generosity of the males toward other males tify the food owner only because we were is illustrated by the gifts. Gifts are normally present before the piece was pooled. Simi- observed when the owner has almost fin- larly, in active-passive sharings (31.9% of all ished his meat and then presents the re- episodes) the owner, while masticating, held mains to another individual (Gombe the meat toward another individual. As a descriptions are similar for such gifts; Good- rule, a long piece of meat (i.e., an anterior or all, 1986; Teleki, 1973). In Tal, occasionally posterior half of a colobus) is pooled by the at the beginning or during the meat con- owner with another individual (passive shar- sumption, males may cut off a small piece for ing). In contrast, with a short but large piece themselves (commonly 3-5 vertebrae) and of meat (i.e., the complete rump or thorax of give the much larger piece to a nearby male, a colobus), the owner will have a bite and which gives an impression of generosity. For then hold the piece toward a beggar, allow- the males, sharing seems to have an impor- ing the beggar to have a bite while the owner tant social value, and we hypothesize that is chewing his piece (active-passive sharing). such generosity is related to social status; the Active sharing does not necessarily happen higher a male is in the hierarchy, the more between dyads activeIy engaged in passive generous he is in meat sharing. and active-passive sharing, as both eat their Active sharing accounts in Gombe only for fill. Active sharing occurs regularly (5.4% of four sharings out of 335 observations (Teleki, all sharings). 1973). This frequency is significantly lower Table 17 presents the details of the sharing than in Tai' (X2 = 16,45, P < 0.001). Tai' episodes classified according to the agehex chimpanzees share meat more frequently classes of the owner. Male owners initiate than Gombe chimpanzees, and the sharing 78.6% of the active sharing and 66.6% of the quality seems to differ as well. gifts. The difference between the sharing be- Tai' adult males, as regular owners of large havior of female and male owners is signifi- portions of meat, are the providers of meat cant (X2 = 50.87, df = 3, excluding S1, P < in 72% of the sharing episodes observed (see 0.001). Males tend to share meat more often Table 18). Adult females provide meat in 28%

TABLE 18. Proportion of meat shared, measured in percentage of sharing occurrences observed between the different agdsex classes in Tai'(see Table 17) and Gombe (calculated from Teleki, 1973, p. 147, Fig. 11, including 623 sharing occurrences) Owner of Beneficiary Tai' Gombe meat of sharing (%I (%I Adult male Adult male 21.43 38.47 Adult male Adult female 38.07 40.90 Adult male Subadult 12.51 9.53 Adult female Adult male 8.62 0.51 Adult female Adult female 8.39 1.03 Adult female Subadul t 10.95 9.18 HUNTING BEHAVIOR OF CHIMPANZEES 565 of the episodes, which is, nevertheless, higher threatening group of adult red colobus, as than the frequency of their involvement in soon as he went into the trees. He tried to hunting (only 13%).It is relevant that adult avoid them, but they followed him. After 4 females provide meat to adult males in 61% minutes he broke a fresh branch and threat- of their sharing episodes with adults and that ened them once with it, by waving it towards adult males obtain meat from adult femaIes them. Then he threw the branch at the two in 29% of the cases. nearest colobus, hitting them. The colobus Comparisons with Gombe are not straight- retreated. forward, because Teleki (1973, p. 147) pools requesting and meat sharing behaviors. A In Tai', tools are also regularly used for first look at the interactions between the processing the prey during the consumption. adults shows that males are targets of re- Chimpanzees use them in two different ways questing and meat taking in 98% of the in order to facilitate access to the brain and interactions, whereas Tai males are sig- bone marrow: nificantly less often providers of meat through sharing (in 77% of the sharings 1. To open skulls (proto-tool use). The skull events between adults) (X2 = 101.30, df = 1, of an adult colobus is quite hard, and opening P < 0.001). In opposition, the interactions of it with the teeth requires a lot of strength. In adult males and females with the subadults some cases (3 out of 15 adult skulls observed are highly similar in both populations (X2 = to be opened by the chimpanzees in Tai'), the 0.12, df = 1, P = > 0.05). These comparisons opening process was facilitated by banging are justified only if the meat requests were the skull held in the hand against a hard equally successful in both populations. How- surface, a root or a tree trunk. Tai chimpan- ever, the data to test this assumption are not zees are also regularly observed to use such available. a banging behavior for three species of large These comparisons between Tai' and Gombe or hard fruit (Treculia africana, Strychnos suggest qualitative differences in the shar- aculeata, and Landolphia hirsuta @]I, and the ing patterns between the two population. same activity has been observed in East Af- Sharing in Tai occurs more frequently and rica for Strychnos. Thus we conclude that Tai' more actively between more individuals, and chimpanzees have generalized this move- females seem more often to be important pro- ment to hard skulls. The behavior has been viders of meat for adult males and females. labeled as proto-tool use by Parker and Gib- It may be relevant to note that Tai' chimpan- son (1977). zees generally eat meat on the ground (65% 2. Marrow extraction. A true tool use is of the cases), whereas Gombe chimpanzees performed to extract the marrow from bones. consume meat typically in trees, apparently After having broken the head of the large to avoid the beggars (Goodall, 1986; Wran- bones (femur, tibia, and humerus) with their gham, 1975). teeth, the chimpanzees repeatedly dip small sticks (which they have previously fashioned) Tool use during hunting and meat eating into the soft marrow material of the bones; Wild chimpanzees regularly use tools in they then lick the end of the stick. Out of 28 many different contexts and often manufac- observations, such sticks were used for emp- ture them from vegetable material, i.e., tying bones in 26. In one of the exceptions, a grass, leaves, or wood. In social contexts they juvenile who could not break open the skull may throw clubs and stones at other group tasted some of the brain by poking a stick members to impress or threaten them (see through the foramen magnum. In the second also Goodall, 1986). In Gombe, tool use was case, a stick was used to clean the vertebral observed three times in a hunting context canal of some tail vertebrae of a colobus. (Goodall, 1986; Plooij, 1978). Stones were thrown in order to put the animals at which DISCUSSION they were aimed (one bushpig and two ba- Environmental influences on hunting boons) to flight; the intent was not appar- behavior ently to harm the target and facilitate its Our comparisons of a forest-living popula- subsequent capture. tion of chimpanzees (Tai) with two popula- In Tai, one use of tools as a defense weapon tions living in a more open savanna/ during a hunt was observed: woodland habitat (Gombe and Mahale), lead to the conclusion that the forest population On March 23,1985, Darwin, a young adult seems more efficient in hunting than the sa- male chimpanzee, was surrounded by a vanna ones. Tai' forest chimpanzees tend to 566 C. BOESCH AND H. BOESCH hunt more frequently for larger prey, with a group; Nishida, 1979). In such situations, im- higher rate of success, and they hunt more portant demands act on cognition to solve often in groups with a higher level of orga- orientation tasks (see, for example, stone nization than do chimpanzees living in a transportation in Boesch and Boesch, 1984a). more open environment. Sharing of meat is Our study also shows that forest-living chim- more frequent and more generalized in for- panzees developed nut-cracking behavior, one est-living chimpanzees. These results con- of the most sophisticated tool uses observed trast with the widely accepted influence of in chimpanzees (Boesch and Boesch, 1981 the environment on these behaviors in the 1984b; see also Goodall, 1986; p. 545). In con- evolution of the hominids. Paleoanthropolo- clusion, the “open environment hypothesis” gists were struck by the fact that all remains for hominization should be revised in the of early hominids found in East Africa were light of these results. east of the Rift Valley, in savanna regions, and that all known living pongids live mainly Cross-population differences in a forest environment west of the Rift Val- Clearly not all the differences observed be- ley (see for example, Coppens, 1983; Isaac, tween these chimpanzee populations are re- 1978; Johansen and Edey, 1981; Leakey, lated to environmental differences. The forest 1981; Leakey and Lewin, 1978; Pilbeam, structure imposes constraints, but the solu- 1980). They suggest that the transition from tions adopted by the chimpanzees are func- a forest to a savanna-like environment in tions of their physical and cognitive abilities. early hominids has favored some important One way to discriminate between these pa- physical changes, such as bipedalism, and rameters would be to make a comparison some major behavioral adaptations, includ- involving a close match of as many different ing the appearance of hunting, tool use and independent variables as possible. The hunt- tool making, food sharing, cooperation, and ing behavior illustrates, first, interactions use of a home base. In this perspective, the between two animal species involving mainly data on the behavior of wild chimpanzees in their physical and cognitive abilities. Sec- these two environments could serve as a test ond, we compare the same species of preda- of this hypothesis. We conclude that the su- tor, the chimpanzee (P troglodytes) and its perior huntings performances of the Tai‘ actions relative to one main prey species, the chimpanzees clearly contradicts it. The chal- red colobus (C. badius). Third, biases due to lenges of forest life may have been underes- observational conditions are limited, as the timated relative to those presented in the studies are basically similar, i.e., all are long- savanna. For example, it is easier to solve term studies of natural and spontaneous be- the visibility problem in the savanna by havior of wild-living chimpanzees. standing momentarily upright or climbing a The important and numerous differences tree, whereas this problem may never be we find between the three chimpanzee popu- solved in the dense tropical forest. In the lations already exclude any unique or simple forest, social animals are forced to rely on explanation. We shall discuss only the most acoustic rather than visual means for group obvious differences because they are the least cohesion, for group movements, and some- dependent on observational procedure dif- times for recognition of individuals. Unex- ferences. pectedly, some animal species live in larger One first set of differences seem to be re- groups in the forest than in the savanna, i.e., lated; prey choice, frequency of intentional bushpigs or chimpanzees (in TaY, groups of search for the prey, hunting group size, hunt- seven chimpanzees or more account for 50% ing duration, hunting success, and level of of the encounters and in Gombe for only 18%; organization of the hunt. Tai’ chimpanzees Goodall, 1986). Spatial orientation in a dense hunt in groups for the most common middle- environment with a visibility of 20 m at most, sized mammals (colobus monkeys). Our data as in the Tai’ forest, represents a bigger chal- show that duration of the hunt, hunting suc- lenge in finding more or less patchy food cess, and organization of group hunts are all sources than in a savanna. This factor may related to the number of hunters (see Table be critical when moving in large areas. Tai’ 9). In Gombe and Mahale, where chimpan- chimpanzees live in home ranges larger than zees most often begin to hunt opportunisti- Gombe or Mahale chimpanzees (Tai = 27 cally, the prey choice is less specialized, and km2, Gombe = 9.6-24 km2, according to the the hunting group size is smaller. Three other number of males; Goodall, 1986; Mahale = parameters (hunting duration, success, and 10.4 km2 for K-group and 13.4 km2 for M- organization level) are also smaller, as would HUNTING BEHAVIOR OF CHIMPANZEES 567 be expected if they were related to the num- descriptions of intercommunity encounters in ber of hunters as they are in Tai'. Gombe; Goodall, 1986; Wrangham, 1975). In So the important questions are: 1) why do contrast, in Gombe, there were 0.25 encoun- Tai' chimpanzees hunt in larger groups than ters per month (33 encounters in 132 months; other chimpanzee populations; and 2) does Goodall, 1986). Goodall (personal communi- the much lower hunting success for lone cation) observed that Gombe chimpanzees hunters in Tai' justify this important behav- foraged in much larger coherent groups in ioral change? We assume here that the soli- 1985-87 than during the 1968-74 period, tary hunt, the most commonly observed type when Teleki and Busse made their observa- of hunting in other chimpanzee populations, tions. During the later period, they also is more primitve than the group hunt. tended to hunt in groups more systemati- cally. This increase in group cohesion may be 1. Busse (1978) suggests that the hunting related to an increase in territorial pressure group size is related to the size of the social from neighboring communities. Thus the group prior to encountering a prey. However, probably higher rate at which strangers are because he did not distinguish between hunt- encountered in Tai' may have forced them to ers and bystanders, this relation is obvious. adopt a more cohesive group structure, which If the motivation of adults to hunt was the may subsequently lead to a higher involve- same in both populations, however, then the ment of the males in social life at the cost of size of the hunting group would be directly tool manipulation (Boesch and Boesch, related to the size of the group prior to the 1984b). hunting. As Tai' chimpanzees forage in larger 2. The increase in hunting success with groups than do Gombe chimpanzees, this ex- larger group size observed in Tai' is intrigu- planation may be true. However, data are dif- ing, because a similar increase seems not to ficult to compare. From Goodall (19681, the appear when Gombe chimpanzees hunt in mean foraging group size is 4.83 individuals groups. The higher hunting success of Tai' and the hunting group size is somewhere males seems to be more related to organiza- between 2 and 4 individuals for Gombe. In tion than numbers. When they hunt in Tai', the foraging group size is 10.28 individ- groups, they typically disperse under the uals, and the hunting group size 4.11. It may prey, often out of sight of each other but all be that the greater sociability of the Tai' concentrated on the same prey, waiting until chimpanzees can explain the larger number the progression of the hunt gives them an of hunters, as the ratio of bystanders to hunt- opportunity to act. With the progress of the ers seems to be about the same as in Gombe. hunt they tend to reunite while cornering This leads us to a new question: why are Tai' the prey. Descriptions from Gombe (Goodall, males (the hunters) in larger groups than 1986; Teleki, 1973) generally give the image Gombe males? Boesch and Boesch (1984b) of hunters either pursuing prey from a simi- observed that the Tai' males, contrary to the lar direction or dispersing while pursuing females, prefer to abandon nut-cracking different prey. Jane Goodall (personal com- when there is a risk of losing contact with munication) gained the impression that, by other male group members. Therefore there chasing prey in different directions, Gombe seems to be a pressure for the males to keep chimpanzees increased the confusion of the strong contact among themselves and to for- prey and prevented them from mobbing age in larger groups than do East African against the hunters (Gombe males are afraid chimpanzees. Predation alone cannot explain of aggressive colobus; Table 8). Thus Gombe the higher sociability of the Tai' males. Leop- males, when hunting in groups, seem to dis- ards, the only natural predators of chimpan- organize the prey's defense rather than in- zees, can be repelled without tools by a lone crease their own hunting organization, as do male (personal observation) and are also Tai' males. present in Gombe. The Tai' data are still too incomplete to The pressure of territorial fights could be measure precisely the cost and benefit of both another major factor. Encounters with neigh- strategies (lone versus group hunt), and the boring communities are fairly common (29 second question cannot be answered fully. encounters in 29 months in Tai'). It might But the clear increase of hunting success with well be that the larger a male group is, the larger groups (see Table 9) explains why Tai higher is its chance to win such territorial chimpanzees hunt in cooperative groups fights and the higher is its chance of enlarg- more consistently than do Gombe chimpan- ing its territory and its access to females (see zees. Our observations suggest that group 568 C. BOESCH AND H. BOESCH hunting may facilitate the capture of the 1986). We tend thus to think that the high prey. By increasing the organization of the sharing tendency predated the increase in hunting group, the success is further in- adult prey. Any explanation for sharing creased. If this surmise is correct, then a tendencies can be only tentative, but Tai subsequent elaboration of cooperation would chimpanzees could be influenced by early ex- lead to higher hunting success. We shall test periences in sharing. Nut-cracking behavior this proposition in a future project for the is typical of the West African forest chimpan- four cooperation levels in Tai'. In other words, zees and provides them with a high energetic cooperation could be a self-reinforcing behav- intake (Boesch and Boesch, 1984b), without ior, which, once begun, provides its own feed- requiring much energy expenditure (Gunther back for more complexity. and Boesch, in press). It also involves a high Hunting and meat eating are highly social level of nut sharing between the mother and activities, with much excitement, many dis- her offspring. Such nut sharing between plays, and frequent meat sharing. Therefore mother and infants can last longer than the some group hunting in Gombe and Tai' could youngsters' first 5 years, and the mother may be explained by its social attractiveness as share up to 50% of the nuts she opens during well. the 2 years when the infants beg with great- As hunting group size is related to the level est insistence. Sharing episode frequencies of cooperation in Tai', we should ask if coop- may reach 1-5 per minute (Boesch and eration is a consequence of hunting itself or Boesch, in preparation). Therefore it is possi- results from the preexisting cooperative be- ble that infants raised in such a sharing en- havior in the population. Tai' male chimpan- vironment for years still have, as adults, a zees always cooperate in territorial fights higher tendency to share food. One example (eight observations) and always assist indi- observed after the hunt on December 4,1985 viduals that are giving distress calls either illustrates this idea: because they are facing potential predators (snakes and leopards) or because they are Snoopy, the youngest adult male, dis- trapped in a snare (our observation was very played, unsuccessfully, for an hour within a similar to the description by Fossey [1983] of large group against an adult female who was the gorilla silverback rescuing group mem- eating an adult colobus after having resisted ber). These examples illustrate the possibil- successfully against the dominant males. ity that cooperation among unrelated Suddenly, Snoopy appeared with a large part individuals could have developed in contexts of the upper part of the colobus followed by other than hunting. two adult females, Saphir and Kiri. It was The next major differences we want to look the first time we saw him with such a large at are also related: duration of meat con- piece of meat, but to our surprise, after hav- sumption, size of sharing clusters, frequency ing fed for some time on the meat, he actively of sharing, quality of sharing, and probably gave the head, the fur, and the intact thorax also size of prey. Tai' chimpanzees have a to both females, conserving for himself only tendency to share meat frequently, so that some vertebrae on the neck. Salome, the sus- many chimpanzees are attracted to food own- pected mother of Snoopy, is one of the most ers because the success of request is high. As generous mothers that we observed, sharing more chimpanzees eat more meat, the meat- up to 70% of the nuts she opened with her 5- eating episodes are shorter, and probably year-old son. hunters try to kill the larger prey when they have the choice. This finding may explain In other words, the disposition to sharing why Tai' hunters were never observed to within the family (as observed with the nuts snatch colobus infants away from their moth- in Tai') may spread out to the whole social ers' bellies without attempting to capture the group. Of course this difference is not unre- mother as well. Why do Tai' chimpanzees lated to the environment; without the abun- have such a high sharing tendency? We have dance of nuts, chimpanzees could never have seen that Tai' chimpanzees share more large learned to crack or to share them. If the prey than small ones; large prey size could hypothesis is true, however, social factors influence the sharing frequency. Hence the embedded in the Tai' population may be cru- question is: which of these two factors was cial for the extensive meat-sharing activity. the prime mover? Availability of large prey probably does not explain the proportion of Cultural differences adults hunted, as Gombe chimpanzees ne- We can go on and look for differences be- glect them, often intentionally (Goodall, tween Tai' and Gombe chimpanzees that are HUNTING BEHAVIOR OF CHIMPANZEES 569 even more remote from environmental con- largely generalized the “dipping in holes” straints. These differences were seen in many movement with sticks than other popula- different aspects of meat-eating episodes. tions, and also adopt it in the meat-eating Some could be labeled “culinary prefer- context for bone marrow and brain. ences. ” For example, Tai’ chimpanzees keep In the context of hunting, the wild chim- the rump or the ribcage for last, consistently panzees present a large variety of differences share the brain, and always swallow their either because of environmental factors to leaf wadges. By contrast, Gombe chimpan- which the chimpanzee responds in different zees keep the brain, which they rarely share, ways, or because of social factors inherent ot for last, spit out their leaf wadges, suck the the population concerned, or for reasons not blood of their prey, and eat, with delight, the as yet understood. We think that, in this fecal content of the large intestine. sampling, many examples can be found to Other differences could be labeled “butch- illustrate all the grades of complexity that ering habits.’’ On the one hand, Tai’ chimpan- could be required to fulfil the idea of a cul- zees kill adult prey by disembowelling them. tural difference in a nonhuman primate. They open the skull from the back, power- fully bite and tear apart the four limbs when Sex differences dismembering a prey, and use sticks to ex- Chimpanzee females present the same kind tract the marrow from bones. On the other of involvement and capture rate in hunting hand, Gombe chimpanzees kill adult prey by in Gombe and Tai‘, although they have a “a combination of smashing the prey against smaller involvement than males. However, tree trunks or rocks and tearing at their Tai’ females may reach a much higher status limbs (sometimes breaking them).” (Goodall, than Gombe females during the division of 1986). They open the skull normally from the the prey and gain access to much larger front, sometimes dismember a prey by twist- pieces of meat. Furthermore, they are regu- ing a limb round and round, but were seen lar providers of meat to adult males and fe- once to use leaves to clean a skull. males during the meat-eating episodes. This We find it difficult to propose any ecological observation tends to alter, somehow, the view or sociological reasons that could explain all of the females in hunting; the Tai’ females these different variations in a satisfactory have a more important role in this so-called way. typical male activity. It is difficult for us to The differences in tool use for getting at give a satisfactory explanation of these dif- the bone marrow is of particular interest. ferences between Tai‘ and Gombe. Tai’ fe- Most wild chimpanzee populations were ob- males can be important allies of some males, served to insert sticks into holes to gain ac- i.e., Ondine is a clear associate of Brutus, the cess to food (either insects and insect products alpha male, but was also able to maintain such as honey) or to probe (for other popula- her position in his absence. Therefore it may tions than Ta?, Gombe, and Mahale, see Beck, be that Tai’ females have a higher interest in 1980; Sabater Pi, 1974; McGrew and al., 1979; some social activities. Sugiyama, 1985; Sugiyama and Koman, Whatever the reasons for the great interest 1979, 1987). Gombe chimpanzees are famous of some females in hunting, the view of hunt- for their sophisticated termite-fishing tech- ing as a male activity that is antagonistic to niques, but only Tai’ chimpanzees were ob- female activities in primates (Zihlmann, served to use sticks for meat eating. Tai’ 1981; Zihlmann and Tanner, 1978), does not chimpanzees regularly use small sticks hold true in the case of Chimpanzees. It has (maximum length about 30 cm) in many dif- also been proposed that, in human evolution, ferent contexts; to probe for wood-boring bees males traded meat for sex with estrus fe- (Xylocopa sp. ) (eight observations), to fish for males, an exchange that helps form privi- insects living under the bark of fallen trees leged long-lasting bonds with these males (one observation), to dip for driver ants (two (Lovejoy, 1981, cited in Johanson and Edey, observations), to dip for honey of three spe- 1981; Hill, 1982). We have rarely observed cies of bees (Apis sp. and two Melipona sp., this kind of trade in Tai’ chimpanzees, where 59 observations), to empty the almonds’ resi- males mostly share meat with females with due of three species of nuts (Detarium sene- which they have a good relationship indepen- galensis, Panda oleosa, and parinari excelsa, dently of their sexual cycle. 110 observations), to inspect wounds of other chimpanzees (three observations), and to in- Predation in wild chimpanzees spect dead animals (six observations). Thus Comparisons of all studies of wild chimpan- it seems that the Tai‘ chimpanzees have more zees show that hunting is observed almost 570 C. BOESCH AND H.BOESCH everywhere (see Goodall, 1986); the present problem in defining such words as “hunt” study shows that Tai‘ forest chimpanzees hunt and “cooperation.” We tried to consider only in a more elaborate way than savanna popu- the hunts for which it was possible to be lations. But not all chimpanzees living in reasonably sure that the respective observers forests hunt so much: chimpanzees living in defined these terms similarly. Therefore this the Kibale Forest, Uganda, rarely hunt (one table should not be considered as represen- hunt suspected in 2 years of study; Ghiglieri, tative of all hunts engaged in by a particular 1984). However, this contrasting observation species, especially for social carnivores in was obtained during a short period of time which solitary hunts on small prey tend to with minimally habituated chimpanzees that be underestimated. were rarely followed in their daily move- We may conclude from this table that first, ments (observations were focused at the some social carnivores may be specialized fruiting fig trees). If this result holds true, group hunters, as are wild dogs and hyenas then the suspected divergence between forest for some prey, but the proportion of their and savanna in terms of hunting behavior group hunts are still comparable with Tan would become questionable. However, it is chimpanzees’ group-hunting tendency. More- also possible that Kibale chimpanzees may over, important variations are found in this hunt quite frequently; for example, in TaY tendency according to the prey. Hyenas al- during the first 2 years of our study we saw most always hunt zebras in groups, whereas only seven hunts! with wildebeest this tendency drops to 35% Teleki (1981) and Wrangham (1975) pro- (Kruuk, 1972). Comparably, wolves seem al- pose a mean meat intake in Gombe of about ways to hunt moose in groups (94.5%), 27 g per day for each individual. We think whereas this tendency decreases to 66.6% for that such mean estimations may be mislead- the white-tailed deer and drops to 27.7% for ing in a certain sense as individual meat successful hunts upon caribou (Mech, 1970). intake can vary greatly, some eating hardly Therefore group hunts in social carnivores any or no meat and others (mainly the domi- seem to be an adaptation to the prey defense nant males) eating a large portion of meat potential: zebras always present a coherent each time (see also description in Goodall, group defense while the wildebeest, which 1986). Dominant males may eat more than can easily be singled out, do not. Moose are 300 g of meat per session. Such an intake very powerful and dangerous prey, whereas will be relevant not only qualitatively but deer and caribou rely mostly on flight for also quantitatively for those individuals that their safely. For chimpanzees, such a relation generally are also the hunters. In a chimpan- is more difficult to document, as mxt prey zee population with a hunting rate similar to present a similar defense potential, except Gombe and Tai‘, we can understand why some for bushpigs in Gombe. individuals hunt. The benefits probably com- Second, level of cooperation and group pensate the costs. However, Mahale chim- hunting do not seem to be related in social panzees hunt much less frequently, and meat carnivores: group hunts rarely involve high may play a marginal role in their diet. Thus coordination of the hunters, most of them the supplement of nutrients (proteins, vita- just pursuing the prey in a string (wolves and mins, minerals) provided by meat (Teleki, wild dogs) or as a pack fanning out behind 1981)could be covered by a low hunting rate. them (wild dogs and hyenas). A collaborative hunt has been reported only for lions and Chimpanzee versus carnivore hunting wolves. Schaller (1972) describes precisely Schaller and Lowther (1969) have proposed how hidden lioneses wait for others to drive that the hunting behavior of the social car- the prey against them and only then rush nivores could be a better model for under- toward the prey (29 observations). Among standing the behavior of early hominids than wolves, three hunts are described in which that of primates. Some comparisons of the one or two wolves hid in ambush while wait- carnivores with the primates have sharply ing for another to drive the prey toward them underestimated the hunting behavior of the (Mech, 1970). This disjunction of the fre- latter (Hill, 1982; Thompson, 1975, 1976). In quency of group hunts and level of coopera- Table 19, we present another attempt to com- tion in carnivores suggests that the pare hunting behaviors in social carnivores performance of different roles during a hunt and chimpanzees, using our definitions of the may be somewhat difficult for such animals. group hunt. Such a comparison, including If the comparisons in Table 19 are reliable, numerous different works, stumbles over the higher organization of the hunt as a normal HUNTING BEHAVIOR OF CHIMPANZEES 571

TABLE 19. Hunting strategies of social predators classified according to our definitions of cooperation' Group No. of hunt Similarity Synchrony Coordination Collaboration hunts (96) (%) (%) (%I (%) Primates Chimpanzee 80 92 6.5 11.5 14.5 63 TaP Gombe 86 36 -29 - Mahale 34 23 23 - - - Papio anubis 147 14 t- 14 - Social carnivores Lion 523 52 -c 46 - 5 Hyena' 46,164 91,35 24 91,11 - - Wild dog 54 91 - c- 91 - - Wolf3 103 86.5 f- 78.5 - 2 3 'In some cases, we could not differentiate from the description between different cooperative levels, and we pooled them. The baboon data are from Strum (1981), who presents the most elaborate observations of hunting within this species. The lion data are from Schaller (19721, including all hunts he observed. The spotted hyena data are from Kruuk (1972) and present hunts on two different prey species to illustrate the variability of their hunting behaviors; zebra hunts are the most elaborate in this predator species. The wild dog data are from Goddard and Estes (1969);we include captures they describe of small prey, which are solitary actions made while searching for larger prey. The wolf data are from Mech (1970) ?Zebra and wildebeest hunts, respectively. 3Moose, caribou and white-tailed deer hunts pooled.

We are very grateful to the following per- means of hunting appears only in the chim- sons: Prof. F. Bourlihe, who initially pro- panzees. Some abilities of the higher pri- posed this project to us; Prof. A. Aeschlimann mates may be necessary for cooperation. for his constant support and encouragement; However, the differences seen within the F. Naef, director of the Centre Suisse des chimpanzees require more study to under- Recherches Scientifiques en CBte d'lvoire for stand what factors are responsible for the logistic help; T. Zoroa Tiepkan and his staff increase of the level of cooperation in of the IET station in Tai' for their technical primates. help and friendly support in our camp life; E. In conclusion, we propose that Tai' chim- Boesch, H. Gloor, J. Goodall, R. Rungger, and panzees hunt in the most elaborate way two anonymous reviewers for their critical known for animal hunters. With the results comments; and L. Achermann, B. Marker, of the studies on the Gombe and Mahale and H. Gloor for correcting the English. chimpanzee populations, we shall be able to Finally, we wish to express particular analyze precisely the factors that influence thanks to Prof. H. Kummer for his valuable behaviors such as group hunts, cooperation guidance, critical discussion, and constant sharing, and sex differences. Such a cross- encouragment throughout the study. population analysis within chimpanzees may shed some light onto the enigma of how these factors evolved in the primate order and how they may have appeared in the early LITERATURE CITED hominids. Altman SA and Altmann J (1970) Baboon Ecology: Afri- can Field Research. Chicago: University of Chicago Press. ACKNOWLEDGMENTS Axelrod R and Hamilton WD (1981) The evolution of cooperation. Science 211:1390-1396. This study was supported by grant nos. Badrian N and Malenky RK (1984) Feeding ecology of 3.203.82 and 3.317.86 from the Swiss Na- Pan panzscus in the Lomako Forest, Zai're, In RL Sus- man (ed.): The Pygmy Chimpanzee: Evolutionary Mor- tional Science Foundation to Prof. Hans phology and Behavior. New York: Plenum Press, pp. Kummer and Dr. Christophe Boesch. We are 275-299. greatly indebted to the Ministere des Eaux Beck B (1980) Animal Tool Behavior. New York: Garland et For6ts of the Ivory Coast for their support STPM Press. and for providing facilities for working in the Boesch C and Boesch H (1981) Sex differences in the use of natural hammers by wild chimpanzees: A prelimi- Tai' National Park. This project is integrated nary report. J. Hum. Evol. 10:585-593. into the UNESCO project Tai-MAB, and we Boesch C and Boesch H (1983) Optimization of nut-crack- wish to thank the general supervisor, Dr. ing with natural hammers by wild chimpanzees. Be- Henri Dosso, for his support. havior 83:265-286. 572 C. BOESCH AND H. BOESCH

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