YEARBOOK OF PHYSICAL ANTHROPOLOGY 42:1–30 (1999)

Evolution of Coalitionary Killing

RICHARD W. WRANGHAM Department of Anthropology, Peabody Museum Harvard University, Cambridge, Massachusetts 02138 KEY WORDS chimpanzee; lethal raiding; warfare; assessment

ABSTRACT Warfare has traditionally been considered unique to hu- mans. It has, therefore, often been explained as deriving from features that are unique to humans, such as the possession of weapons or the adoption of a patriarchal ideology. Mounting evidence suggests, however, that coalitional killing of adults in neighboring groups also occurs regularly in other species, including wolves and chimpanzees. This implies that selection can favor components of intergroup aggression important to human warfare, including lethal raiding. Here I present the principal adaptive hypothesis for explaining the species distribution of intergroup coalitional killing. This is the ‘‘imbalance- of-power hypothesis,’’ which suggests that coalitional killing is the expression of a drive for dominance over neighbors. Two conditions are proposed to be both necessary and sufficient to account for coalitional killing of neighbors: (1) a state of intergroup hostility; (2) sufficient imbalances of power between parties that one party can attack the other with impunity. Under these conditions, it is suggested, selection favors the tendency to hunt and kill rivals when the costs are sufficiently low. The imbalance-of-power hypothesis has been criticized on a variety of empirical and theoretical grounds which are discussed. To be further tested, studies of the proximate determinants of aggression are needed. However, current evidence supports the hypothesis that selection has favored a hunt-and-kill propensity in chimpanzees and humans, and that coalitional killing has a long history in the evolution of both species. Yrbk Phys Anthropol 42:1–30, 1999. ௠ 1999 Wiley-Liss, Inc. TABLE OF CONTENTS

Coalitionary Killing among Chimpanzees and Other Species ...... 4 Species distribution of coalitionary killing ...... 5 The lethal-raiding problem ...... 5 Lethal raiding by chimpanzees ...... 5 Territorial defense ...... 6 Border patrols ...... 7 Deep incursions ...... 7 Coalitionary attacks ...... 8 Coalitionary kills ...... 8 The Imbalance-of-Power Hypothesis ...... 11 Explaining chimpanzee violence ...... 11 The significance of power imbalances ...... 12 Origins of power imbalances ...... 12 Group territoriality and the benefits of lethal raiding ...... 14 Sex differences in territoriality and aggressiveness ...... 16 Bonobos: exceptions that support the rule? ...... 17 The imbalance-of-power hypothesis and the evolution of human warfare ...... 18 Challenges to the Imbalance-of-Power Hypothesis ...... 20

௠ 1999 WILEY-LISS, INC. 2 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999 Uncertainty in the chimpanzee data ...... 20 The claim that biology is irrelevant for human warfare ...... 21 Implications of the Imbalance-of-Power Hypothesis ...... 22 Chimpanzee and human psychology ...... 22 The complexity of war ...... 23 The relation between lethal raiding and hunting ...... 24 Morality ...... 25 Conclusion ...... 26 Acknowledgments ...... 26 Literature Cited ...... 27

Two related but distinct hypotheses have terms of predation by carnivores and tapho- proposed that warfare has its origins in nomic processes, and the supposed bone and pre-human violence. The first is no longer horn weapons were better explained as frag- supported. This was the so-called ‘‘killer ments produced by carnivores chewing bone ape’’ hypothesis, which stated that warfare (Brain, 1981; Cartmill, 1993). The killer ape springs from an aggressive instinct that hypothesis fell into general disrepute in the began among australopithecines and contin- 1970s, even though the notion that warfare ued into humans (Dart 1953; Ardrey, 1961, evolved out of complex hunting patterns has 1966; Lorenz, 1966; Tiger, 1969). Raymond not completely died (Morris, 1977; Ferrill, Dart based this idea on South African homi- 1985). nid fossils, which he interpreted with in- In the 1980’s a second, unrelated, set of creasing pessimism after the Second World ideas arose, which I collectively call the War until eventually concluding that Austra- chimpanzee violence hypothesis (CVH). Like lopithecus africanus not only hunted other the killer ape hypothesis, the CVH proposes mammals but also killed adult conspecifics that human warfare is built on pre-human (Dart, 1953; Dart and Craig, 1959; Cartmill, tendencies. In contrast to the killer ape 1993). At the time of these ideas, intraspe- hypothesis, however, the CVH does not posit cific killing was considered to be absent in a prior history of hunting, nor an aggressive other wild mammals (including chimpan- instinct. These and other differences make zees Pan troglodytes) (Lorenz, 1966). There- the killer ape hypothesis irrelevant to the fore, killing by australopithecines was con- CVH (Table 1). The remainder of this paper sidered part of a uniquely hominid suite of is concerned with the CVH, and not with the characteristics. Lorenz (1966) lent ethologi- killer ape hypothesis with which it has cal authority to Dart’s ideas by suggesting sometimes been confused (Sussman, 1997). how killing could evolve. He proposed, for The CVH proposes that selection has fa- example, that the use of weapons, such as vored a tendency among adult males to pebble tools, could overcome natural inhibi- assess the costs and benefits of violence, and tions against killing conspecifics. Lorenz to attack rivals when the probable net ben- (1966) thus followed Dart in arguing that efits are sufficiently high. It suggests that human warfare had evolved from australopi- this tendency occurs as a result of similar thecine aggressive instincts. conditions in the lives of chimpanzee and The killer ape hypothesis provoked vigor- human ancestors, including a fission-fusion ous attacks (e.g., Ashley Montagu, 1968). system of grouping, and intergroup hostility. Much criticism was directed at theoretical It also raises the question of whether lethal components, such as the claim that humans raiding had a common origin in the ancestor have an innate aggressive drive that needs of chimpanzees and humans around 5–6 periodic expression. It was empirical evi- mya, or whether it evolved later and indepen- dence that felled it, however. Most impor- dently in each line. tantly, the fossils suggestive of intraspecific The CVH was stimulated by observations violence were convincingly reinterpreted in of male chimpanzees collaborating to kill or Wrangham] EVOLUTION OF COALITIONARY KILLING 3 TABLE 1. Comparison of the killer ape and for at least 5 million years. This idea of an chimpanzee violence hypotheses ancient origin of warfare is supported by the Chimpanzee rarity of coalitionary lethal violence toward Killer ape violence hypothesis hypothesis adult conspecifics in other primates, and by evidence that subsequent to the split with Lethal violence Important in Important in human evolu- human evolu- gorillas Gorilla gorilla (Pilbeam, 1996), chim- tionary history tionary history panzees and humans share a common ances- Significance of Critical pre- Possible conse- hunting cursor to intra- quence of tor around 5–6 mya. specific vio- intraspecific In the first part of this paper, the evidence lence violence; not a for coalitionary killing by chimpanzees and necessary pre- cursor the nature of their intergroup aggression Mechanism of Instinct Strategic assess- are examined. The principal adaptive expla- aggression ment Putative reason Inadvertent Assessment that nation linking chimpanzee and human vio- for intraspe- breakdown of costs of elimi- lence is then reviewed. This is the imbalance- cific violence natural inhibi- nating rival of-power hypothesis, which states that tions are low Chimpanzees Nonviolent Strategically vio- coalitionary kills occur because of two fac- considered to lent tors: intergroup hostility, and large power be Killing among Assumed to be Known to occur asymmetries between rival parties. After animals other absent considering separately the costs and ben- than hominids Reliance on fossil Critical Relevant, not efits of lethal raiding among chimpanzees, evidence critical how the imbalance-of-power hypothesis also First ancestor of Australopithe- Unknown applies to bonobos (Pan paniscus) and to humans sup- cines posed to have humans is assessed. coalitionary In the third part, objections and problems violence Evolutionary Include group Group selection are considered. Arguments are discussed mechanisms selection appears that deny the relevance of biological argu- favoring vio- unnecessary ments for understanding human warfare, lence including the following claims: warfare is wholly cultural (e.g., Keeley, 1996); modern war is too complex for individual aggression brutally wound other adults (Goodall et al., to be important (Hinde, 1993); and, nothing 1979). Most such attacks were directed to- ward members of neighboring communities, useful can be learned by studying species in patterns reminiscent of human war raids other than humans, because humans are (Goodall, 1986). As a result, various authors already known to be violent (Leach, 1968; raised the possibility of functional parallels Gould, 1996). and/or evolutionary continuities linking Criticisms directed specifically at the im- chimpanzee violence and human warfare balance-of-power hypothesis are also dis- (Trudeau et al., 1981; Otterbein, 1985, 1997; cussed. These include concerns about the Goodall, 1986;Alexander, 1987, 1989; Wrang- validity of the chimpanzee data (e.g., Power, ham, 1987, 1999b; Ghiglieri, 1988; van Hooff, 1991; Sussman, 1997) or about the interpre- 1990; Hamburg, 1991; Knauft, 1991; Man- tation of human data (e.g., Knauft, 1991; son and Wrangham, 1991; Boehm, 1992; van Sponsel, 1996), and claims that data on der Dennen, 1995; Wrangham and Peterson, bonobos undermine the use of chimpanzees 1996; Boesch and Boesch, 1999). For ex- as a reference species for early hominid ample, Otterbein (1997, p 253) noted that ancestry (Zihlman, 1997; Stanford, 1998a). similarities between chimpanzee communi- They also include accusations of genetic ties and human bands suggest that ‘‘early determinism (e.g., Regal, 1998; Sussman, man . . . is likely to have been organized into 1997) or societal bias (e.g., Sussman, 1997). localized groups of related males, groups These are important considerations, but that engaged in intergroup conflict.’’ If so, they do not invalidate the comparative ap- Otterbein concluded, warfare has been con- proach. Therefore this article ends with a tinuous in human and pre-human ancestry brief discussion of the implications of the 4 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999 imbalance-of-power hypothesis for human species never form coalitionary alliances in psychology, warfare, and morality. any context. But coalitions are not a suffi- cient condition for coalitionary killing. Thus COALITIONARY KILLING AMONG many primates form coalitions without any CHIMPANZEES AND OTHER SPECIES evidence of adult-killing [e.g., Cercopithecus Species distribution of coalitionary killing aethiops (Cheney et al., 1988)] or with fatal Contrary to initial assumptions (Lorenz, fighting known only from dyadic interac- 1966), research in recent decades has re- tions [e.g., Cebus capucinus] (Miller, 1998). vealed that intraspecific killing occurs in a Indeed, the only nonprimate mammal for variety of species, commonly following pat- which coalitionary violence is known to be terns explicable by natural selection theory. commonly responsible for adult deaths is For example, among primates infanticide is the wolf Canis lupus. In at least three sites, widely reported, typically committed by non- adults are known to kill other adults at high relatives (Hausfater and Hrdy, 1984; Palom- rates [Denali (Alaska), Isle Royale (Michi- bit, 1999). Among spiders, killing of adults gan), and Minnesota] (Mech et al., 1998). occurs predictably when resources of high For example in Denali, 39–65% of adult value are at stake (Austad, 1983). Among mortality was due to intraspecific killing, ants, large imbalances of power increase the based on 22 intraspecific killings recorded probability of lethal intercolony aggression from 17–20 packs (Mech et al., 1998). This is (Ho¨lldobler, 1981; Adams, 1990). Obvious the least disturbed study site of wolves. In parallels can be found among humans (Daly northeastern Minnesota, 43% of wolves not and Wilson, 1988; Bueno de Mesquita, 1981, killed by humans were killed by other wolves. 1985; Bueno de Mesquita and Lalman, 1992). Killings tended to occur in buffer zones To some extent, therefore, patterns of hu- (where territories met), which wolves mostly man killing appear to follow the ordinary avoided (Mech, 1994). These data suggested patterns of lethal aggression found in other to Mech et al. (1998) that intraspecific kill- species. ing is a normal consequence of wolf territori- Not so ordinary, however, is the way that ality. human killing occurs. Among humans most Occasional coalitional killing of adult con- killing occurs in warfare, where the predomi- specifics in neighboring groups has also been nant style of violence is coalitionary. In most recorded among other social carnivores [li- animals, by contrast, even where aggression ons Panthera leo, spotted hyenas Crocuta occurs at high rates, lethal violence is dyadic crocuta, cheetahs Acinonyx jubatus (Kruuk, (one versus one) rather than coalitionary 1972; Caro and Collins, 1986; Goodall, 1986; (many vs. one, or many vs. many). During Packer et al., 1988; Grinnell et al., 1995)] rut-fighting among male pronghorn ante- and at least one group-territorial bird [Tas- lope (Antilocapra americana), for example, manian native hen Gallinula mortierii (Put- 12% of 82 fights over mating rights to es- land and Goldizen, 1998; A. Goldizen per- trous females led to the death of one or both sonal communication)]. However, the males (Byers, 1997). Likewise, in different frequency of killing has not been reported populations of red deer Cervus elaphus, 13– for these species. Outside of mammals, only 29% of adult male mortality came from social insects are known to kill conspecifics rut-fighting (Clutton-Brock et al., 1982). regularly with coalitional aggression (van Many similar examples occur. Deaths tend der Dennen, 1995). to occur in intensely escalated contests in These data suggest that animals can be which both opponents expose themselves to divided into three major categories. First high risk of injury, typically because ‘‘a are species in which intraspecific killing of major part of a contestant’s lifetime repro- adults is rare (e.g., less than 1% of all adult ductive success is at stake’’ (Enquist and deaths). Most species fall into this category. Leimar, 1990). But killing is never coalition- Second are those where killing occurs more ary in these species. frequently [often 10% of more of deaths The explanation for the widespread ab- (Enquist and Leimar 1990)], but entirely in sence of coalitionary violence is trivial. Most dyadic interactions. In these species, fatal Wrangham] EVOLUTION OF COALITIONARY KILLING 5 fighting is dangerous. In the third category, Abundant evidence routinely attests to the killing is also frequent, but differs by being blood-lust of the participants. polyadic (coalitionary). Furthermore, as ar- Against the notion that men have a ready gued below, fatal fighting need not be danger- appetite to attack their enemies, combat- ous for the killers. ants in modern warfare are often reluctant Ants dominate this last category, which to fight (Hinde, 1993; Ehrenreich, 1997; contains probably less than 10 mammalian Grossman, 1999). This front-line lack of species and perhaps no other vertebrates. aggressiveness is understandable because Chimpanzees and humans (or at least, cer- in modern warfare, unlike intergroup aggres- tain populations of these species) are the sion in primates, soldiers are organized hier- only primates known to be frequent coalition- archically and are ordered into battle by ary killers. A possible additional candidate their superiors, regardless of their personal is the western red colobus monkey (Colobus motivation. Participation in raids among badius), for which at least two and possibly pre-state societies, however, is normally vol- four coalitionary kills were recorded by Sta- untary (Keeley, 1996). Thus, reluctance of rin (1994). In these cases, coalitions of fe- soldiers under orders does not undermine males attacked and killed males attempting the more widespread phenomenon of male to enter their groups. eagerness for fighting. It is likely that lethal raids also occur in The lethal-raiding problem some of the carnivore species that engage in In chimpanzees, humans, and some other intraspecific killing of adults. For example, animals, coalitionary killing can occur in the lethal raiding is suggested by the report of context of lethal raiding. Lethal raids are an Mech et al. (1998) that neighbors killed unusual form of aggression because they do three members of a wolf pack, two others not escalate from a conflict. Instead, parties disappeared unseen, and the defeated pack’s of allied males collectively invade a neighbor- territory was taken over by the killers. There ing territory, seek one or more vulnerable are also reports of spotted hyenas making neighbors, apparently assess the probability incursions into neighboring territories to of making a successful attack, conduct a attack neighbors (Goodall, 1986). Among ‘‘surprise’’ attack that leaves one or more invertebrates, patterns similar to lethal raid- victims dead or dying, then return to their ing occur in a variety of ants (Ho¨lldobler, own territory. ‘‘Surprise’’ refers to the attack 1981; van der Dennen, 1995). occurring without any initial conflict, with- Among species other than humans, how- out escalation from a lower level, and with- ever, lethal raids have been most clearly out the victim interacting with the oppo- reported in chimpanzees. Evidence of chim- nents until the attack starts. panzee raiding has been fundamental for Thus, lethal raids indicate an appetite for the development of the CVH. I therefore hunting and killing rivals that is akin to review the chimpanzee data in detail. predation. By contrast, most animal con- Lethal raiding by chimpanzees flicts escalate in a stepwise manner that allows both opponents to assess each other Although lethal raiding among chimpan- and to withdraw when the risks of losing zees has been described more clearly than appear too high (Archer and Huntingford, for any other mammal, few cases have been 1994). The ‘‘appetite for lethal raiding’’ there- completely observed. Furthermore, all the fore requires explanation in different terms detailed observations come from a single from escalated conflicts. site, Gombe National Park in Tanzania Among humans, lethal raids are wide- (Goodall, 1986). Of course, lethal raids are spread in all forms of warfare. For example, expected to be rare, as they must be in any Keeley (1996) regards small raids and am- long-lived, slowly reproducing species. Nev- bushes as ‘‘the commonest form of combat ertheless, the concentration of observational employed in primitive warfare’’ (see also evidence at Gombe has suggested to critics Turney-High, 1949; van der Dennen, 1995; of the CVH that lethal raiding may have Maschner and Reedy-Maschner, 1998). been induced in Gombe by unnatural condi- 6 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999

TABLE 2. Territorial behavior in chimpanzees and bonobos1 P. t. schweinfurthii P. t. verus P. paniscus Gombe Mahale Kibale Budongo Ta¨ı Wamba Lomako Territorial defense ϩ ϩ ϩ ϩ ϩ ϩ ϩ Border patrols ϩ ϩ ϩ ? ϩ Ϫ Ϫ Deep incursions ϩ ϩ ϩ ? ϩ Ϫ Ϫ Coalitionary attacks ϩ ϩ ? ? ϩ Ϫ Ϫ Coalitionary kills ϩ ϩ ϩ ϩ Ϫ Ϫ Ϫ Border avoidance ϩ ? ϩ ? ϩ Ϫ Ϫ Peaceful intercommunity association Ϫ Ϫ Ϫ Ϫ Ϫ ϩ ϩ

1 All long-term studies are included except for Bossou, where the community is isolated by agricultural land from its nearest chimpanzee neighbors. ‘‘Coalitionary kills’’ refers to adult victims only. ‘‘Coalitionary attacks’’ means non-lethal attacks by several males on a single victim. Data are from: Gombe (Goodall, 1986); Mahale (Nishida, 1979, 1986; Nishida et al. 1985); Kibale (Chapman and Wrangham, 1993); Budongo (V. Reynolds, personal communication); Ta¨ı (Boesch and Boesch, 1999); Wamba (Kano, 1992; Hashimoto et al., 1998); Lomako (White, 1996).

TABLE 3. Chimpanzees and bonobos: lethal violence by site1 Site Subspecies Adult deaths Infanticides Years of study Gombe P. t. schweinfurthii 6 (3) 6 (3) 38 Mahale P. t. schweinfurthii 1 (6) 4 (6) 33 Kibale P. t. schweinfurthii 2 (0) 1 (0) 11 Budongo P. t. schweinfurthii 1 (1) 1 (1) 8 Bossou P. t. verus 0 (0) 0 (0) 22 Ta¨ı P. t. verus 0 (0) 0 (0) 19 Wamba P. paniscus 0 (0) 0 (0) 24 Lomako P. paniscus 0 (0) 0 (0) 15 Total 10–20 12–22 170

1 Numbers include (for adult deaths) kills recorded on the basis of direct observation and/or fresh bodies as well as (in parentheses) those from suspicious disappearances (see Table 5), or (for infanticides) kills observed directly or (in parentheses) inferred from context (Arcadi and Wrangham 1999, updated for Mahale by Nishida (personal communication). ‘‘Years of study’’ is number of years from beginning of continuous study until 1998. Bonobo studies have been intermittent.

tions such as reduced habitat, or provision- Six components of chimpanzee intergroup ing, and therefore that lethal raiding is aggression are especially relevant to lethal uncharacteristic of chimpanzees more gener- raiding: territorial defense (showing evi- ally (Power, 1991). dence of hostile intergroup relationships), There are six study sites in which habitu- border patrols, deep incursions, coalitionary ation of chimpanzees is sufficiently good to attacks, coalitionary kills, and border avoid- allow multi-hour observations of known indi- ance (Table 2). viduals traveling throughout the home range (Tables 2 and 3). These include four studies Territorial defense. This has been re- of the eastern subspecies (P. t. schwein- ported in all studies in which intercommu- furthii), none of the two central subspecies nity relationships have been described, based (P. t. troglodytes and P. t. vellerosus), and two on some combination of: counter-calling be- of the western chimpanzee (P. t. verus). In tween parties of neighboring males; rapid one of the P. t. verus studies, at Bossou, the travel toward a site where an opposing party study community is ‘‘semi-isolated’’ by loss has been detected; avoidance of opposing of habitat, separated by several kilometers parties that were obviously larger; charging from the home ranges of its closest neigh- displays directed toward an opposing party bors (Sugiyama, 1989; Sugiyama et al., of males; or one party chasing another 1993). Consequently there is no possibility (Nishida, 1979; Goodall, 1986; Boesch and of territorial behavior or intercommunity Boesch, 1999; V. Reynolds personal commu- interaction from Bossou. This leaves five nication; Wrangham et al., in preparation). studies of chimpanzees, varying in duration For example on nine occasions chimpanzees from 8 to 38 years, that permit observation in Ta¨ı have been seen in ‘‘back-and-forth of intergroup interactions (Table 3). attacks’’ with neighbors, in which all males Wrangham] EVOLUTION OF COALITIONARY KILLING 7 rush toward the opponents, giving loud at- (Wrangham, 1975, Table 5.9). By contrast, tack calls, and opponents may flee up to 400 in the 5 years following the extinction of the m. In a further nine cases, males in the front Kahama community (1978–1982), border pa- line of attack were supported by loud calls trols continued at a rate of 18 per individual from females in a rear line (Boesch and per year (range 9–27; calculated from Good- Boesch, 1999). all, 1986, Table 17.1). Table 2 shows that border patrols have Border patrols. These are an intrinsic been reported also from Mahale, Kibale, and component of lethal raiding, because they Ta¨ı. Border patrols have not been described put a party of individuals in a position to in detail from Mahale, but key elements of stalk and to hunt a neighboring victim. border patrols have been reported—includ- Border patrols are visits to a peripheral ing scouting, and silent and cautious travel, sector of a home range by a party of males mainly by males, in border areas (Nishida, that monitors the area. They are initiated 1979, 1990; Nishida et al., 1985). The Kibale without any immediate contact with mem- evidence comes from border patrols seen at bers of the neighboring community, and are Kanyawara, involving parties of males inter- often undertaken with little or no feeding. mittently checking their territorial bound- According to Goodall (1986) and Boesch and aries (Wrangham et al., in preparation). In Boesch (1999), they include some or all of Ta¨ı, border patrols occurred in 29% of 129 the following features: (1) cautious and slow territorial actions, normally involving at travel around or across the border, including least four males, and included all of the long periods of gazing toward the neighbor- elements listed above (Boesch and Boesch, ing home range; (2) nervousness shown to- 1999). ward unexpected sounds; and (3) inspection of signs of other chimpanzees, such as dis- Deep incursions. These were included as carded food wadges, feces, nests, or aban- part of border patrols by Goodall (1986), but doned termite-fishing tools. I distinguish them because they involve Border patrols were first reported at deliberate travel into the neighboring terri- Gombe in 1971 by JD Bygott, who was the tory rather than merely checking of the first researcher to conduct regular all-day border area. Deep incursions are character- observations of individual males (Bygott, ized by (1) substantial penetration into the 1979). Most patrols at Gombe were by males neighboring territory, e.g., for one kilometer of the principal study group, the Kasekela or more; (2) silent and cautious travel dur- community, but the Kahama males pa- ing periods of moving outwards from own trolled also (Bygott, 1979; Wrangham, 1975). territory; and (3) noisy and vigorous dis- Border patrols do not occur every time a plays on return to their own territory. Deep party reaches the boundary area. In Gombe, incursions have been well described at for example, patrols occurred during 28% of Gombe and Ta¨ı. Boesch and Boesch (1999) 134 boundary visits made by parties from found that ‘‘many patrols were probably the Kasekela community from 1977 to 1982 aimed at finding and attacking strangers. . . (calculated from Goodall, 1986, Table 17.1). . . . (they) were impressive by the intensity Their frequency appears to vary as a func- with which the males searched for strang- tion of relations between particular commu- ers, not only entering deep (into) their terri- nities. Thus, during and after the 1974– tory, but once even heading backwards to 1977 period, during which the Kasekela find the neighbors.’’ Deep incursions lasted community killed the males of the neighbor- up to 6 hours, and on 5 of 129 territorial ing Kahama community, Kasekela border interactions in Ta¨ı, led to attacks. All incur- patrols were disproportionately directed to- sions were led by males. wards the Kahama territory (Goodall, 1986). The recorded frequency of border patrols by Coalitionary attacks. These are interac- Kasekela males was highest in 1972–1973, tions in which observers assess that the when 13 border patrols occurred in 58 days intent of those in the aggressive party is to of observation, i.e., a rate of 82 per year hurt or to kill one or more victims. They can 8 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999 occur within the territory of either the ag- an adolescent male and nulliparous female gressors or the victims, or in the boundary near the territorial border, but later re- area. In Gombe, coalitionary attacks have treated when confronted by four adult males included both interactions between the well- from the neighboring community, who chased habituated Kasekela and Kahama communi- the aggressors for 700 m (M. Muller, per- ties, as well as other communities. In late sonal communication). 1974, for example, a party of three Kahama Coalitionary attacks also occur within com- males encountered a male and female found munities. For example, a bullying and insub- to the south of their territory (the ‘‘Kalande’’ ordinate young adult male (Jilba) in Ma- community). Two of the Kahama males hale’s M-group was attacked so severely by grabbed and attacked the Kalande male, but six males and two females that it took him 3 he escaped without serious injury (Goodall, months of traveling alone before he recov- 1986). ered sufficiently to rejoin the community In Mahale, cases included both attacks by (Nishida, 1994). M-group toward K-group males, and vice versa. For example, Nishida (1979) recorded Coalitionary kills. No cases have been three K-group males chasing an M-group reported of dyadic violence leading to the male for 200 m. Two of the pursuers gave up, death of an adult chimpanzee. However, but the third caught the victim, forced him lethal coalitionary attacks on adults have to the ground, bit his thigh, stamped on him, been reported from all four study sites of the chased him as he tried to escape, but then eastern subspecies (Table 4). Table 4 lists suddenly gave up. known and inferred cases. The largest sam- In Ta¨ı, Boesch and Boesch (1999) reported ple comes from Gombe and includes five a category of attack which they called ‘‘com- observed brutal attacks followed by the dis- mando,’’ in which a party of males (with or appearance of the victim (four male, one without females) penetrated into the neigh- female), and one case of a fresh corpse of an boring range and attacked one or more unidentified female, considered to have been neighbors, i.e., a combination of a ‘‘deep killed by the Kahama males. Goodall (1986) incursion’’ with a coalitionary attack. The summarizes the observations as follows: the Ta¨ı males sometimes ‘‘waited and listened attacks lasted at least 10 min each; the silently for hours before they attacked. We victim was always held to the ground by one twice saw the study community being victim or more of the assailants while others at- of a commando attack, in one of which tacked; the victim was dragged in at least Macho (an adult male) escaped with 19 two directions, eventually gave up resisting, wounds’’ (Boesch and Boesch, 1999). A sec- and was essentially immobilized by the end ond form of coalitionary attack was the of the attack. ‘‘lateral attack’’ (seen six times), in which a In Mahale, Nishida et al. (1985) recorded party moved laterally while looking in the the deaths of all six adult males of K-group direction of strangers, then approached, community between 1969 and 1980. In Nishi- chased, and on at least one occasion caught da’s words, the observers ‘‘speculate that at and attacked one of the opponents. This least some adult males, particularly So- appeared to be a tactic for increasing the bongo and Kamemanfu, were killed by M- imbalance of power by isolating a victim group’s chimpanzees. Severe fighting was from the rest of the party (Boesch and occasionally witnessed between males of K- Boesch, 1999). group and M-group in (their area of over- In Kibale, no complete coalitionary at- lap). . . . M-group’s males were sometimes tacks have been seen, but twice parties of seen to penetrate into the core area of K- neighbors have silently charged toward iso- group’s range from 1974 onwards’’ (Nishida lated males of the Kanyawara study commu- et al., 1985, p 288). The males who disap- nity, then veered off on seeing observers, peared were all healthy, not senile. In one suggesting that coalitionary attacks were case, M-group males were known to be very averted by the presence of humans. In a near to K-group males; there were many third case, five Kanyawara males attacked outbursts of calls; and the next day another Wrangham] EVOLUTION OF COALITIONARY KILLING 9

TABLE 4. All reported intraspecific kills of adult chimpanzees1 Aggressor’s Victim’s Result Site Date Community Party Community Party Victim’s ID Ref. Death Gombe 1974 Kasekela 7M, 1F Kahama 1M Godi Goodall (1986, p. 50) Death Gombe 1974 Kasekela 3M, 1F Kahama 3M, 1F De´ Goodall (1986, p. 50) Death Gombe 1975 Kasekela 5M Kahama 1M Goliath Goodall (1986, p. 50) Death Gombe 1977 Kasekela 6M Kahama 1M Sniff Goodall (1986, p. 51) Death Gombe 1975 Kasekela 4M Kahama 1F Madam Bee Goodall (1986, p. 51) Death Mahale 1995 M-gp gang M-gp 1M Ntologi Nishida (1996) Death Budongo 1998 Sonso gang Sonso 1M Zesta (ϩ2 K. Fawcett (per- injured) sonal commu- nication) Death Kibale 1992 Rurama gang Kanyawara Յ1M Ruwenzori KCP Death Kibale 1998 Kanyawara gang Sebitole Ն1M Unknown KCP Death Gombe 1972 Kahama ? Kalande? Ն1F 1F Wrangham (1975) Death Gombe 1977 Kasekela 5M Kahama Ն1M Charlie Goodall (1986, p. 50) Death? Mahale 1996 M-gp ? M-gp 1M Jilba M. Huffman (personal com- munication), Hofer et al. (1998) Death? Mahale 1970-83 M-gp ? K-gp ? Some males? Nishida et al. (1985) Death? Gombe 1981 Kalande ? Kasekela ? Humphrey Goodall (1986, p. killed? 51) Attack Gombe 1974 Kahama 3 M Kalande 1M, 1F M attacked, Goodall (1986, p. caught by 2, 49) escaped Attack Gombe 1980 Kalande ? Kasekela 1F Passion Goodall (1986, p. (inferred) 51) Attack Mahale 1974 K-group 3M M-group 1M 1 on 1 fight Goodall (1986, p. 51)

1 Parties for aggressors and victims show the number of adult males (M) and females (F). Letters in bold show the victim’s sex. KCP (citation for Kibale deaths) is records of the Kibale Chimpanzee Project.

K-group male was missing (Kasonta) (T had been removed. Nine Kanyawara males Nishida, personal communication). had been patrolling the border on the previ- In Kibale, after 3 days during which males ous evening, all of whom were present the from the Kanyawara and Rurama communi- next morning (17 hours later), and several of ties had been counter-calling at each other whom beat on the victim’s body and dragged in a hostile manner, a Kanyawara male it about (M. Muller, personal communica- (Ruwenzori) was found freshly dead in the tion). border area. His body, huddled face down at Like coalitionary attacks, coalitionary kills the bottom of a slope around which the also occur within communities. At Budongo vegetation had been beaten down, showed in 1998, an adult male was killed by other clear evidence of a violent attack by chimpan- males of his own community (K. Fawcett, zees. In an unrelated incident, the personal communication). Intracommunity Kanyawara males were followed by observ- killing is also thought to have occurred in ers to the fresh corpse of an individual from Mahale, where adult male Ntologi was found a neighboring community (Sebitole) who had dead in the center of M-group’s territory apparently been killed by chimpanzees the with numerous wounds on his body. His previous evening. There were numerous death followed several coalitionary attacks wounds on the front of his body, his trachea on him by his former subordinates—after had been ripped through, and both testicles his defeat as alpha-male of M-group 10 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999

TABLE 5. Chimpanzee coalitionary kills of adults1 Kill seen, Aggressor Victim’s or fresh Suspicious Site community community corpse disappearance Reference Gombe Kasekela Kahama 5 2 Goodall (1986) Gombe Kahama Kalande 1 Goodall (1986) Gombe Kalande Kahama 1 Goodall (1986) Kibale Rurama Kanyawara 1 KCP Kibale Kanyawara Sebitole 1 KCP Mahale M-group K-group 6? Nishida et al. (1985) Mahale M-group M-group 1 1 T. Nishida (personal communication) Budongo Sonso Sonso 1 K. Fawcett (personal communication) Total 10 10?

1 Intra-community aggression is indicated by the victim and aggressors being in the same community. KCP, records of the Kibale Chimpanzee Project.

(Nishida, 1996; T. Nishida, personal commu- tion time. Thus, Figure 1 shows that in nication). relation to observation time, the number of The chimpanzee data are summarized in observed and suspected kills appears simi- Table 5. Tables 4 and 5 also list suspicious lar in the four schweinfurthii study sites. disappearances. These are cases where ob- The idea that as observation years accumu- servers believed the most likely explanation late, more killing will be seen, is supported for disappearances was that they were killed by the data on infanticides, which show a by neighbors, because: (1) those who disap- similar trend (Fig. 1). In summary although peared were healthy and not senescent; (2) the evidence needs to be substantiated by other causes of death appeared improbable; continuing observation, current evidence is and (3) the disappearances occurred at a that in all four populations of the eastern time and place where there were patently subspecies, adults kill each other occasion- hostile relationships with a neighboring com- ally through coalitionary violence. Figure 1 munity. suggests a rate of approximately 0.25 adults Although the reported episodes of lethal killed per year. coalitionary violence are still few, the kill- On the other hand, there is no evidence of ings are noteworthy because they have been lethal intraspecific aggression toward either reported from four sites and, in relation to adults or infants from either of the studies of total observed adult deaths, they appear to the western subspecies, i.e., from Ta¨ı or be demographically significant. In Gombe, Bossou. Because the Bossou community has data reported by Goodall (1986) indicate no neighbors and few males, low rates of that for adult males in Kasekela and Ka- aggression are not surprising. However, as hama, the proportion of adult male mortal- demonstrated by Figure 1, lethal coalition- ity from intraspecific coalitionary aggres- ary aggression would be expected to have sion was 30–40%. Although fewer kills have been seen at Ta¨ı: the study is now 21 years been seen elsewhere, it seems likely that old and should therefore have produced evi- this variation is partly a function of observa- dence of about five killings each of adults

Fig. 1. Chimpanzee intraspecific killings by study site. S, P. t. schwein- furthii; v, P. t. verus. See Tables 3 and 4 for data and sources. Wrangham] EVOLUTION OF COALITIONARY KILLING 11 and infants if this population conformed to The known kills at Gombe occurred be- the schweinfurthii pattern. Although the tween 1973 and 1977, during a period of sample sizes are small, the fact that Ta¨ı intense hostility between two communities chimpanzees show all components of lethal that had recently split from a single commu- raiding but no coalitionary kills suggest that nity, and which were each dominated by two the nature of aggressive relationships dif- alpha-males with mutually hostile relations fers between Ta¨ı and the eastern popula- (Goodall, 1986). Border patrols by Kasekela tions (below). males were directed mostly toward the Ka- Finally, border avoidance is expected if hama community during this period, which individuals are aware that the border is ended with the extinction of the Kahama physically dangerous. Low frequency of use community (Goodall, 1986). Thus, unusual of border areas has been documented by demographic and social conditions applied Boesch and Boesch (1999), who found that in to elicit this particular bout of lethal raiding. Ta¨ı 75% of time was spent in the central 35% Therefore, nonlethal raiding is a routine of the range. When Gombe or Kibale component of the chimpanzee behavioral (Kanyawara) chimpanzees do visit border repertoire. Coalitionary killing is less com- mon. However it has been recorded in four areas, they tend to do so in parties that are out of five sites. This raises the question of relatively large (Gombe) and contain a higher why chimpanzees have an appetite for ago- proportion of males than normal nistic interactions with members of neighbor- (Kanyawara) (Bauer, 1980; Chapman and ing communities, and why they sometimes Wrangham, 1993). Finally, high prey densi- kill opponents. ties in border areas have been reported for the main prey species of chimpanzees, red THE IMBALANCE-OF-POWER colobus (Colobus badius), both in Gombe HYPOTHESIS (Stanford, 1998b) and Kibale [Ngogo (D. Explaining chimpanzee violence Watts, personal communication)]. Similarly Many reasons have been advanced to ac- among wolves, prey densities are higher in count for chimpanzee lethal raiding, includ- border areas between territories, a result of ing: male–male bonds, hostility toward out- avoidance of those areas by wolves (Mech et siders, cooperative group living, cooperative al., 1998). Border avoidance by territory hunting skills, power imbalances when par- holders has not been reported in other non- ties from neighboring communities meet, human species, but is presumably wide- large and overlapping home ranges, high spread in humans. cognitive ability, and innate killing potential In summary, there are five study sites (reviewed by van der Dennen, 1995). The (Gombe, Mahale, Ta¨ı, Kibale, Budongo) in only attempt at a cost-benefit analysis that which chimpanzees have neighbors and explains the species distribution of lethal where intercommunity interactions have raiding, however, is the imbalance-of-power been at least partly described or observed hypothesis. This hypothesis was implied by (Table 2). At the three best-documented sites Goodall (1986), then elaborated by Manson (Gombe, Mahale, Ta¨ı) patterns of territorial and Wrangham (1991), Wrangham and Pe- interaction appear similar: all of them show terson (1996), and Wrangham (1999b). The territorial defense and border patrols by imbalance-of-power hypothesis proposes that adult males, with violent coalitionary at- the function of unprovoked intercommunity tacks on neighbors. All these patterns are aggression (i.e., deep incursions and coali- similar to the data on wolves, which is the tionary attacks) is intercommunity domi- only other nonhuman mammal with fission- nance. By wounding or killing members of fusion grouping and group territories in the neighboring community, males from one which intergroup interactions have been community increase their relative domi- well described. These points suggest that all nance over the neighbors. According to the the major elements of lethal raiding are imbalance-of-power hypothesis, the proxi- routinely present in populations of chimpan- mate benefit is an increased probability of zees. winning intercommunity dominance con- 12 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999 tests (nonlethal battles); this tends to lead to when a party of three or more males encoun- increased fitness of the killers through im- ters a lone victim (M Wilson et al., personal proved access to resources such as food, communication), supporting the observa- females, or safety. tional data from Gombe. These observations The imbalance-of-power hypothesis con- make sense because to date, there appear to trasts with proposals that chimpanzees are be no records of any aggressors receiving exceptionally capable of conducting attacks, serious wounds. or win particularly large rewards from inter- In light of such data, several authors have group competition. proposed that it is the ability of a gang of three or more males to overwhelm a lone The significance of power imbalances victim, at low risk of injury to themselves, Both within and between primate groups, which at least partly explains why chimpan- contests tend to be won by the larger of two zees are so ready to attack (Goodall, 1986; coalitions, though variables such as domi- Manson and Wrangham, 1991; Wrangham nance rank and geographic location are also and Peterson, 1996; Boesch and Boesch, important (Cheney, 1986; Chapais, 1995). 1999). The logic is that a victim can be held Coalition size appears even more important down or otherwise disabled by two or more, for interactions among chimpanzees from while another aggressor can impose damage different communities. In the four longest at will. This idea that the low cost of lethal studies of chimpanzees, the principal deter- aggression elicits lethal raiding is central to minant of the nature of intercommunity the imbalance-of-power hypothesis (Manson interactions is not the geographic location and Wrangham, 1991; Wrangham and Peter- but the relative size and composition of son, 1996). parties when they encounter each other. Origins of power imbalances This conclusion is based on direct observa- tions at Gombe, Kibale, Mahale, and Ta¨ı All chimpanzee populations have fission- (Bygott, 1979; Nishida, 1979; Goodall, 1986; fusion grouping patterns, with individuals Boesch and Boesch, 1999; Wrangham et al., sometimes alone and sometimes in parties in preparation), as well as playback experi- (Fig. 2), and adult males more gregarious ments at Kibale (M Wilson et al., personal than mothers (Wrangham, 1999a). Demo- communication). For example, Boesch and graphic, social and ecological variables influ- Boesch (1999) found that small parties of ence party size (Boesch, 1996). For example, males (1–3) mainly checked for the presence party size increases both with the number of of strangers by drumming and listening to females having sexual swellings, and with the response (67% of 18 occasions). Middle- the amount of fruit in the habitat (Nishida, sized parties (4–6 males) tended to make 1979; Goodall, 1986; Boesch, 1996; Wrang- incursions into the neighboring territory ham, 1999a). Parties appear to be con- more often (37% of 76 observations). Large strained by fruit availability as a result of parties (7–9 males) tended to attack the scramble competition, with larger parties strangers (63% of 30 observations). More formed more when fruit is sufficiently abun- generally, at all sites, the probability that a dant to allow gregariousness (Chapman et party will advance, exchange displays, or al., 1995). retreat appears to be well predicted by Neighboring communities can experience whether it is larger than, equal to, or smaller markedly different levels of fruit supply, a than the opposing party (Boesch and Boe- result of differences in fruit-tree density, or sch, 1999). Relative party size is also a in fruiting success (Chapman et al., 1997). critical variable among lions (McComb et al., This means that, on occasion, neighboring 1994; Grinnell et al., 1995). communities may contain parties of differ- The evidence therefore suggests that chim- ent mean size. The community in which panzee parties are bolder when they contain parties are able to be larger can then make relatively more males. In addition, playback low-risk raids to attack neighbors. This socio- experiments at Kibale support the hypoth- ecological connection has been observed in esis that males are more likely to attack Mahale, when the M-group community Wrangham] EVOLUTION OF COALITIONARY KILLING 13 attacked by a coalition at low risk to the aggressors. This means that populations (or seasons) with fewer encounters between soli- taries and large groups should have fewer violent interactions. Data on wolves at De- nali (Alaska) offer a test of this prediction. The ratio of the number of parties contain- ing three or more wolves to the number of solitary individuals was higher during win- ter (5.2) than summer (0.1). This means that solitaries were much more likely to encoun- ter a large party during winter than sum- mer. As expected, winter was also the season when intraspecific kills were more likely (a sevenfold increase in probability, from 22 Fig. 2. Party size distribution among chimpanzees dated kills) (data calculated from Figs 5.4 and bonobos. Data sources: eastern chimpanzee P. t. and 5.7, Mech et al., 1998). schweinfurthii: Kibale, Kanyawara community 1994– If a similar effect applies to chimpanzee 1996. Western chimpanzee P. t. verus: Ta¨ı (Boesch, 1996); Mt. Assirir (Tutin et al., 1983). Bonobo P. panis- populations, and if the fact that Ta¨ı has had cus; Wamba (Kuroda, 1979); Lomako (White, 1988). All a low kill rate is meaningful (rather than populations show substantial variation in party size over time and between communities. Comparable data stochastic), Ta¨ı should have larger, less fis- for Gombe and Mahale were not found. sioned parties than at Gombe, Mahale, or Kibale. Preliminary data suggest this predic- tion is qualitatively correct, because Boesch would make seasonal forays into the terri- (1996) found that among six chimpanzee tory of the K-group community, supplanting populations, the mean party size was high- K-group parties and sometimes attacking est at Ta¨ı (8.3, compared to a mean of them (Nishida, 1979). 5.2 Ϯ 0.8 at the other five sites, including Why are chimpanzees (compared to other Gombe, Mahale, and Kibale). The percent- species) particularly vulnerable to the pres- age of lone individuals was also lowest in Ta¨ı sures of scramble competition that lead to a (4%, compared to 14% Ϯ 3% for the three fission-fusion, rather than a stable-troop populations with data, Boesch, 1996, Table system of grouping? Wrangham et al. (1996) 8.2). These data thus indicate consistently argued that the important characteristic of larger parties in Ta¨ı than elsewhere, compat- chimpanzees is that even when fruits are ible with evidence that Ta¨ı is a relatively scarce, individuals continue to search for productive habitat (Boesch and Boesch, them. Consistent with this hypothesis, chim- 1999). panzees (unlike gorillas) are restricted to Further data will test whether party size areas that contain year round fruits, and and the frequency of high-intensity aggres- spend significantly greater proportion of sion do indeed co-vary among sites, and how their feeding time eating ripe fruits than do often chimpanzee populations tend to have sympatric frugivorous monkeys (Wrangham small parties and high rates of aggression. et al., 1998). This strategy of constant fruit Two points suggest that the high frequency search is presumably forced on chimpanzees of intense aggression seen at Gombe and by species-specific digestive adaptations, suggested by the other eastern chimpanzee such as the rate of food passage through the populations may be unusual for the species gut and the ability to ferment long-chain as a whole. First, skeletal trauma indicative carbohydrates (Milton, 1987). Whatever its of intraspecific aggression has been found at origins, it exposes them to relatively intense higher rates in a sample of chimpanzee scramble competition. crania from Gombe than from elsewhere The imbalance-of-power hypothesis states (Jurmain, 1997). Second, the four P. t. sch- that violence is facilitated by vulnerability, weinfurthii study sites (Gombe, Mahale, because lone individuals can be vigorously Kibale, and Budongo) are all located at the 14 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999 extreme east of the species geographical occurs in chimpanzees. Others are (1) that range, where dry seasons are relatively long. the benefits of raiding rise so steeply with These eastern populations may therefore be increased party size that it pays individuals living under relatively harsh conditions of to participate for selfish reasons or (2) that food availability compared to more western chimpanzees have evolved exceptional coop- sites. erative abilities in contexts other than le- In summary, chimpanzees are vulnerable thal raiding (C. van Schaik, personal commu- to particularly intense scramble competi- nication). This remains an important tion, apparently because of their digestive problem, as it does for much of human adaptations to ripe fruit. This competition behavior (Boehm, 1999). forces them to travel alone or in small On the other hand, the occurrence of parties when fruits are scarce. Patchy fruit territoriality among chimpanzee communi- distribution can mean that one community ties, and of occasional imbalances of power has abundant supplies, while its neighbors between parties from neighboring communi- have few. Demographic differences between ties, are easily explained. First, current communities (i.e., differences in the number theory suggests that home ranges are eco- of adult males) may also mean that parties nomically defensible if individuals can eas- in one community can be dominant over ily cross their home range in a day. Chimpan- those in the neighboring territory. Such fac- zees can do so (Lowen and Dunbar, 1994; see tors can account for differences between also van Schaik, 1996). Second, a system of populations or communities in the number fission-fusion grouping can explain why lone of males in parties, and hence for differences individuals occasionally encounter larger in their vulnerability to attack by coalitions coalitions, and are therefore vulnerable to of neighbors. attack. But neither long day ranges nor fission- Group territoriality and the benefits fusion grouping can explain lethal raiding, of lethal raiding in which individuals seek opportunities to Understanding the selective advantage of attack (as opposed to responding to inva- aggression is more complicated for intercom- sion, escalated contest for resources, etc.). munity than interindividual relationships, As Goodall noted, for example, there have because any fitness benefits gained by a rise been three major invasions at Gombe and in intercommunity dominance are shared Mahale. ‘‘Kasekela males took over Kahama among individuals within the community. range, Kalande males pushed deep into This might be expected to favor free-riders Kasakela range, and M-group moved into (individuals who would benefit from lethal K-group range. During all these invasions raiding without taking part), which would adult males (and some females) were killed lead to a suboptimal level of collective action or disappeared. Even if it is argued that the (van Schaik, 1996; Nunn, 1999). In fact, Kasekela males were merely trying to re- however, there is no evidence of defection claim an area to which they previously had among raiding chimpanzees (Goodall, 1986; free access, the assertion does not explain Wilson et al., personal communication) [(or, the northward thrust of the Kalande commu- for that matter, among lions in similar inter- nity or the takeover by the M-group at community contexts (Grinnell et al., 1985)]. Mahale’’ (Goodall, 1986, p. 528). How lethal raiding escapes the free-rider Two kinds of hypothesis have been pre- problem is not understood. One possibility is sented to account for such incursions, proxi- that intercommunity conflict has been so mate competition and dominance drive. intense that selection has occurred at the First, aggression may be proximately elic- between-group level (Boehm, 1999). How- ited by resource competition, such as for ever, this is unlikely because it would re- mates, food, or land (Manson and Wrang- quire very frequent group extinctions with ham, 1991). This hypothesis is strongly sup- few survivors. Another is that free-riders are ported by some observations, such as the policed by others in the community (Boehm, incursions by Mahale’s M-group into the 1999), but no evidence suggests that this K-group range. These occurred in a seasonal Wrangham] EVOLUTION OF COALITIONARY KILLING 15 rhythm coincident with fruit shortages in weakness of a dominant due to aging, wound- M-group’s range and abundant fruits in ing, loss of allies or loss of confidence in- K-group’s range (Nishida, 1979). Again, in creases the rate of attack by a subordinate certain circumstances raiding might help (Bygott, 1979; de Waal, 1982; Goodall, 1986; males to recruit young females: this possibil- Nishida, 1994). Selection is expected to fa- ity is suggested by evidence at Gombe that vor the effort to rise in dominance because severe attacks on the mothers of nulliparous dominant individuals (or groups) tend to females in neighboring communities are have high fitness, and accordingly, individu- sometimes followed by the young females als opportunistically take advantage of any joining the aggressor’s community (Goodall, perception of changes in power asymmetry 1986). (Chapais, 1995). Alternatively, aggression may be elicited Sometimes, admittedly, the expectation merely by the opportunity to reduce the that higher dominance leads to higher fit- coalitionary power of the neighbors (Manson ness is not met. Thus, in around half of the and Wrangham, 1991; Wrangham and Peter- studies between dominance and reproduc- son, 1996). According to this ‘‘dominance tive success within primate groups, there drive’’ hypothesis, no resources need be in was no relationship. However, in the other short supply at the time of the raid. Instead, half, dominants had higher fitness than unprovoked aggression is favored by the subordinates (Harcourt, 1987; de Ruiter and opportunity to attack ‘‘economically,’’ that is, van Hooff, 1993; Ellis, 1995). This means at low personal risk. If raiding leads to the that even though increased dominance does wounding or death of a neighboring male, not always lead to higher fitness, it pays on the neighboring community’s competitive average. ability is substantially reduced. For ex- Therefore, according to the dominance- ample, if the neighboring community has 10 drive hypothesis, a necessary and sufficient males, its fighting power is reduced by 10%. condition for intercommunity aggression is This reduction lasts for a considerable time, a perception that an opponent is sufficiently because the system of male philopatry means vulnerable to warrant the aggressor(s) at- that a dead male can be replaced only via tacking at low risk to themselves. births within the community, which is a slow The dominance drive hypothesis appears process. The aggressors’ probability of win- useful for explaining why carnivores share ning future intercommunity contests lethal coalitionary violence with chimpan- (battles, not raids) will therefore be signifi- zees. As expected by both the proximate cantly increased by killing a neighboring competition and dominance drive hypoth- male. The increase in relative fighting power eses, fission-fusion grouping and intergroup can be expected to enable a community to hostility occur in these species (Table 6). The enlarge its territory, as suggested by prelimi- proximate competition hypothesis also pre- nary evidence of a correlation between the dicts, however, that the type of food supplies, number of males and territory size at Gombe mating system and/or coalitionary bond (Stanford, 1998b). Over the long term, there- should be similar in allowing benefits to be fore, if fitness is correlated with territory gained from raiding or killing neighbors. size, successful raiding is expected to in- However, the four species of carnivores in crease the raiders’ fitness. which lethal coalitionary violence has been This ‘‘between-community dominance recorded show various combinations of mat- drive’’ hypothesis for explaining aggression ing systems and coalitionary bonds, all differ- between groups is therefore similar to the ent from those found in chimpanzees (Table ‘‘within-community dominance drive’’ hy- 6). For example, intergroup transfer is in pothesis, which contributes to explaining some species primarily by males, in others patterns of aggression between individuals primarily by females. Therefore, lethal vio- within dominance hierarchies (Popp and lence cannot be uniformly explained as re- DeVore, 1979; Chapais, 1995). Within chim- sulting from competition over females. Be- panzee communities, for example, male ag- cause the carnivore species in Table 6 vary gression occurs predictably over status; in the type of benefits to be gained by 16 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999

TABLE 6. Intergroup aggression in fission-fusion species with group territoriality1 Spotted Chimpanzee Human Wolf Lion hyena Cheetah Battles Y Y Y Y Y ? Kill adults Y Y Y Y Y Y Lethal raid Y Y Y Y? Y? ? Food supplies Dispersed Variable Clumped Clumped Clumped Dispersed/ clumped Coalitionary Males Males Pair ϩ Helpers Females; Males Females Males bonds among

1 ‘‘Kill adults’’ is shown separately from ‘‘Lethal raid’’ because, in hyenas and lions, it is not clear if killing of neighbors occurs with lethal raids, or merely when invaders are discovered and killed by residents. ‘‘Dispersed’’ food supplies imply that individual food- patches are not defensible, whereas ‘‘clumped’’ foods can be individually defended (e.g., carcasses). Coalitionary bonds are bonds in which adults support each other in aggression against others. Sources for mating system and coalitionary bonds are Kruuk (1972) (spotted hyenas), Grinnell et al. (1995), and McComb et al. (1994) for lions, and Mech et al. (1998) for wolves. For other sources, see text. intergroup dominance, the dominance drive high rates, and there is no evidence of a sex hypothesis explains similarities in their ten- difference in aggressiveness (Mech et al., dency to use lethal violence more easily than 1998). Why, therefore, are males the princi- the proximate competition hypothesis. pal perpetrators of aggression in chimpan- The proximate competition and domi- zees and humans? nance drive hypotheses are closely related, Traditional explanations are that males because in both cases, the ultimate benefits are more expendable, or that males have of dominance are increased success in re- more to gain simply because they have higher source competition. The proximate competi- variance in fitness than females do (re- tion hypothesis is favored if raiding is elic- viewed by van der Dennen, 1995). However, ited by the presence of stealable resources, such general explanations do not account for or if benefits accrue immediately after a species variation in the intensity of female raid. On the other hand, unprovoked deep participation. Nor does the degree of sexual incursions and attacks on males without dimorphism in body size, because among any obvious reward are better explained by nonhuman primates, sexual dimorphism in the dominance drive hypothesis. To differen- body size is not correlated with female in- tiate these hypotheses more clearly, data are volvement in intergroup aggression (Man- needed on the proximate stimuli that elicit son and Wrangham, 1991). aggression. Since current information sug- Male bonding, which is especially pro- gests that chimpanzee raids are often initi- nounced among chimpanzees and humans, ated without the raiders perceiving mates or has often been proposed to be an important food sources, the dominance drive hypoth- influence (reviewed by van der Dennen, esis appears relevant to explaining the tim- 1995). This idea is supported by the fact that ing and direction of lethal raiding. both in humans and nonhuman primates, populations with more patrilocal residence Sex differences in territoriality (or male philopatry) have relatively greater and aggressiveness tendency for aggressors to be male (Adams, Among chimpanzees, males have to date 1983; Manson and Wrangham, 1991). been the only observed killers and aggres- The ultimate origins of male bonding are sors in intergroup interactions, and males still debated. In chimpanzees, males are are also more likely than females to be more gregarious than mothers, possibly be- victims (Table 4). Among humans, warriors cause, as a result of carrying and waiting for are also overwhelming male (Adams, 1983). infants, mothers travel slowly (Wrangham, This contrasts with spotted hyenas, where 1999a). The relative mobility and gregarious- females are more aggressive than males ness of males means that they can use allies (Kruuk, 1972; Frank, 1986; East and Hofer, to dominate access to their home ranges, 1991; Henschel and Skinner, 1991); and excluding other males and thereby forcing with wolves, where both sexes are killed at male philopatry. As a result, a system evolves Wrangham] EVOLUTION OF COALITIONARY KILLING 17 in which it pays to eradicate males from the community range borders. Among bono- neighboring communities (Wrangham, bos, by contrast, interactions are seen at 1999a,b). range borders without the components of According to this logic, therefore, male lethal raiding. bonding has two effects. First, it contributes Second, relatively peaceful intercommu- to the development of male philopatry and nity relationships in bonobos may be an the benefits of excluding nongroup males, incidental result of a reduction in the level of thereby raising the stakes in territorial en- within-community violence compared to counters. Second, it makes available allies chimpanzees. For example, fewer violent that enable a larger party to dominate a behavioral interactions of all kinds occur smaller party. among bonobos: no sexual coercion, no infan- None of this means, however, that male ticide, no brutal fights among males or fe- participation is a necessary condition for the males competing for dominance, no male evolution of lethal violence in territorial beatings of females (Wrangham and Peter- interactions. As wolves, spotted hyenas and son, 1996; Furuichi et al., 1998). The same is ants show, coalitionary territoriality can be true in captivity (de Waal and Lanting, carried out by both sexes, or even primarily 1997; Stanford, 1998a). Collateral evidence by females. The comparative evidence, there- comes from a survey of cranial and postcra- fore, suggests that lethal raiding in chimpan- nial skeletal trauma that concluded that in zees and humans cannot be attributed to the chimpanzees (and gorillas), but not in bono- fact that bonds among adults are primarily bos, there was evidence of serious risk from among males. interindividual aggression (Jurmain, 1997) and from the generally less robust and less Bonobos: exceptions sexually dimorphic morphology of bonobos that support the rule? than chimpanzees (Zihlman and Cramer, Intercommunity relations among bonobos 1978; Shea, 1984). Pending further data, sometimes involve fights between large par- bonobo males consistently appear to be less ties, but as a species they appear to be violent than chimpanzees. substantially less hostile to each other than Accordingly, a possible hypothesis is that are chimpanzees (reviewed by Wrangham selection may have favored a generally less and Peterson, 1996; Stanford, 1998a). First, aggressive male temperament in bonobos, bonobos have never been seen to engage in as opposed to a loss of motivation specifically lethal raiding, nor indeed in any compo- for lethal raiding. The reasons why male nents of such behavior (Table 2). Second, bonobos are generally less aggressive than they can include markedly peaceful interac- male chimpanzees could derive from the tions, in which individuals from neighboring dominance of males by powerful female– communities rest, travel, copulate, play, and female coalitions, or the greater importance groom together (Idani, 1991; White, 1996). of mothers than other males as allies for In contrast, peaceful interactions involving individual males, or other social dynamics males of neighboring communities have not occurring within communities (Kano, 1992; been seen among chimpanzees. Parish, 1996; Wrangham and Peterson, 1996; In view of the anatomical and phyloge- de Waal and Lanting, 1997). The important netic similarities between chimpanzees and point is that the reduced tendency for lethal bonobos these differences are remarkable. raiding would be viewed as an incidental Three kinds of explanation suggest them- consequence of a more general reduction in selves. male aggression. This hypothesis is chal- First, the facts may be misleading; more lenged, however, by species that have aggres- prolonged observation may reveal lethal sive intergroup interactions despite having raiding in bonobos (Stanford, 1998a). How- peaceful relationships within groups (e.g., ever, this eventuality appears unlikely be- female lions). Furthermore, the tendency to cause at Gombe, Mahale, Ta¨ı, and Kibale engage in lethal raiding seems unlikely to be components of lethal raiding were seen selectively neutral, considering its poten- shortly after individuals were observed near tially large effects on dominance relation- 18 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999 ships between groups and the time and however, remains to be proven. In Lomako, effort spent on raids. I therefore conclude male bonobos spend increased time alone that the low tendency for lethal raiding in during periods of fruit scarcity (White, 1998). bonobos is not merely an incidental conse- White (1998) suggests that these males quence of the benefits of within-community choose to travel alone in order to track the peacefulness. increasingly dispersed female parties, but The third kind of explanation is that according to the imbalance-of-power hypoth- among bonobos, important components of esis, it should be dangerous for them to do lethal raiding has been specifically selected so. However, if solitary travel is a social against. Under what circumstances could option rather than a strategy dictated by this occur? According to the imbalance-of- ecological pressures, it may be possible for power hypothesis, lethal raiding is favored bonobos to restrict their solitary periods to by a combination of coalitionary territorial- times and locations when they can assess ity and imbalances of power sufficient to that they are safe. More quantitative data allow one party to kill victims of the rival will be needed to test such ideas. community with impunity. Since lethal raid- One of the only other species of primate in ing is absent even though coalitionary terri- which lethal raiding might be expected from toriality occurs among bonobos, the imbal- the imbalance-of-power hypothesis are spi- ance-of-power hypothesis predicts that der monkeys Ateles spp., because spider compared to chimpanzees, bonobos must monkeys have a fission-fusion grouping sys- experience greatly reduced power imbal- tem like chimpanzees: individuals some- ances between rival parties. times travel alone and sometimes in larger In general, variance in bonobo party size parties. Furthermore, males are more gre- is less than among chimpanzees, even when garious than mothers, and they form coali- average party size is similar (Chapman et tionary bonds with each other against neigh- al., 1994). This is as expected from the boring groups (Chapman et al., 1995). On imbalance-of-power hypothesis. However, the other hand, spider monkeys are wholly the more critical question is how often bono- arboreal, which may reduce their ability to bos are forced to travel alone, because indi- use coalitionary aggression. Among baboons viduals can be killed (at minimal cost to the Papio anubis, for example, coalitions of low- aggressors) only when they are found alone ranking males are effective in defeating a by a rival party. The two principal bonobo single higher-ranking male on the ground, study sites both indicate that lone travel is but not in trees (Smuts, 1986). Further data rarely forced by ecological pressures. In on the effect of arboreality on power asymme- Wamba, bonobos usually range as one or two tries between coalitions and solitaries is large mixed parties averaging more than ten therefore desirable. individuals (Hashimoto et al., 1998). In Lo- The imbalance-of-power hypothesis mako, where feeding competition appears and the evolution of human warfare more intense than in Wamba, females re- main in multi-female parties even during Peace is the normal human condition, in the seasons when fruit is least available the sense that most human groups, for most (White, 1998). Such observations suggest of the time, are not at war (Ferguson, 1989; that compared to chimpanzees, the intensity Sponsel, 1996). Nevertheless, ethnographic of feeding competition is substantially re- and historical records clearly show that war- duced among bonobos (Chapman et al., 1994; fare is a frequent practice (Keeley, 1996; Wrangham et al., 1996; White, 1998). As a Manson and Wrangham, 1991; van der Den- result, extreme imbalances of power appear nen, 1995). Increasingly, archaeological data unlikely to occur between parties meeting suggest that violence has often been a statis- from neighboring communities. tically important source of death, and it is Whether the apparent difference in the sometimes possible to infer that the violence intensity of feeding competition between was coalitionary (Keeley, 1996; Larsen, chimpanzees and bonobos is sufficient to 1997). In small-scale societies, the common- satisfy the imbalance-of-power hypothesis, est form of war interaction is a raid (e.g., Wrangham] EVOLUTION OF COALITIONARY KILLING 19 Turney-High, 1949; Keeley, 1996; Maschner ers’ ability both to defend a range and to and Reedy-Maschner, 1998). Even if hu- form alliances. mans are routinely peaceful, therefore, war Both intergroup hostility and a fission- needs to be explained. Because warfare in- fusion grouping system are universal in cludes a variety of types of interaction (such contemporary human populations, whether as raids and battles), it will require multiple tribal or nation-state (Rodseth et al., 1991). explanations. Whether these features were characteristic The myriad hypotheses proposed to ex- of humans in prehistory is unknown. How- plain why humans practice raids and other ever, with the exception of brain size, human forms of warfare fall into three general morphology has changed relatively little dur- classes. Maladaptive hypotheses suggest ing the last 1.9 million years (Wolpoff, 1998), that warfare results from an originally adap- suggesting that the essential ecology of hu- tive aggressive tendency that, as a result of man prehistory may been rather stable prior subsequent developments such as the inven- to agriculture. The essence of theories about tion of weapons, became disadvantageous fission-fusion grouping in chimpanzees is even to the winners (e.g., Lorenz, 1966). that fission is a response to high costs of They suffer from the theoretical problem scramble competition (Chapman et al., 1995); that if warring tendency is indeed disadvan- scramble competition is expected to be more tageous, it has such large effects that it intense in species that depend on rare, high- should be selected against rapidly. However, quality foods (Janson and Goldsmith, 1995), it seems unlikely that winners fare badly. and humans appear adapted to high-quality Neutral hypotheses are currently more foods (Milton, 1987; Leonard and Robertson, popular, though they also suffer from the 1997). Following this line of argument, fis- selective-disadvantage problem. They sug- sion-fusion grouping is expected to have been characteristic of human evolutionary gest that warfare should be regarded as history. deriving merely from a capacity, or poten- Based on the ubiquity of xenophobia and tial, resulting from our cognitive creativity. ingroup-outgroup bias in contemporary Neutral hypotheses consider warfare to be populations, intergroup hostility is normally elicited by environmental and social stimuli assumed to have been routine in human that have no evolutionary significance (e.g., prehistory. The likelihood of intergroup hos- Bock, 1980; Keeley, 1996; Gould, 1996). They tility in prehistory is supported also by its are often based on the (erroneous) premise prevalence among nonhuman primates (Che- that behaviors that vary among populations ney, 1986). The form of hostility can be cannot be explained in terms of natural inferred as being territorial, because among selection without assuming genetic differ- primates, territories tend to be found in ences between populations (see Discussion). species with long day ranges in relation to The imbalance-of-power hypothesis exem- home ranges, and are predicted to occur plifies a third kind of hypothesis that views more easily where groups are split into warfare as adaptive and rooted in genetic multiple parties (Mitani and Rodman, 1979; predispositions. It suggests that raiding de- Lowen and Dunbar, 1994). The long day rives from the advantages of gaining inter- ranges of contemporary forager men [e.g., 9 group dominance and an ability to assess km (Bailey, 1991)] and the probability of power imbalances in an environment of inter- fission-fusion foraging suggest that territori- group hostility and power imbalances be- ality would have been possible where home tween parties from neighboring communi- ranges were not immense. ties. As in chimpanzees, it raises the question The imbalance-of-power hypothesis is thus of why territories are defended by males compatible with conventional views of hu- rather than females. By analogy with the man prehistory. It can in theory be chal- argument for chimpanzees, male rather than lenged by evidence that recent prehistoric female territoriality derives from the high ancestors foraged in stable parties, or had cost of travel experienced by mothers ways of reducing power imbalances between (Wrangham, 1999a). This cost reduces moth- rival parties, or had little intergroup hostil- 20 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999 ity, though such evidence would in practice the forest has been disturbed by encroach- be difficult to obtain. ment or logging (Kibale, Ngogo, and On cladistic grounds, various authors have Kanyawara). hypothesized that lethal raiding in humans However, the relative lack of observations and chimpanzees shared a common origin of violence in the early years of chimpanzee around 5–6 mya, and has been present studies cited by Power (1991) is easily under- continuously in the subsequent evolution of stood without reference to the effects of each species (Wrangham, 1987; Ghiglieri, disturbance. Where there was no provision- 1988; Wrangham and Peterson, 1996; Otter- ing, early observations were relatively few bein, 1997). This hypothesis is currently compared to later years, and they were untestable. A key issue for human ancestry mostly of poorly habituated individuals, more is whether australopithecine ancestors of concerned about humans than each other humans foraged in temporary parties (i.e., [e.g., contrast early observations by Ghi- with fission-fusion) or in stable groups. An- glieri (1984), Kibale, Ngogo community, Isa- swers to such questions are needed before birye-Basuta (1989), Kibale, Kanyawara we can be confident whether lethal raiding community, Reynolds and Reynolds (1965), in chimpanzees and humans represents a Budongo, and Sugiyama (1973), Budongo] synapomorphy or a homoplasy. with those based on well-habituated indi- CHALLENGES TO THE viduals by Watts (1999) (Kibale, Ngogo com- IMBALANCE-OF-POWER HYPOTHESIS munity), Wrangham et al. (1992) (Kibale, Kanyawara community), and Newton-Fisher Uncertainty in the chimpanzee data (1997) (Budongo). With increased observa- The evidence of lethal raiding in chimpan- tion of habituated individuals, studies of the zees comes from few cases and a small Kanyawara, Ngogo, and Budongo communi- number of populations, some of which have ties conform to the essential Gombe-Mahale experienced significant anthropogenic influ- model of dominance-motivated and strategi- ences. This has led to doubts about the cally violent males. Power (1991) appears importance of lethal raiding as a species not to have appreciated the difficulty of trait among chimpanzees (Power, 1991; Suss- observing dominance behavior and violence man, 1997). among poorly habituated and little known For example, Power (1991) accepts that individuals. the descriptions of chimpanzees in Gombe, Provisioning complicates the issue be- Mahale, and Kibale as violent and status- cause it confounds increased observability striving are accurate, but regards the behav- with a concentrated food resource that is iors as nonadaptive consequences of exces- liable to promote aggression. In the case of sive ecological stress. A central concern for Gombe, Goodall’s introduction of banana Power (1991) is why there was a shift in the feeding in 1962 led to the chimpanzees perception of chimpanzee society from peace- becoming habituated quickly, so individuals ful to violent, beginning in the 1970s based were watched at close quarters during the on observations after the first 5 years of first decade. Once the chimpanzees were Goodall’s study (1960–1965). She argues that habituated, they were observed almost en- the 1960s view of chimpanzees living in a tirely in the banana-feeding area (ca. 50 ϫ 50 peaceful society was the ‘‘correct’’ one, and m, approximately 1/5,000th of their terri- that subsequent observations of violence tory of 12 sq km or more), and not followed reflect a social environment stressed by vari- toward territorial boundaries. Much aggres- ous kinds of human-induced disturbance. In sion during intense banana-feeding years of support, she cites the fact that intense ag- 1965–1969 was clearly directed toward ob- gression was seen rarely in early chimpan- taining bananas (Wrangham, 1974). During zee studies, especially at Gombe, Budongo, 1969, banana feeding was reduced, and fewer and Kibale (Ngogo community). She notes aggressive incidents occurred in the banana- that in studies where violence has been feeding area (Wrangham, 1974). reported, either humans have provided food Power (1991) argued that a particularly for the chimpanzees (Gombe, Mahale), or important feature of the banana-feeding sys- Wrangham] EVOLUTION OF COALITIONARY KILLING 21 tem was that, after 1965, chimpanzees were not easily viewed as ‘‘distinct.’’ The series of frustrated by their lack of control. She char- attacks that began in January 1974 oc- acterized the observation period 1960–1965 curred almost a decade after the start of the as ‘‘naturalistic’’ (implying undisturbed by problematic banana-feeding-system, and humans) because the bananas were given took place several kilometers from the feed- freely, i.e., they were never withheld. From ing station. Power (1991) implies that this 1965 onward, in attempts to reduce the long delay can be accommodated by the banana-induced aggression of the 1962– hypothesis of a permanent behavioral/psy- 1965 era, bananas were made available in chological reorganization, but clearly the metal boxes equipped with doors that were behavior must also be viewed in the context controlled in various ways by observers. It of ongoing social tensions (see Goodall, 1986 was this system that Power argued caused for an account of the relationships between chimpanzees to express their potential for the alpha-males of the Kasekela and Ka- aggression, because it frustrated them. hama communities that may have helped In the absence of controlled experiments, precipitate the aggression). no hypothesis can be rejected. Two points The incidence of aggression in the banana- relevant to the imbalance-of-power hypoth- feeding-area was closely related to the num- esis can be made, however. First, even if the ber of bananas that chimpanzees obtained. frustration-aggression hypothesis is correct Party size increased in the feeding area in explaining why the Kasekela community compared to the natural habitat, but within attacked the Kahama community, it does minutes of the chimpanzees leaving the not explain why coalitional lethal aggres- feeding area the expected party size was sion was elicited relatively easily in these restored (Wrangham, 1994). These and simi- chimpanzees, or why it occurs in males, lar results show that there were indeed whereas it has not been seen in any other short-term influences related to the availabil- species faced with similarly frustrating con- ity of bananas, but no long-term influences texts. [For example, baboons obtained ba- have been detected (Wrangham, 1974; Good- nanas regularly at the banana-feeding-area, all, 1986). The accumulation of data from though observers tried to prevent them other, nonprovisioned sites continues to chal- (Wrangham, 1974). There has been no hint lenge the view that chimpanzees are natu- of any behavior resembling lethal raiding in rally averse to violence. these baboons, despite intense study in sub- The claim that biology is irrelevant sequent years]. Thus, as Power (1991) her- for human warfare self says, whether or not feeding frustration contributed to the social tensions at Gombe, Some critics reject evolutionary explana- there remains a problem to be explained. tions of warfare out of hand, based on the Why are male chimpanzees easily prompted misconception that the only behavioral pat- to adopt intense coalitionary violence as a terns explicable by biology are ‘‘instincts,’’ solution to social problems? i.e., behaviors that are obligatory and/or Second, the frustration-aggression hypoth- invariable. According to this logic, since esis has much against it. The idea is that warfare is not ‘‘instinctual,’’ biological adap- ‘‘frustration causes a distinct behavioral tations cannot explain the propensity for change in the condition of an organism’’ war (Keeley, 1996; Sussman, 1997; Regal, (Power, 1991, p 3). This implies that under 1998). This error seems remarkable, be- natural conditions, chimpanzees are not cause behavioral ecologists have long naturally frustrated, which is clearly not stressed that psychological adaptations are true. For example, intense aggressive compe- expected to respond in a contingent way to tition occurs regularly in all study sites over appropriate contexts (e.g., Hrdy, 1990; prized foods, such as meat (Goodall, 1986; Barkow et al., 1995; Krebs and Davies, Boesch and Boesch, 1989). On the other 1997). In the words of Otterbein (1997, hand, whatever behavioral change occurred p 272), ‘‘Man is neither, by nature, peaceful among the post-1965 Kasekela chimpanzees nor warlike. Some conditions lead to war, (compared to chimpanzees at other sites) is others do not.’’ 22 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999 The imbalance-of-power hypothesis is en- are the places to seek answers about popula- tirely compatible with the observations that tion variations. many people live wholly peaceful lives, that The assumption that evolutionary analy- some cultures have periods of peace lasting sis implies genetic determinism seems oddly for several generations, and that some chim- old-fashioned in an era when we are begin- panzee populations have no lethal raiding ning to understand the nuances of psycho- for long periods. Indeed, such variation is to neuroendocrinological adaptations of differ- be expected. The imbalance-of-power hypoth- ent species and sexes. To Keegan (1993, p 3), esis conforms well to some theories of peace, war reaches into ‘‘the most secret places of such as the idea that nonviolence is an the human heart, places where self dissolves adaptive response by societies to violence by rational purpose, where pride reigns, where stronger neighbors (Dentan, 1992). Admit- emotion is paramount, where instinct is tedly, males are expected by this hypothesis king.’’ If psychology can describe those se- to take advantage of power over neighbors, cret places, it is the task of evolutionary especially when unfettered by social or cul- anthropology to explain how they arose. tural constraints. They are also expected to IMPLICATIONS OF THE probe for weaknesses in perceived oppo- IMBALANCE-OF-POWER HYPOTHESIS nents, and to be willing to fight in a wide variety of circumstances where elevated sta- Chimpanzee and human psychology tus is predictable or perceived opponents Different versions of the CVH pay varied will be wounded or destroyed at low cost. attention to such factors as cognitive ability, But the essence of the imbalance-of-power weapons, brain size, male-bonding, territori- hypothesis, like other behavioral hypoth- ality, sexual dimorphism, and imbalances of eses for large-brained mammals, is that power (van der Dennen, 1995). They are expression of the behavior depends on con- united, however, in providing adaptive ra- text. Whether or not an individual employs tionales for chimpanzee and human vio- violence is expected to depend on the proxi- lence, and therefore in proposing that lethal mate stimuli, about which we still know raiding has a substantial evolutionary his- little. What leads individuals to classify tory, possibly since our split from a common others as ‘‘opponents’’? How do social and ancestor with chimpanzees. ideological pressures affect the ease with The implication is that there has been which men, or women, respond to incite- selection for a male psyche that, in certain ments to violence? How is ‘‘dominance’’ per- circumstances, seeks opportunities to carry ceived? How are risks perceived, for in- out low-cost attacks on unsuspecting neigh- stance, when Ego is embedded within a bors. The psychological mechanisms that hierarchy of alliances? How do institutional would make such a complex function pos- war relationships influence individual neu- sible have not been studied, but a partial list roendocrinology, and vice versa? Such ques- might include: the experience of a victory tions are critical for understanding who thrill, an enjoyment of the chase, a tendency becomes violent, and when. for easy dehumanization [or ‘‘dechimpiza- In the current context, the imbalance-of- tion,’’ (Goodall, 1986), i.e., treating nongroup power hypothesis suggests that selection members as equivalent to prey], and deindi- has favored certain emotional predisposi- viduation (subordination of own goals to the tions in males that cause aggressive behav- group), ready coalition formation, and sophis- ior to be elicited relatively easily under ticated assessment of power differentials. certain circumstances. The challenge of de- Sex differences can be expected in at least fining the eliciting circumstances is the prov- some of these traits, unless developmental ince of disciplines that probe local varia- constraints interfere. Some features of a tions, including not only biological lethal-raiding psychology are not easily pre- anthropology, but also social psychology, be- dicted, such as the mechanisms by which havioral ecology, social anthropology, cul- expected costs and benefits are assessed. tural ecology, or social ecology. Those disci- A sharp alternative to the CVH is the plines, together with biological anthropology, standard social science model (SSSM), that Wrangham] EVOLUTION OF COALITIONARY KILLING 23 human males have no inherent propensity human groups incorporate more levels of to take advantage of power differentials. social dynamics (Hinde, 1993). At each such Instead, according to the SSSM, humans level, ‘‘level-specific properties’’ influence and merely have a capacity for violence, and are influenced by adjacent levels. For ex- since ‘‘the range of possible cultural results ample, group processes and institutional is not explicable by natural selection’’ (Bock, influences modify the motivational ideals of 1980; p. 76; cf. Gould, 1996), evolutionary individuals. In modern nation-states, the history is claimed to be irrelevant. This line military-industrial-scientific complex tends of thinking has several problems. It does not to precipitate and maintain war, e.g., by account for the species distribution of coali- producing increasingly sophisticated weap- tional aggression. It treats biology and cul- onry. War as an institution can in theory be tures as alternatives, rather than as mutu- maintained by the inertia of subinstitutions, ally interacting influences. It does not such as the belief that capitalism needs account for the predictability of human ag- militarism for its continued growth (Hinde, gressive patterns, and it is easily subsumed 1993). Dynamics like these mean that a under adaptive theories of violence, which propensity for lethal raiding cannot be trans- can account both for the fact that individu- lated directly into an explanation of the als choose to manipulate others (whether complexities of human warfare. through ideology or other ways) and for the Is a propensity for lethal raiding of the fact that they are so easily manipulated. chimpanzee type at all relevant to human warfare? Among people living in small politi- The complexity of war cally independent groups, lethal raiding ap- Even in the complex human world, some pears strikingly similar to the patterns of the processes that regulate aggression among chimpanzees. In both cases, small among large groups are analogous to those parties of males aim to make undetected that occur at the individual or face-to-face incursions into the ranges of neighbors, at- level (Hinde, 1993). Both at the large group tack unsuspecting victims, and retreat with- and the individual level, for example, per- out being drawn into a battle (Turney-High, sonal relations between leaders of opposing 1949; Chagnon, 1992; Keeley, 1996). Al- groups can play an important role, with a though the psychological processes remain threat to the interests or values of the actor undescribed, the imbalance-of-power hypoth- being capable of instigating aggression. Pla- esis might suggest that selection has fa- catory signals or actions (e.g., donations) are vored various complex traits, such as a used to deter aggression. Aggressiveness tendency to classify others as in-group or can be augmented by a greater asymmetry out-group, to regard members of out-groups of power, or reduced by a probability of as potential prey, to be alert to (or search for) punishment. Hinde (1993) found more than power asymmetries between in-group and 20 such analogies, of varying significance out-group parties, and to be ruthless in and distinctness, linking the behavioral in- attacking out-group parties when the per- teractions among individuals and large ceived power asymmetry is sufficiently great. groups. This suggests that, in some ways, A list of traits such as these can in theory the logic of aggressive interaction among describe an evolutionarily selected ‘‘propen- individuals can be applied to large groups. sity for lethal raiding.’’ Nevertheless, so many cultural and lin- Such traits appear obviously relevant to guistic novelties complicate warfare that the some aspects of intergroup relations (e.g., connection to biology can appear tenuous at the planning and execution of military en- best. Among factors such as the number of gagements). Among humans, the complexity military and strategic options available, the of society means that individual propensi- ability to discuss options and manipulate ties sometimes have less direct impact on others, the adoption of cultural goals, and social outcomes than among chimpanzees. the unpredictable potential for shifting alli- Nevertheless, the imbalance-of-power hy- ances, a particularly important trait distin- pothesis may explain why culturally derived guishing humans from chimpanzees is that information is used in certain ways. For 24 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999 example, as a result of cultural beliefs or humans and chimpanzees than in other social pressure, individuals can either primates. Furthermore, the behaviors shown broaden or contract their concept of where by chimpanzees toward mammalian prey an in-group/out-group boundary falls, or of are partly similar to those they show toward how important it is. Idealogues can per- conspecific victims, including quiet stalking suade their followers that sufficient power during a hunt, intense arousal during the asymmetry exists to make attacks on an attack phase (pilo-erection, intimidation dis- outgroup worthwhile. Culture can thus ma- plays), and ambivalence toward the victim. nipulate the information an individual uses In contrast, the behaviors shown by special- to assess whether an attack is desirable. ized carnivores toward their prey are not The imbalance-of-power hypothesis can like those directed toward conspecifics. For be reconciled with the power of culture; example, social carnivores do not show signs therefore, if human males have a tendency of excitement when killing prey, and tend to to search for, and take advantage of, power use a killing bite (van der Dennen, 1995). asymmetries sufficient to enable them to Such observations suggested to Eibl- safely kill rivals, while social pressures Eibesfeldt (1975) and Goodall et al. (1979) modify the concept of ‘‘rival,’’ ‘‘ally,’’ and that among chimpanzees similar motiva- ‘‘sufficient power asymmetry.’’ It accordingly tional factors may be involved in intraspe- suggests an explanation for why human cific killing and hunting. Eibl-Eibesfeldt males become dangerous when they obtain, (1975) specifically proposed that, ‘‘Motiva- or believe they have, large power advan- tionally, hunting behavior in chimpanzees tages over others. (Whether, in novel circum- has probably been derived from intraspecific stances, they use such power adaptively is aggression’’ (translated and quoted by van an open question.) It also suggests the impor- der Dennen, 1995, p 192). The essential logic tance of systems that reduce power asymme- is that if hunting had arisen independently, try, such as intergroup alliances through it should be expected to show more similari- trade, marriage or treaty. ties to the patterns displayed by social carni- When large power asymmetries do not vores. Van Hooff (1990) agreed, suggesting occur, relationships between groups are of- that if selection favored the ability to hunt ten peaceful, as expected from the imbalance- and kill conspecifics, the psychological of-power hypothesis (Knauft, 1991; Bueno mechanisms that evolved would be easily de Mesquita and Lalman, 1992; van der co-opted toward obtaining meat. Dennen, 1995). Even when there is a bal- Note that these ideas are opposite to the ance of power, however, lethal battles and killer ape hypothesis. The killer ape hypoth- wars can occur among humans, in contrast esis suggested that intraspecific violence to the pattern among chimpanzees (Singer, evolved from hunting, whereas Eibl-Eibes- 1989, Boehm, 1992). Coalitionary aggres- feldt (1975), Goodall (1986), and van Hooff sion occurring between opponents with bal- (1990) proposed that hunting evolved from anced power requires other kinds of explana- intraspecific violence. As van der Dennen tion than the imbalance-of-power hypothesis, (1995) notes, the relationship between intra- such as the cultural exaggeration of motivat- specific killing and hunting probably now ing forces or the development of self-decep- involves multiple directions. For example, in tive assessment strategies (Boehm, 1992; some human populations hunting may pro- van der Dennen, 1995; Wrangham, 1999b). vide practice for warfare (Otterbein, 1997). Disentangling these relationships will there- The relation between lethal raiding fore not be easy. and hunting Nevertheless the idea that violence begat Both lethal raiding and hunting are car- hunting is useful because it suggests a new ried out primarily by adult males acting in way to solve a puzzle about bonobos, namely, coordinated groups: both involve otherwise that bonobos show no evidence of monkey unusual actions such as searching for large hunting. Thus, no monkey hunting or mon- prey, stalking, chasing, seizing, wounding, key eating has been recorded at the long- and killing; both are more elaborated in term bonobo sites (Wamba and Lomako), or Wrangham] EVOLUTION OF COALITIONARY KILLING 25 in the shorter-term studies of Yalosidi, Lake suggests that compared to chimpanzees, Tumba, or Lilungu, even though these stud- bonobos have a weaker motivation to hunt ies have provided sufficient data to record, monkeys. for example, termite-eating in four of the As a third possibility, therefore, Wrang- sites (Thompson, 1997). Bonobos do eat meat ham and Peterson (1996) noted that the lack occasionally, however, which they obtain in- of monkey hunting among bonobos might be dividually by seizing young antelope. The explained as a consequence of their low lack of monkey hunting by bonobos is strik- interest in intraspecific killing. For ex- ing given that they not only prey on terres- ample, if bonobos evolved from a chimpanzee- trial mammals, but also complete with each like ancestor, they began with a tendency for other to eat meat, and sometimes interact lethal raiding which was lost or inhibited socially with monkeys, in grooming and when they acquired relatively stable par- play. On three occasions, they have even ties. Accordingly, the evolution of inhibitions kidnapped young monkeys during play, but against lethal raiding may have inadver- not eaten them (reviewed by Wrangham and tently caused monkey hunting to be inhib- Peterson, 1996). ited also, if the two patterns are indeed Stanford (1998b) suggested that the rea- motivationally related (Eibl-Eibesfeldt, 1975; son why male bonobos hunt rarely is that van Hooff, 1990). This proposal implies that hunting has a low pay-off, because they tend monkey hunting is motivationally more simi- to lose meat to females. Against this, low- lar to lethal raiding, and relatively distinct ranking male chimpanzees often lose meat from the killing of terrestrial ungulates. to high-ranking males, but still hunt fre- Important similarities between monkey quently (Goodall, 1986). Furthermore it is hunting and lethal raiding could include the monkey hunting, rather than meat eating, necessity for coordination and planning and that appears to be lacking in bonobos, not the ability to assess an adequate power only among males but also among females. imbalance between predators and prey. Stanford’s proposal is therefore not sup- In summary, it is admittedly speculative ported. to propose that monkey hunting has been Another possible explanation for the lack lost in bonobos as a result of selection against of observations of monkey hunting by bono- propensities relevant to lethal raiding. How- bos is stochastic. Hunting traditions might ever, this idea appears to explain the facts vary among bonobo populations. If so, mon- better than alternative hypotheses. It sug- key hunting may be observed in the future, gests a correlation between group hunting in populations that have not yet been stud- and lethal raiding in humans, chimpanzees ied. However, there is no evidence that any and bonobos that challenges traditional chimpanzee population fails to hunt mon- thinking, and draws attention to the need keys, provided monkeys are present. Chim- for further data. panzees prey on monkeys in at least 12 sites, Morality including all the long-term sites [Gombe, Mahale, Ta¨ı, Kibale, and Budongo (Goodall, This paper suggests that violent propensi- 1986)] as well as seven lesser-known popula- ties of a particular kind have been positively tions [Chambura, (B. Fahey, personal com- selected among male chimpanzees and hu- munication; Kahuzi-Biega, DRC (Basasose mans. Biologically, this is unsurprising. Like- and Yamagiwa, 1997), Lope´, Gabon (Tutin wise, propensities for particular types of and Fernandez, 1993), Mt. Assirik, Senegal altruistic and cooperative behavior have (McGrew et al., 1979), Outamba-Kilimi, Si- probably also evolved through selection, and erra Leone (Alp, 1993), Sapo, Liberia (Ander- are neither more nor less important biologi- son et al., 1983), Tongo, DRC (A. Lanjouw, cally than violence. personal communication). There is, there- But anthropologists’views on violence tend fore, a strong contrast between the wide- to be interpreted politically. For example spread occurrence of monkey hunting in Otterbein (1997) labeled anthropologists as chimpanzees and its absence in bonobos. ‘‘Hawks’’ or ‘‘Doves’’ according to whether Contrary to the stochastic hypothesis, this they consider evolutionary biology relevant 26 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 42, 1999 or irrelevant to warfare (Otterbein, 1997). understand about the evolutionary origins Although Doves (e.g., Power, 1991; Suss- and persistence of intergroup violence, the man, 1997) suggest that Hawks (e.g., Ghi- better we can predict and avert it. glieri, 1984; Goodall, 1986; Wrangham and Peterson, 1996) are culturally biassed in CONCLUSION thinking that male chimpanzees strive ag- Despite some important unsolved prob- gressively for status and use violence adap- lems, chimpanzee lethal raiding appears tively in intergroup interactions, there is no generally well explained by the imbalance-of- evidence for a positive correlation between power hypothesis, which states that success- anthropological Hawkishness (in the Otter- ful attacks on rivals are favored because bein sense) and political beliefs. Indeed, they increase the dominance status of the some notable ‘‘anthropological Hawks’’ have aggressors. A combination of three points been prominent in the search for peace. For likewise suggests that selection has favored example, Hamburg (1991) argued for the unprovoked intergroup violence in human importance of biological similarities in chim- males: the prevalence of human war raiding, panzee and human violence, and in the same the similarities of chimpanzee and human spirit co-chaired a multi-year effort to re- lethal raiding, and the ability of the imbal- duce the frequency and intensity of interna- ance-of-power hypothesis to explain the tional violence (Carnegie Commission, 1997). mammalian distribution of lethal violence. There is no moral high ground to be held by Until an alternative model exists, chimpan- virtue of being an anthropological Dove. zees and humans are, therefore, best re- Admittedly, any theory of violence has garded as species in which a dominance moral implications, because biological analy- drive by male groups has been positively ses can be misused. But no theory, however selected. benign or malevolent or whether based on If this conclusion has merit, anthropology biology, psychology, or culture, is immune to has given inadequate consideration to coali- co-option by ideologues and propagandists. tionary violence as a force in human evolu- While German military philosophy was tion. As anthropologists, we have a duty to backed by Darwinism in the First World acknowledge the horrors of our evolutionary War, French military philosophy was backed past, partly for the sake of truth, and partly by Bergson’s theory of creative evolution to consider how such behavior can be avoided (Tuchman, 1962). On either side, opposing in the future. By combining primatological, theories of evolution were used to bolster the paleontological and behavioral-ecological ev- waging of war. Military organizations can be idence, anthropologists can provide espe- expected to deceive themselves and their cially rich tests of evolutionary hypotheses. followers using any available materials These will offer a solid base from which (Wrangham, 1999b). evolutionary anthropology can work with I see no better course than to follow Dar- other disciplines to understand cultural win (1871, p 405): ‘‘. . .we are not here con- variation and the proximate stimuli that cerned with hopes or fears, only with the elicit violence. truth as far as our reason permits us to ACKNOWLEDGMENTS discover it.’’ Lethal violence appears strik- ingly frequent among chimpanzees and hu- I thank Clark Larsen and the American mans, and appears explicable by relatively Association of Physical Anthropologists for simple adaptive rules. Current evidence sug- inviting me to speak at the AAPA annual gests it has been a major selective pressure meeting in Salt Lake City, April 1998, which for significant periods of chimpanzee and prompted this paper. Irven DeVore, Robert human evolution. Until lethal violence is Hinde, Bill McGrew, and Karen Strier gener- shown to be a strange new phenomenon, we ously offered extended comments. For help- should consider it sufficiently ancient to ful critiques I am grateful also to Chris- have influenced the temperaments of both topher Boehm, Christophe Boesch, Nancy species, particularly of males, in ways that DeVore, Martin Muller, Vernon Reynolds, should be taken seriously. The more we Carel van Schaik, and Michael Wilson. Chris- Wrangham] EVOLUTION OF COALITIONARY KILLING 27 tophe Boesch, Katie Fawcett, Martin Muller, Boesch C. 1996. Social grouping in Ta¨ı chimpanzees. In: McGrew WC, Marchant LF, Nishida T, editors. Great Toshisada Nishida, Vernon Reynolds, and ape societies. 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