The Biopolitics of Primates

 

Johan M.G. van der Dennen (University of Groningen, the Netherlands)

 

Dedicated to the memory of Adriaan Kortlandt, animal psychologist, who died in 2009

 

Introduction

In this chapter, I use the term ‘biopolitics’ to mean evolutionarily-informed political science. Politics has been characterized as “Who gets what, when, and how” (Lasswell, 1936), but rather than about material possessions, politics is understood to be about power, more specifically about collective power, especially differential group power competition, hierarchy and stratification in power distribution, and the universal struggle to enhance power, and to maintain or challenge/destroy this status quo. Politics “should be found in any system of nature in which conflicts of interest exist among cooperating organic units” (G. Johnson, 1995: 279). My main focus will be competitive intergroup relations in monkeys and apes, or as I (van der Dennen, 1995) called it ‘Intergroup Agonistic Behavior’ (IAB). I also briefly treat interindividual and intercoalitionary agonistic behavior when relevant.

 

Early Reports from the Zoo

The London Zoo Massacre: A vivid demonstration of the violence that can occur in primates was provided by a remarkable series of observations by Zuckerman (1932). In 1929-30 Zuckerman had made a detailed study of a large colony of hamadryas baboons (Papio [c.] hamadryas) at the London Zoo in Regent’s Park. Violence was a constantly recurring event, and any major disturbance of the equilibrium (such as the death from disease of a high-ranking male) caused the society to collapse into an anarchic mob, capable of orgies of wholesale carnage (Russell & Russell, 1968).

Zuckerman’s baboon colony was not the only zoo community to indulge in orgies of slaughter. Hall (1964) reported that “fighting broke out in a group of 17 baboons at the Bloemfontein Zoo (in South Africa) when an ‘alien’ adult male and adult female were introduced into their midst”, as a result of which most of the animals were killed or died of their injuries.

Reynolds (1971) observed 24 rhesus monkeys (Macaca mulatta) at Whipsnade Park in the English county of Bedfordshire. Out of 4616 ‘social interactions’ at Whipsnade, 2347 (more than half) were quarrels.

At the Zürich Zoo, where Kummer (1957) (see also Kummer & Kurt, 1965) made observations on 15 hamadryas baboons, bites serious enough to cause nasty wounds were commonplace (Russell & Russell, 1968).

After the initial observations of the baboon colony at the London Zoo by Zuckerman, the age-old ‘simian’ prejudices (cf. Morris & Morris, 1966) generally seemed confirmed: apes and monkeys were perceived as vicious, violent, ferocious and lascivious creatures.

 

Diablo, the Monkey ‘Warlord’

In 1938, when Carpenter (1942, 1964) shipped his rhesus monkeys from India to Cayo Santiago (an islet of 40 acres off Puerto Rico), the land and sea transport was carried out under frightful conditions. There was food shortage on the voyage, and the mothers were under such stress that that most of them fought their young for food, and at least eight mothers killed their babies. During their first year on the islet, there was heavy mortality from lethal fighting, and indeed they seem to have remained unstable and relatively aggressive long afterwards. In particular, the social machinery of leadership selection had been grievously upset. Hence in one of the bands that now formed there arose an aggressive dictator called Diablo, more autocratic and bad-tempered than even those of the zoo colonies.

Diablo’s band waged what amounted to a war of conquest; eventually they had the run of the whole islet. The normal band territorial system had completely broken down. Carpenter removed Diablo, and peace returned to the island (Russell & Russell, 1968).

 

Primates in their Natural Habitat: The first generation of primatologists

Yerkes & Yerkes (1929) had been studying the behavior of individual monkeys for many years in America. Under their auspices, Carpenter went to the island of Barro Colorado in Central America. Here in 1931-3, he studied the societies of howler monkeys (Alouatta palliata). Carpenter also made field studies of red spider monkeys (Ateles geoffroyi) in the Coto region on the borders of Panama and Costa Rica; and in 1937 he was watching gibbons (Hylobates lar) on the hillsides of Thailand. The Cayo Santiago colony has been studied by Carpenter himself (1939-40), Altmann (1956-8) and Koford (1958 onward).

Soon after the end of the Second World War, enterprising Japanese scientists (notably the Primate Research Group of Kyoto University) began to study the monkeys on the islet of Koshima and on Takasakiyama. In 1956 the Japan Monkey Center was set up at Inuyama.

Together with other early observations by (in alphabetical order): Altmann, Bygott, Carpenter, Cheney, DeVore, Fossey, Galdikas, Gartlan, Goodall, Haddow, Hall, Hamburg, Hrdy, Itani, Izawa, Jay, Kawanaka, Koford, Kortlandt, Kummer, Mason, Moynihan, Nishida, Nissen, Reynolds, Saayman, Schaller, Seyfarth, Silk, Southwick, Struhsaker, Sugiyama, Suzuki, Teleki, Ullrich, Washburn, Wrangham, among many others, these studies revealed the intricate societal structures, the affiliative systems and the rather peaceful modus vivendi (with some spectacular exceptions) of monkey and ape populations. Violence, indeed, appeared to be a density-dependent phenomenon in captive populations, enhanced by an unbalanced sex-ratio.

Also early observations in the wild suggested that chimpanzees (Pan troglodytes) lived in an amiable web of friendships and occasional get-togethers (Nissen, 1931; Reynolds & Reynolds, 1965; Goodall, 1968, 1971; Albrecht & Dunnett, 1971). The apes were now regarded as gentle, peace-loving creatures, especially after the publications on bonobos (Pan paniscus) by Parish (e.g., 1994, 1996) de Waal (e.g., 1984), and Parish & de Waal (1999).

 

In retrospective the twenty most important discoveries in primatology from a biopolitical and ‘biocriminological’ perspective (not in chronological order) are:

 

  1. Discovery of infanticide in many primate species (Bygott, 1972; Hrdy, 1977, 1979; Hausfater & Hrdy, 1984, van Schaik & Janson, 2000; Goodall, 1977, 1986);
  2. Discovery of carnivority: meat eating and (cooperative communal) hunting in chimpanzees by Goodall and her team in 1960; Goodall, 1977, 1986; McGrew, Tutin & Baldwin, 1979; McGrew, Tutin, Baldwin, Sharman & Whiten, 1978; Meat-eating has also been observed in bonobos (Ihobe, 1992);
  3. Discovery of cannibalism in chimpanzees (Goodall and her team, 1960; Suzuki, 1971; Bygott, 1972; Goodall: Passion and Pom. A female chimpanzee Passion, and her daughter Pom were seen to kill and eat three Kasakela infants and almost certainly killed seven more newborns over a period of four years (P. Miller, 1995: 107).
  4. Discovery of violence against disabled (polio) and ‘alien’ chimpanzees (ethnocentrism and xenophobia: Goodall); Goodall (1971 et seq.) reported the case of a polio-stricken chimpanzee, partly paralyzed, who was being ostracized, mobbed and harassed by the other chimpanzees (Goethe [1939] designated this kind of behavior as ‘Anstossnehmen’). Bygott (1979) suggested that there would be strong selection for chimpanzee males to be rapidly aroused to attack strangers, particularly males, on sight. Wilson & Wilson (1968) noted that in a captive chimpanzee colony, the most severe attacks were directed at new individuals who had just been introduced into the enclosure, and aggression against strangers and introducees has been noted in many other primate studies (e.g., Hall, 1962; Southwick, 1967; see review in van der Dennen, 1995) in the wild, as well as in the semi-captive colony at the Arnhem Zoo (de Waal, 1982). Especially in relation to human ethnocentrism and xenophobia, it is particularly interesting that in the non­human primates generally – as in humans – intragroup cohesion and intergroup hostility may be correlated (Cheney, 1992).
  5. Discovery that chimpanzees attack an animated stuffed leopard with sticks and clubs (Kortlandt, 1965, 1975, 1999). “Common chimpanzees can throw projectiles with an overarm movement and use big sticks as clubbing weapons against a stuffed leopard model. Surprisingly, nothing of this kind seems ever to have been observed in the gracile chimpanzee” (i.e., the bonobo) (Kortlandt, 1999: 35).
  6. Discovery of degrees of female ‘promiscuity’ and consortship (‘honeymooning’) in chimpanzees (by Goodall and her team in 1994);
  7. Discovery of rape (coercive or forcible copulation) in orangutans (Pongo pygmaeus) (Pitcairn, 1974, 1975; Galdikas, 1979; MacKinnon, 1979; among others);
  8. Discovery of ‘battle-type’ warfare in baboons, macaques, and other primate species (Cheney, van Hooff, Pusey, Hrdy, Smuts, among others; see Appendix)
  9. Discovery of ‘raiding-type’ ‘primitive warfare’ in chimpanzees (Goodall, Wrangham, Nishida, among others; see Appendix). Lethal male raiding indicates “an appetite for hunting and killing rivals that is akin to predation” (Wrangham, 1999: 5). Van der Dennen (1995: 171) already argued that ‘battle-type’ warfare and ‘raiding-type’ warfare corresponded with two discernible motivational systems, the one tilting toward fight/flight motivation, the other toward predation;
  10. Discovery of females as ‘warriors’ in a number of female-philopatric species; female participation in Intergroup Agonistic Behavior (e.g., Macaca mulatta: Hausfater, 1972; Hanuman or gray langurs [Presbytis (Semnopithecus) entellus]: Ripley, 1967: see Appendix).
  11. Social behavior of muriquis or woolly spider monkey (Brachyteles arachnoides) (Strier). Despite their evolutionary and ecological distance from the apes, muriquis nonetheless exhibit striking similarities in various aspects of their social behavior. Most notable are the highly affiliative, egalitarian relationships that philopatric males maintain with one another and with females in their groups (Strier, 2001: 80); Wrangham (2006) called the muriqui “Perhaps the most dramatic primate example of a non-violent species”. Strier, Ziegler & Wittwer (1999) proposed that both the lack of significant changes in testosterone levels with the onset of the copulatory season and the lack of prebreeding increases in cortisol may be related to the low levels of overt aggression displayed by male muriquis over access to mates (see also Strier, 2001 et seq.). Although aggressive interactions were rare among group members, the males showed strong solidarity when they took a stance in encounters against males from neighboring muriqui groups (Strier, 2001: 81). A striking feature of muriqui mating patterns is that males make no overt moves to interfere with one another's sexual activities and do not try to harass females into copulating with them. Instead, males wait patiently until a receptive female favors them with a chance to mate. Fighting among males is futile in the face of unmonopolizable mates, so muriqui males rely for fertilizations on female favoritism and perhaps on more subtle forms of competition (Strier, 2001: 84).
  12. Discovery of raiding behavior in male spider monkeys (Ateles geoffroyi yucatanensis) (Aureli, Schaffner, Verpooten, Slater & Ramos-Fernandez, 2006), revisiting the debate on the relationship between cooperative hunting and warfare. Like chimpanzees, spider monkeys are characterized by male philopatry, a high degree of fission-fusion dynamics, strong associations between males, and male cooperative territorial defense;
  13. Discovery of sex as reconciliation in bonobos (Parish, 1994, 1996; de Waal, 1984; de Waal & Lanting, 1998), and the debate of which Pan-species can serve as model for human phylogeny: “Current debates are therefore not about which species is closer to us, the bonobo or chimpanzee, but rather which one more resembles the last com­mon ancestor” (de Waal, 2001: 44-45).
  14. Discovery of empathy, sharing, reconciliation, reassurance behavior, consolation, compassion, ‘morality’, etc. (e.g., de Waal & Roosmalen, 1979; Preston & de Waal, 2003; Arnold & Aureli, 2006; de Waal, 2010) in chimpanzees and other primates, and mirror neurons (by Rizzolatti and his team) in macaques;
  15. Discovery of many sexual dimorphisms in behavior paralleling those in humans, including dominance, rough-and-tumble play (e.g., Dolhinow & Bishop, 1972; Hamburg & Goodall, 1974), and even juvenile play in vervet monkeys (Cercopithecus [Chlorocebus] aethiops) and rhesus macaques (experiments by Alexander & Hines, 2002; Hassett, Siebert & Wallen, 2008);
  16. Discovery of coalitionary killing in the western red colobus monkey (Colobus badius). At least two and possibly four coalitionary kills were recorded by Starin (1994). In these cases, coalitions of females attacked and killed males attempting to enter their groups (Wrangham, 1999: 5). Before this discovery only male chimpanzees in the wild (Goodall, 1979, 1986; Goodall et al., 1979), and in semi-captivity (de Waal, 1982) had been known to use coalitionary lethal power politics.
  17. Discovery of primate hunting and meat eating in bonobos (Surbeck & Hohmann, 2008; Surbeck, Fowler, Deimel & Hohmann, 2009). Before this time, bonobos were supposed not to hunt communally, and, especially, not to hunt monkeys;
  18. Discovery of ‘incipient culture’: tool making and tool use traditions in (mainly female) chimpanzees (by Goodall and her team in 1960) and orangutans (e.g., Goodall, 1970, 1986; McGrew, 1974, 1992; McGrew, Marchant & Nishida, 1996; Nishida, 1987; Wrangham, McGrew, de Waal & Heltne, 1994);
  19. Discovery of spear-like implements for hunting by female savanna chimpanzees (Pan troglodytes verus) in Fongoli (Senegal), where high levels of in-group and out-group directed aggression has not been observed (Pruetz & Bertolani, 2007); once again questioning the link between hunting and conspecific killing.
  20. Social deprivation experiments in rhesus monkeys by Harlow, Suomi, Sackett, and colleagues (Harlow’s Wisconsin deprivation research): Early isolation in young monkeys leads to hyperaggressiveness, self-mutilation, hypersexuality and bizarre sexual behavior, and pathological social behavior (e.g., Deets & Harlow, 1971). Motherless mothers: Monkeys raised without their mothers and other conspecifics show inappropriate aggression, making poor social companions, and as a result, when placed into social settings, they are not pre­ferred as social partners. They are generally shunned by other monkeys, and end up low in social dominance rank (Higley, 2003: 26).

 

Biopolitics and primatology have been bedfellows from the very beginning of their disciplines (e.g., Masters, 1975, 1983, 1989; Willhoite, 1976; Peterson & Somit, 1980; Schubert, 1983; Somit, 1990; Schubert & Somit, 1982; Schubert, 1991; Schubert & Masters, 1991; G.Johnson, 1995; Somit & Peterson, 1995, 1998; etc.)

 

Interindividual Agonistic Behavior

Threat, attack, and submissive behavior patterns (together termed “agonistic behavior”) are most likely to occur among chimpanzees under the following conditions (Goodall, 1968, 1971, 1986; Hamburg, 1972; 1978; Bygott, 1974; 1979; Tutin, 1975; Pusey, 1979; Trudeau, Bergmann‑Riss & Hamburg, 1981): (1) In daily interactions involving dominance (status); (2) following the recent occurrence of other attacks; (3) in long‑term changes in status or dominance, particularly among males; (4) when apparent unexpressed aggression toward higher ranking mem­bers is redirected at a lower status individual; (5) in the protection of infants by adults of both sexes, but especially by females; (6) in defending against potential predators; (7) in killing and eating animals of other species; (8) in terminating severe disputes among subordinate animals; (9) in association with a presumably painful injury; (10) in exploration of strange or threatening areas; (11) in meeting relatively unfamiliar chimpanzees; (12) in circumstances were highly valued resources are in short supply; (13) by resident females towards newly transferring females; and (14) when relatively unfamiliar animals are crowded in the presence of highly valued resources.

Additionally, Bygott (1979) reports that the most frequent context of attacks and displays occurred at meetings between two individu­als who had been separated for at least half an hour.

Some further generalizations on agonistic relationships offered by Bygott (1974) are: (1) A male’s ability to dominate other males in agonistic interactions changes with age, peaking at physical maturity. (2) Attacks between adult males of the same community are very rare; (3) Adult males are dominant in all their fights with all other age/sex classes. (4) Two or more adult males may form a coalition (usually between brothers) that strengthens the status of both members over that which could have been obtained individually. (5) Agonistic interactions are rarer than expected between relatives of different age/sex classes. (6) Agonistic relationships between females are rare.

Now, such patterns and contexts of agonistic behavior may be taken to be fairly representative for the primates in general, though every species shows its own species‑specific profile of agonistic patterns and contexts. A characteristic form of agonistic behavior lacking in chimpanzees, is, for example, the ‘stink fight’ of ring-tailed lemurs (Lemur catta) (Jolly, 1966), the aggressive ‘herding’ of females as described in Papio [c.] hamadryas (Hall & DeVore, 1965; Kummer, 1968; 1971), or violent rape as described in the orangutan (e.g., Pitcairn, 1974). Furthermore, primate species differ in their degree of showing breeding seasonality of aggression, different types of dominance hierarchies, sexual harassment, territoriali­ty, coalitions, social organization, and so on. But, by and large, the agonistic repertoire as described above gives a fairly accurate picture of primate agonistic behavior in general.

Bernstein & Ehardt (1985) reviewed data from a wide variety of species and report that aggression, on average, accounts for 2‑5 percent of a monkey's daily activity budget (Higley, 2003: 19). The most frequent context in which aggression is seen is in defense of sta­tus (i.e., dominance) (Higley, 2003: 20; Walters & Seyfarth, 1987: 307-308).

In both captive and free‑ranging animals, old scars and wounds, at times even severe wounds, are frequently observed, especially among adult males (Rawlins & Kessler, 1986; Southwick, Beg & Siddiqi, 1965; Steenbeek, Piek, van Buul & van Hooff, 2000). Females also receive wounds, especially during the breeding season (Higley, 2003: 20-21).

The social setting in which aggression is most likely to occur is probably when two troops meet, or, in captivity, during the intro­duction of unfamiliar monkeys into an existing group, or the formation of a new group (Altmann, 1962; Bernstein, Gordon & Rose, 1974; Carpenter, 1974; Cheney, 1981; Poirier, 1974; Southwick, Beg & Siddiqi, 1965; Southwick, Siddiqi, Farooqui & Pal, 1974) (Higley, 2003: 22; see review in van der Dennen, 1995). While severe aggression is more frequent between strangers, among some particularly aggressive individuals, and within groups of some species, it occurs among virtually all primate species studied.

While it is often believed that primate males are more aggressive than females, among many species of Old World primates, rates of aggression are surpris­ingly similar between the sexes. Nevertheless, some sex differences have been demonstrated (Higley, 2003: 24).

Socially living primates must learn not only to recog­nize social cues of aggression but also to restrain and control their own impulses whenever necessary. In fact, achieving high dominance status within a troop may depend on inhibiting aggression as much as on expressing it (Higley, 2003: 26).

The frequency of aggression between two individuals does not necessarily reflect the overall aggressiveness of their social relationship. Since rates of aggression are to some extent dependent on the frequency with which indi­viduals come into contact, animals that associate regu­larly may appear to be more aggressive toward each other than ones that spend less time together. For example, same-sexed peers and kin often show high rates of aggres­sion. However, these individuals usually interact less aggressively than others when the frequency of associa­tion is taken into account (e.g., toque macaques [Macaca sinica]: Dittus, 1977; Japanese macaques [Macaca fuscata]: Kurland, 1977). Similarly, in species with one-male groups, male-male aggression may occur relatively infrequently simply be­cause there is little opportunity for such interaction. Where males do come into contact, however, their interactions are often extremely hostile (e.g., blue monkeys [Cercopithecus mitis]: Cords, 1987; redtail monkeys [Cer­copithecus ascanius]: Cords, 1984, 1987; black-and-white colobus [Colobus guereza]: Oates, 1977; gray lan­gurs: Hrdy, 1977). In multimale groups of baboons, aggression between adult males does not always occur at high rates, but since aggression often accounts for an extremely high proportion of their inter­actions, their relationships appear to be relatively hostile (Walters & Seyfarth, 1987: 308).

 

Infanticide: It is now well documented that Hanuman langurs, after driving a leader male from his females, may systematically kill all infants in the troop (Sugiyama, 1967; Mohnot, 1971; Hrdy, 1974, 1977, 1979). In this way the new leader ensures that most subsequent infants will carry his genes. Of interest are Fossey’s (1979) observations relating to infanticide in the mountain gorillas: three infants were killed as a result of aggressive intergroup interactions. Moreover, on one occasion the lone silverback responsible left with the mother of the dead infant a week later.

Killing of infants has also been reported in chimpanzees (Suzuki, 1971; Bygott, 1972; Goodall et al., 1979), patas monkeys (Erythrocebus patas) (DeVore, pers. comm. in Fossey, 1979), purple-faced langurs (Presbytis senex senex) (Rudran, 1973), redtail monkeys (Simonds, 1977; Struhsaker, 1978), hamadryas baboons (Kummer, Götz & Angst, 1974), crab-eating macaques (also known as Java monkeys) (Macaca fascicularis) (Washburn & Hamburg, 1968), and Verreaux’s sifaka (Propithecus verreauxi verreauxi) (Littlefield, 2010).

Infanticide and cannibalism often go together, e.g., in chimpanzees (Bygott, 1972; Goodall, 1977 et seq.; Kortlandt, 1980; Nishida, 1980; Kawanaka, 1981; Itani, 1982; Schubert, 1983).

Confusing paternity in order to mitigate male aggression is thought to be a common female anti-infanticidal tactic (Strier, 2001: 88).

 

Sexual coercion and harassment: On sexual coercion in primates see Muller & Wrangham (2009). The most plausible alternative to the fertilization tactic theory of rape is the sexual coercion hypothesis proposed by Smuts & Smuts (1993; Clutton-Brock & Parker, 1995). In some species, according to this line of thought, rape may be an evolved male mechanism whose primary aim is not fertilization in the present, but control – for the ultimate purpose of fertilization in the future.

Sexual coercion looks also to be the underlying reason of male battering of female chimpanzees, because the mating patterns in Gombe demonstrate how particularly effective male domination is. As Goodall (1986: 481) described it: “Almost always, unless he is crippled or very old, an adult male can coerce an unwilling female into copulating with him”. So the rule seems to be: Coercion works. Furthermore, Goodall stated: “Males may attack females seemingly in order to drum into their victims, again and again, that theirs is a male-dominated society” (see also Silk, 2002a).

Many pri­mate females have active tendencies toward polyandrous mating, and show opportunistic sexual proceptivity when encountering new males. Harassment by males of sexually attractive females is most salient among the Old World primates, especially in species in which males are much bigger than females or females range alone at least some of the time. Possessive mate guarding is also most pro­nounced in this taxon, as is the incidence of males in­juring females. In other primates, females seem to have much more control over their mating behavior, including the identity of their mating partners. It is therefore likely that unusual sexuality of catarrhine females evolved in part to reduce the risk of infanticide (van Schaik, 2002: 938).

 

Reconciliation and consolation: Reconciliation (e.g., Silk, 1997, 1998, 2002b; de Waal, 2006; 2009) should not be exaggerated. For example, Daniel, Santos & Cruz (2009) recorded 190 agonistic interactions and subsequent post-conflict behavior in a captive group of brown capuchin monkeys (Cebus apella). Only 26.8% of these conflicts were reconciled. Reconciliation was more likely to occur between opponents that supported each other more frequently and that spent more time together (supporting the ‘valuable relationship’ hypothesis).

Similarly, Kempes, de Castro & Sterck (2008) found that both (aberrantly) aggressive rhesus monkeys and human children do not reconcile.

One can deduce from de Waal’s work that reconciliation and “other-regarding behavioral dispositions” are only an in-group phenomenon. Adang (1999) has emphatically pointed out that there is no – never – reconciliation after episodes of intergroup violence in chimpanzees.

Commenting on de Waal’s Peacemaking, Somit (1990: 578-579) aptly remarked: “Unfortunately, the kind of intragroup personal conflict with which de Waal deals is profoundly different, qualitatively and quantitatively, from conflicts involving large-scale societies and states… While our politicians and diplomats might personally be more than willing to embrace the bonobo strategy, ‘make love, not war,’ I doubt that it affords a very promising method of resolving international rivalries and ethnic antagonisms”.

Consolation may be considered related to reconciliation, but, nonetheless, evidence for consolation (requiring empathic ability) among primates is sparse indeed (Bernstein, 1999: 74). Interestingly, in contrast to the former statement, Fraser (2008) found that in chimpanzees the best predictor of reconciliation was the absence of consolation and the best predictor of consolation was the absence of reconciliation, suggesting that consolation may act as an alternative to reconciliation. Thus, valuable partners may reduce post-conflict stress levels in recipients of aggression through consolation when reconciliation fails to occur. Furthermore, mathematical modeling suggests that reconciliation and other affiliative patterns may arise as a side effect of the dominance style (egalitarian vs. despotic) in macaques (Puga-Gonzalez, Hildenbrandt & Hemelrijk (2008).

Romero & de Waal (2011) confirmed the occurrence of appeasement (i.e., postconflict affiliation by a bystander toward an aggressor) in captive chimpanzees. They also found that appeasement occurred more often in the absence of reconciliation than after its occurrence suggesting that appeasement may act as an alternative to reconciliation when the latter fails to occur.

 

Opportunistic coalitions

Over the past decades a continuing study of a semi-captive group of chimpanzees at the Arnhem Zoo in the Netherlands has focused on the importance of (opportunistically) coalitional behavior by males in the establishment and maintenance of, and change in, the primary (i.e., the male) dominance structure of the group (van Hooff, 1973a,b; de Waal, 1978 et seq.; Adang, 1999). An independent female dominance structure is much more stable (see also McGuire, 1982; Hrdy, 1981). Dutch ethologists (Noë, de Waal & van Hooff, 1980; de Waal & van Hooff, 1981; de Waal, 1982 et seq.) and American political scientists alike have discussed the dominance behavior of the Arnhem chimpanzees in terms of cost-benefit analysis, and explicitly as political behavior homologous to that readily observable among contemporary humans (e.g., Schubert, 1983).

It appears that rather than staying with relatives, adult males are opportu­nistic in their relationships, making and breaking alliances for individual advantage as the relative power of each male waxes and wanes (Pusey, 2001: 24).

Adult males of many primate species frequently form al­liances to dispute existing dominance relationships. This occurs, for example, when two males chal­lenge a third who outranks them both (squirrel monkeys [Saimiri sciureus]: Baldwin & Baldwin, 1981; baboons: Hall & DeVore, 1965; Packer, 1977; Rasmussen, 1981; Collins, 1981; Smuts, 1985; Japanese macaques: Stephenson, 1975; chim­panzees: Goodall, 1968; de Waal, 1982; Nishida, 1983). Although such alliances often involve unrelated males, they may consistently involve the same individuals (e.g., Packer, 1977; Saayman, 1971; Nishida, 1983; Walters & Seyfarth, 1987: 316).

Plavcan, van Schaik & Kappeler (1995) found that, among primate species in which the outcome of fights is typically determined by coalitionary fighting, selection for weaponry (canines) is reduced. In particular, male chimpanzees have relatively small canines for their body size, consistent with other evidence that success in battle for chimpanzees depends on coalition size rather than individual weaponry (Wilson, Britton & Franks, 2002; van Schaik, 2002: 937).

 

Polyadic social aggression

Monkeys and apes often resolve fights based on the willingness of others to join on one side or another. In this way size and strength become much less important than the size and reliability of social alliances in resolving fights between individuals. It is no longer the largest and most powerful that will be the victor, but now perhaps the most socially adept (Bernstein, 1999: 74).

Chapais (1991: 200-202) distinguished six functional categories of (triadic, polyadic) aggressive alliances: protective alliances, revolutionary alliances, conservative alliances, resource-specific coalitions, defensive coalitions, and xenophobic alliances, which include all instances in which individuals belonging to the same group jointly threaten or attack one or more members of another group. Such coalitions may develop in three contexts: territorial defense, the repelling of potential immigrants, and dominance interactions between groups. Note that this category of xenophobic alliance is exactly the same as my category of intergroup agonistic behavior (IAB).

Coalitions have the character of transactions which continue as long as both participants derive net benefits and/or have more influence or freedom of movement than in other possible combinations. In an analysis of coalition formation and agonistic third-party intervention in a semi-captive group of chimpanzees, de Waal (1978; cf. also 1982) distinguished a category of protective support, in favor of the weaker party in a conflict, and a category of opportunistic support, in favor of the stronger party.

An increasing number of experimental studies provide evidence that dyadic social relationships in primate groups are affected by third group members and the relations among these (e.g., Kawai, 1965; Kummer, Götz & Angst, 1974). With respect to agonistic interactions complexity appeared to be the rule rather than the exception in two well-established captive groups of Java monkeys: in each group more than half of all aggressive actions occurred during interactions with three or more participants. A detailed description and categorization of the types of agonistic interaction among Java monkeys has been given by de Waal, van Hooff & Netto (1976).

Two of these types of polyadic interactions have in common that one or several group members intervene aggressively in a conflict between others:

(1) Agonistic interactions in which the aggressive party is attacked are called Reactor-Alliances (RA) and the intervening individual is referred to as a protective-aggressor.

(2) If it is the aggressive party that receives support, the interaction type is called an Actor-Alliance (AA), in which we can distinguish the role of start-aggressor and one or several join-aggressors (de Waal, 1978).

From each group of monkeys about 1,000 agonistic interactions were recorded. The analysis of the data revealed that the tendency to join aggressors is considerably higher than the tendency to protect aggressees. In one group 135 AA’s and 44 RA’s occurred, in the other group 110 AA’s and 53 RA’s.

The more altruistic form of support, which depends on a relationship of attachment and affiliation, and the more opportunistic kind, where intervention depends on the possible advantage gained by the supporter, have also been found in other primate species (e.g, Netto & van Hooff, 1986; See Harcourt & de Waal, 1992). Triadic and polyadic interactions appear to be common in primates generally (Walters & Seyfarth, 1987: 309).

 

The paramount majority of all agonistic interactions is apparently an intragroup phenomenon, but some species also exhibit intergroup agonistic behavior which may be more or less collective and orchestrated. This will be the subject of the next paragraphs.

 

Battles

Observing hamadryas and gelada (Theropithecus gelada) baboons in Abyssinia, Sanderson (1955) witnessed “terrific battles, amounting almost to organized warfare – with surprise raids, the taking of prisoners, wide maneuvers, and other grossly human tactics” (quoted in Bigelow, 1969: 31; and Young, 1991: 411).

Van Hooff (1990) vividly portrayed a massively escalated agonistic episode between two olive baboon (Papio [c.] anubis) groups of about 100 and 150 individuals respectively, at Gilgil, Kenya. He described it as an impressive, sometimes earpiercing, spectacular and cacophonous event, lasting more than an hour. A great many animals from both groups formed two, more than 100 meters-long frontlines, mutually threatening and making small sorties, meanwhile alternately ‘jerk-looking’ at their own neighbors (for reassurance) and at the adversaries, and making a hell of a noise (‘shriekbarking’). Suddenly, presumably when one or more adversaries shrank and retreated, a number of animals simultaneously rushed forward in a massive assault. Almost immediately the entire frontline dashed forward and drove the other party before it for some hundred meters, accompanied by a crescendo of shriek­barking by the entire chasing group. Gradually the chase lost impetus, slowed down, the frontlines coming to a standstill and again the parties confronted each other. After a while a similar surging attack would be repeated in the same or the reverse direction, the groups chasing each other to and fro. The end of the confrontation was less spectacular: it ended not in a decisive final chase, but petered out because gradually less and less individuals participated in the forays. This was especially true for the intruding group, which finally withdrew to its own home area. Most notable, according to van Hooff’s account, was the small amount and short duration of physical contacts during the fight. There were no visible injuries, which was probably more due to the animals’ fear to be engulfed by the opponents, rather than to any magnani­mous restraint or inhibition. Furthermore, there were no indications that the manifestly synchronized action could be ascribed to one or more coordinating leaders. It was also striking that the females were just as active in the skirmishes, if not more so, than the males (see also Wrangham & Peterson, 1996).

See especially Smuts, Cheney, Seyfarth, Wrangham & Struhsaker (1987) for pictorial evidence of similar ‘pitched battles’ in a number of other primate species such as vervet monkeys, redtail monkeys, gray langurs, gelada baboons, and rhesus macaques.

DeVore & Hall (1965), describing intergroup relations in chacma baboons (Papio [c.] ursinus) of Nairobi Park and Amboseli, indicated that in many cases groups would drink at the same waterhole with no indication of disturbance. However, groups were not always equally ‘relaxed’ when near each other. Fighting between groups trying to settle in the same sleeping trees has been seen at Amboseli (DeVore & Hall, 1965).

Fighting between bands of hamadryas baboons – the most ‘sexist’ society among the primates – is also conducted by the males. It consists almost entirely of spectacular bluffing, during which the opponents fence at each other with open jaws and slap swiftly back and forth with their hands. Film analysis showed that in spite of appearances, physical contact seldom occurs. Only when one male turns and flees he is apt to receive a scratch on the anal region (Kummer, 1968, 1971). Kummer (1968) described a battle royal between bands of hamadryas over bait he provided.

Recently (2009), two troops of hamadryas baboons have been filmed going to war, with hundreds of monkeys entering into a pitched battle (http://news.bbc.co.uk/earth/hi/ earth_news/newsid_8400000/8400019.stm). The fight, filmed by the BBC Natural History Unit, appeared to be triggered by male baboons attempting to steal females from the harems of rivals. Usually, the two troops live relatively peacefully alongside one another on a 1 km-long cliff in the Awash National Park in Ethiopia. In baboons, as well as other primate species, male individuals may participate in intergroup encounters merely to survey the mating opportunities in another group (Kitchen & Beehner, 2007: 1565).

According to Kitchen & Beehner (2007: 1552), primate groups do not always, or even often, act as cohesive, cooperative units against extra-group animals, even when all members would benefit from excluding rivals. How individuals respond when facing extra-group rivals can vary within a species and even among members of the same group.

Cases in which males cooperate to defend a group or territory are rare among mammals in general (Pusey, 2001: 19).

 

Raids

Raids into neighboring territories may occur for different reasons, including the increase of foraging and mating opportunities directly or indirectly through the killing of neighboring rivals. Lethal raids have been mainly observed in humans and chimpanzees, with raiding males being reported to search purposefully for neighbors.

Bygott (1974, 1979), Wrangham (1975, 1979); Goodall (1979 et seq), Goodall et al. (1979), Nishida, 1979, 1980), and Itani (1982) were the first to report on the intercommunity relationships of the Gombe (Tanzania) population of chimpanzees (Pan troglodytes schweinfurthii), especially episodes of what Goodall literally called “primitive warfare”. Parties of up to ten adult males, sometimes accompanied by females and subadults, quite regularly patrol the boundaries, keeping close together, silent and alert, often stopping to listen intently, apparently actively searching for signs of neighbors. Sometimes they climb a tree to scout the ‘hostile’ territory of the adjacent community, just like a human reconnaissance party might do (the original community had begun to divide into two separate communities about 1970). If no members of the neighboring community are detected, the patrol may stealthily intrude into the ‘enemy’ territory. When a fairly large ‘enemy’ party is encountered both parties may engage in vocal and gestural agonistic displays, or one of them may charge and chase the other away, or both give up and return to their core areas. At other times, a party, upon spotting ‘enemies’, may flee, thus avoiding encounter. When, however, small parties or single ‘enemy’ chimpanzees, particularly anestrous females, are encountered by the ‘warriors’, these may be severely and viciously attacked and killed. Goodall (1979) described several such lethal episodes in some (gruesome) detail. “It seems”, she continued, “that we have been observing a phenomenon rarely recorded in field studies – the gradual extermination of one group of animals by another, stronger, group. Why these brutal attacks? The northern males were not defending their own territory, since all the attacks except one were deep within the southern community home range. On the other hand, the aggressor males, before the community split, had access to the area that the southern community took over. If they were merely trying to reclaim territory they had lost, then they have certainly succeeded”. Subsequently, Goodall (1986) reported observations of five lethal attacks, and some 13 more that left the victims – including adults and infants of both sexes – severely wounded and bleeding profusely. Why, she wondered, would the aggressors attempt to kill, maim or injure their victims instead of merely chasing them away?

By the end of the Four Year War, the Kahama community – seven males and three adult females and their young – had been annihilated. Researchers witnessed five of the attacks, in which Kasakela chimpanzees tore at their victims’ flesh with their teeth as if they were common prey (P. Miller, 1995: 106).

Bygott (1979) and Goodall et al. (1979) emphasized that the males actively seek out agonistic interactions with the adjacent community during their patrolling. Also Nishida (1979, 1980) and Itani (1982) have observed similar group antagonism in chimpan­zees, which was described by Itani as a “skirmish in a war”. On the patrolling behavior of some ‘warrior groups’ Itani also reports: “they looked as if they were aiming for the best chance of encountering another group”, or as if they were looking for an opportunity to ‘hunt down’ conspecifics and inflict fatal injuries (Manson & Wrangham, 1991). Furthermore, the attacks were all characterized by “unusual brutality and persistence” (Bygott, 1979), and the observers could not escape feeling that the aggressors were ‘intentionally’ trying to kill their victims. All observed lethal attacks were unprovoked and lasted at least ten minutes. The victim was deliberately held down by some of the attackers, and subjected to a treatment more brutal than any found in intracommunity aggressive episodes. As Itani (1982) phrased it: “antagonistic interactions of a group versus an individual, or a group versus another group, with the intent to kill, is peculiar to chimpanzee society”.

Interestingly, intercommunity encounters involve mostly males. Females (usually while in estrous) sometimes accompany males on patrol, but they do not typically initiate ‘hostilities’ (Goodall et al., 1979; Wrangham, 1975). Another intriguing observation is that the intense excitement shown by the aggressors during and after the attacks rather easily ‘spills over’ into hunting and killing other primates (red colobus or baboons), which might suggest that at least in some instances similar motivational mechanisms may be involved in both intraspecific violence and interspecific predation (Eibl-Eibesfeldt, 1975; Bygott, 1979; Kortlandt, 1980; Vogel, 1989). Many observers have consensually remarked how arousing it is for primates to participate in and to observe the killing and eating of a prey animal. Some observers have described meat-eating by chimpanzees to be a social, almost a ritualized, experience for the participating animals (Kortlandt, 1980).

Possibly brief attacks on females encountered in overlap zones between neighboring communities attract rather than repel the females concerned (Goodall et al., 1979); some young unhabituated females not only remained within the home range but gradually moved into the core area despite occasional attacks (Pusey, 1979). The male gang attacks on the old male Goliath are particularly puzzling, both in view of his extreme old age and his history of long and peaceful associ­ations with the aggressor males. He could in no way be considered a reproductive competitor (Bygott, 1979).

It appears that the violence of the chimpanzee ‘warriors’ is especially severe towards old, lactating, and anestrous females, and considerably less severe towards females in estrus, i.e., those with high reproductive value. “In particular, young nulliparous females are not attacked severely and instead may be escorted by or forced to travel with the aggressors (Wolf & Schulman, 1984; Goodall, 1986). This makes sense as part of a male reproductive strategy because such females are destined to transfer to a new group and are therefore potential mates. While lethal attacks were likely to be directed against solitary males and anestrous females, estrous females seem to be considered an attractive and alienable resource which can be transferred into the attackers’ group” (Manson & Wrangham, 1991).

Similarly, Ghiglieri (1984, 1987, 1988) reported on the Kibale Forest chimpanzee society in which cooperatively territorial and murderous males were observed to kill the adult males of a smaller group and then absorb their reproductive females. See also Nishida, Hiraiwa-Hasegawa, Hasegawa & Takahata (1985) for an account of the Mahale Mountains National Park chimpanzees.

Ghiglieri (1987) and Alexander (1989) speculated that this strategy may be a pattern common to the human-chimpanzee-bonobo (huchibo) clade. Especially, the combination of male-male cooperation, territoriality and female transfer has been singled out as the starting condition for lethal intergroup aggression (Goodall, 1986; Ghiglieri, 1987, 1988; Alexander, 1989; Manson & Wrangham, 1991; van der Dennen, 1995).

Manson & Wrangham (1991) referred to the chimpanzee ambush-like pattern of stealth, stalking and ‘stabbing in the back’ as “lethal male raiding”, and asserted that it is similar to armed raiding in small-scale human societies.

Goodall (1986: 492-493) stated: “it is clear that interactions between males of neighboring communities are typically hostile”. She outlined the common aspects in the five attacks she witnessed:

 

(a) the attacks were all long – the shortest lasted at least ten minutes, and three continued more than twice as long; (b) all were gang attacks, during which the aggressors sometimes assaulted the victim one after the other, or two to five assailed the victim simultaneously; (c) all the victims were, at some point, held to the ground by one or more of the aggressors while others hit and pounded; (d) all the victims, in addition to being hit, stamped on, and bitten, were dragged first in one direction, then another;... and (g) during each incident the observers, all thoroughly experienced in chimpanzee behavior, believed that the aggressors were trying to kill their victims... because the attacked showed some of the patterns which, while commonly seen during the killing of large prey, have not been seen during intracommunity fighting – as when one of Goliath’s [adult male] legs was twisted, when a strip of flesh was torn from Dé’s [adult male] thigh, or when Satan [adult male] drank the blood pouring from Sniff’s [adolescent male] nose. Moreover, in all cases the attacks continued until the victims were incapacitated (Goodall, 1986: 529; see also Thayer, 2004: 171).

 

From the five long-term sites where chimpanzees live with neighbors (Budongo, Gombe, Kibale, Mahale, and Taï) we now have more than 180 years of data on 11 communities. The 5 sites yield direct evidence of kills of neighbors in 33 cases, and a further 16 suspected victims. They also show repeated interest in killing, evidenced from patrolling of borders to attempts to isolate, immobilize and intensely batter or cut a victim (Wrangham, 2006).

At Mahale, Nishida and his colleagues (Nishida, 1979, 1990; Nishida, Hiraiwa-Hasegawa, Hasegawa & Takahata, 1985) confirmed Goodall’s insights about chimpanzee warfare and discovered that the chimpanzee community they called the K-group gradually became extinct over 15 years as almost all of its adult males died in conflict with the neighboring and much larger M-group. As the number of males was reduced the females were incorporated, at first gradually and then later en masse, into the invading group, as was most of the K-group’s territory, where they mated with the victorious males (Nishida, Hiraiwa-Hasegawa, Hasegawa & Takahata, 1985: 289-292). Kibale also provides evidence of coalitionary kills (Thayer, 2004: 175; Wrangham, 1999: 9; see also Ghiglieri, 1999: 174-175).

Border patrols, deep incursions, ‘commando’ attacks, ‘lateral’ attacks’, coalitionary kills (but not ‘back-and-forth’ attacks), as described in some (gruesome) detail by Goodall (1986), Nishida (1979, 1990), Nishida, Hiraiwa-Hasegawa, Hasegawa & Takahata, 1985; Ghiglieri, 1999; Boesch et al. (2007, 2008), Boesch & Boesch-Achermann (1999/2000), Wrangham (1999), and Thayer (2004: 167-176), are probably all variants of the general phenomenon of coalitionary, intercommunity lethal raiding in chimpanzees.

Among the western subspecies of chimpanzee, Pan troglodytes verus, there is no evidence of lethal intraspecific aggression toward adults or infants from the Bossou or Taï research stations (Thayer, 2004: 175).

According to current data, as reported by Wrangham, Wilson & Muller (2006), the average conservatively estimated risk if violent death for chimpanzees (271 per 100,000 per year) falls in the same order of magnitude as the median value for rates of death from warfare among subsistence-society hunters and farmers (164 and 595 per 100,000 per year, respectively – data compiled mainly by Keeley, 1996). Thus, the authors concluded, among chimpanzees the risk of death from violence appears roughly similar to the risk experienced by humans living in subsistence societies.

On chimpanzee ‘warfare’ see also: Alexander, 1987, 1989; Arnhart, 1995; Boehm, 1992; Boesch & Boesch, 1989; Boesch & Boesch-Achermann, 1999/2000; Boesch et al., 2008; Crofoot & Wrangham, 2009; Diamond, 1992; Gat, 2006; Ghiglieri, 1987, 1988, 1989; Goodall, 1986: 530-34; Hamburg, 1991; Kelly, 2005; Knauft, 1991; Lund, 1995; Manson & Wrangham, 1991; McGrew, 1991; Otterbein, 1985, 1997; Roscoe, 2007; Sherrow & Amsler, 2007; Smith, 2007; Stanford, 1998, 1999; Thayer, 2004; Trudeau, Bergmann‑Riss & Hamburg, 1981, 1981; van der Dennen, 1995; van Hooff, 1990; Watts, Muller, Amsler, Mbabazi & Mitani, 2006; Williams, 2000; Williams, Oehlert & Pusey, 2004; Williams et al., 2008; Wilson & Wrangham, 2003; Wilson, Mjungu & Pusey, 2011; Wilson, Wallauer & Pusey, 2004; Wrangham, 1987, 1991, 1996, 1999; Wrangham & Peterson, 1996; Wrangham & Pilbeam, 2001; Young, 1991; among many others. It is still hotly debated whether lethal male raiding in chimpanzees and humans represents a synapomorphy (shared derived trait) or a homoplasy (convergent evolution) (Wrangham, 1999: 19-20; Wilson & Wrangham, 2003: 385). Explanations of chimpanzee raiding will be presented below.

 

Aureli, Schaffner, Verpooten, Slater & Ramos-Fernandez (2006) reported on the first cases ever witnessed of raiding parties of male spider monkeys (Ateles geoffroyi yucatanensis), a species expected to show such a behavioral tendency, given its similarity with humans and chimpanzees in critical socio-ecological characteristics, such as fission-fusion social dynamics and male-male bonding. Despite the high degree of arboreality of spider monkeys, all seven witnessed raids involved the males progressing single file on the ground in unusual silence. This is remarkably similar to the behavior of chimpanzees. The circumstances around the raids suggest that factors such as reduced mating opportunities, number of males relative to that in the neighboring community, and the strength of bonds among males could play a role in the timing of such actions. The raids did not appear to be aimed at finding food, whereas there is some indication that they may directly or indirectly increase reproductive opportunities. Although no killing was observed, the possibility cannot be excluded, according to the authors, that spider monkey raids may be aimed at harming rivals if a vulnerable individual were encountered. The similarity of spider monkey raids with those of chimpanzees and humans supports the notion that lethal raiding is a convergent response to similar socio-ecological conditions.

“As there is no evidence for cooperative hunting in spider monkeys, the similarity of spider monkey raids with those of chimpanzees and humans could challenge the proposal of a link between group hunting and lethal raids based on the occurrence of both in humans and chimpanzees, but of neither in bonobos (Goodall et al., 1979; van Hooff, 1990; van der Dennen, 1995; Wrangham, 1999)” (Aureli, Schaffner, Verpooten, Slater & Ramos-Fernandez, 2006: 494-495).

 

Bonobos

Little is known yet about bonobo (Pan paniscus) (also known as pygmy chimpanzee and gracile chimpanzee) intercommunity encounters. Encounters are mostly avoided, but when they occur they appear mildly antagonistic, ranging from peaceful mixing in the border area (and mainly confined to adult males giving branch-dragging displays), to nonlethal fighting (no observation has been made of participants killed in intergroup fights), and violent clashes sometimes leading to bloody wounds (Badrian & Badrian, 1984; Kano, 1984; Kano & Mulavwa, 1984; Knauft, 1991; Wrangham & Peterson, 1996). There appears to be intergroup dominance attenuating agonistic contacts (Kano, 1987; Kitamura, 1983; Nishida & Hiraiwa-Hasegawa, 1987).

Kuroda (1980), Kano (1987) and Hohmann & Fruth (1994) observed that male bonobos were the most aggressive sex during intercommunity encounters (Vervaecke, 2002: 61).

The “make love, not war” species, the “ape from Venus” does not exist. In the wild, bonobos appear to be much less ‘sexy’ and much more aggressive (including intergroup violence) than in semi-captivity (Vervaecke, 2002: 123). Even the first signs of hunting of black mangabeys by bonobos have been observed, as well as the consumption of monkey meat (Surbeck & Hohmann, 2009; Surbeck, Fowler, Deimel & Hohmann, 2008).

Stanford (2001) pointed out that the contrast between bonobos and chimpanzees, as painted by de Waal (e.g., 2001) derives largely from a comparison of wild chimpanzees with captive bonobos. In bonobos, intercommunity encounters, too, often involve aggression, and within the group it is often the females who are ‘demonic’ (Slurink, 2002: 272).

Shea (1983) has argued that a variety of bonobo morphological traits are neotenous and pedomorphic compared to common chimpanzees, and proposed that these changes resulted from selection for reduced sexual dimorphism in morphology and behavior. Wrangham & Pilbeam (2001: 11-12) elaborated Shea’s idea with the specific suggestion that reduced sexual dimorphism functioned to reduce aggressive behavior by adult males.

Anatomically and behaviorally the bonobo (pygmy chimpanzee) may be characterized as a slender and pre-pubescent (pedomorphic or juvenilized) form of chimpanzee. Taxonomically speaking the two species Pan troglodytes and Pan paniscus should be renamed the robust and the gracile chimpanzees (Kortlandt, 1999: 25) (and humans the ‘third’ chimpanzee as suggested by Diamond [1992]).

Gracile chimpanzee society is a maternocracy. Another conspicuous characteristic of gracile chimpanzee society is its friendliness and peacefulness in combination with substantial male-female equality (e.g., de Waal & Lanting, 1998). Generally, young mammals are much more tolerant, amicable and playful among themselves than adults. So it might be that the friendliness and peacefulness among the adult graciles is more or less an accidental byproduct of their anatomical and behavioral pedomorphism or juvenilization of the species (Kortlandt, 1999: 35-36).

Kano (1987) made the intriguing suggestion that in the pygmy chimpanzees the “in-group feeling” among females is very strong, and therefore aggressive male expansion of territory is not connected with an increase in available females, and thus does not pay off.

This suggestion was supported by Boose & White (2011) who reported that “factors related to female cohesion reduce the benefit of male direct and coalitionary aggression and instead have selected for alternative individual male strategies utilizing indirect agonism”.

Even contrasts between the male‑dominated hierarchies of chimpanzees, on the one hand, and the more egalitarian relationships of bonobos, on the other hand, come down to the degree to which individuals or groups can monopolize resources, such as food and mates, that are important to reproduction (Strier, 2001: 78-79).

 

Gorillas

Four subspecies of gorilla are recognized today: Western lowland gorilla (Gorilla gorilla gorilla), Cross River gorilla (Gorilla gorilla diehli), Grauer’s gorilla (Gorilla berengei graueri), and the mountain gorilla (Gorilla berengei berengei, formerly Gorilla gorilla berengei). The mountain gorilla is the best studied today, followed by the lowland gorilla.

The wild mountain gorilla is organized into age‑graded‑male troops. Dominance behavior is very low‑keyed and overt aggression nearly nonexistent (Schaller, 1963; 1965a,b; Fossey, 1971 et seq.). Groups respond in variable and unpredictable ways when they meet. Usually the encounters are peaceful, but mutual aggression and aversion, and sometimes severe violence, also occur on occasion (Watts, 1989; Sicotte, 1993; Robbins, Sicotte & Steward, 2001; Bermejo, 2004; Doran-Sheehy, Greer, Mongo & Schwindt, 2004; Magliocca & Gautier-Hion, 2004; Robbins & Sawyer, 2007; Robbins, 2010).  Fossey has stressed the impor­tance of the personal idiosyncracies of the dominant males (silverbacks), who control the movements of the group.

Robbins (2010) has pointed out that female dispersal occurs only during intergroup encounters. Therefore intergroup encounters are key events for male-male competition and female choice. Males ‘herd’ females away from opposing group. Furthermore, she observed that: (1) Interactions with solitary males are more likely to be aggressive and/or result in avoidance behavior (both Karisoke [Virunga] and Bwindi [Impenetrable National Park, Uganda]). Solitary males are more likely to take risks for mate acquisition. Males with groups need to focus on mate retention; (2) The degree of aggression between males during encounters at Karisoke is positively correlated with the number of potential migrant females (nulliparous and cycling females; Sicotte, 1993); and (3) Groups may make large shifts in their ranging patterns following aggressive encounters. (Watts, 1989).

Furthermore, Robbins reported 3 cases of mountain gorilla silverbacks suffering severe injuries and dying following intergroup encounters (Watts, 1989; Williamson, pers. comm.; Grueter, pers. comm.). All were solitary males attacked by multimale groups, and all were relatively young silverbacks, which is suggestive of imbalance of power. She also reported 2 cases of Western lowland silverbacks suffering severe injuries and dying following intergroup encounters (Jeffery, Abernethy, Tutin, Anthony & Bruford, 2007; Cipolletta, 2006, in press). Both were older group-living silverbacks. It has been suggested that Western lowland gorillas tend to have more ‘peaceful encounters’ than mountain gorillas. Bradley et al. (2004) found through genetic analysis that neighboring males were often, but not always relatives. Therefore they proposed the ‘male network hypothesis’: low levels of aggression may be due to males interacting differently towards relatives in neighboring home ranges.

 

The Extent of Animal Intergroup Agonistic Behavior (IAB)

Collective defensive behavior is highly differential among animal species. Most of them have neither defense nor offense in their repertoire. Collective defense is exemplified by avian mobbing of raptors, and collective territorial defense in some other bird species. In some avian species territorial defense involves regular patrolling of territorial boundaries and chorus vocalizations by all group members in concert.

Among social carnivores, a number of species show coordinated lethal attacks. In wolves (Canis lupus), family-based packs occasionally invade neighboring packs’ territories, attacking residents; Mech (1977) found that intraspecific conflict accounted for 43% of deaths not caused by humans. Among spotted hyenas (Crocuta crocuta), who, like wolves, live in family-based, territory-holding groups, intruders into a clan’s territory are likely to be attacked and killed, and smaller clan subgroups patrol the territory boundaries, confronting other ‘patrols’ (Kruuk, 1972, 1975). Neighboring clans sometimes engage in pitched battles over carcasses of prey that one or the other of the groups has killed.

In lions (Panthera leo), which also live in groups (prides) based on a group of related females and one or more associated males, interpride encounters occur, but lethal injury is rare. When invading males are attempting to take over a pride, there may be lethal injuries, though once one male cedes reproductive rights, aggression typically stops. New males are likely to commit infanticide (Bertram, 1976, 1978; Packer, 1986; Packer & Pusey, 1982, 1983, 1984).

Collective defense would be rather pointless if there were no threat or danger of offense. In the remainder of this chapter I shall focus on these (mainly primate) species in which intraspecific intergroup agonistic behavior, involving observed offensive episodes, has been clearly ascertained.

 

General Observations on Intergroup Agonistic Behavior (IAB)

 

1. The overwhelming majority of gregarious and social mammalian species does not have IAB in its behavioral repertoire. It is likely that these species lack the requisite social and cognitive skills, such as a coalitional psychology, to cooperate polyadically for the sake of orchestrated intergroup competition: a high-risk/high-gain strategy.

 

2. The majority of species in which IAB has been documented belong to the primate order (see Appendix). The intergroup behavior of primates is extremely variable – in terms of frequency and intensity of encounters (even between different populations of the same species), the resources being contested, and the sex of the participants – and ranges from very relaxed and ‘peaceful’ to lethal raiding (J.L. Brown, 1975; Eberhard & Candland, 1981; Cheney, 1987; Crofoot & Wrangham, 2009). Some level of intergroup aggression occurs during intergroup encounters in most primates whether or not they actually defend a home range (Fashing, 2001: 219).

Escalated aggression rarely occurs during intergroup conflicts in group-living primates. One possible explanation is that the costs of aggression, compared to the potential benefits, are high (Crofoot & Wrangham, 2009: 18). An alternative explanation is that the collective action problem inherent in group-level resource competition presents an obstacle to high individual investment (Nunn & Lewis, 2001; Nunn & Deaner 2004; Kitchen & Beehner 2007; Crofoot & Wrangham, 18-19), though protecting reproductive investment seems, at least partly, to explain the willingness of individuals to defend the group, even when others do not assist (Kitchen & Beehner, 2007: 1566-1567). Also dominance rank explains behavioral variation in agonistic intergroup participation in several species (capuchins [Cebus capucinus]: Fragaszy, Visalberghi & Fedigan, 2004; Tibetan macaques [Macaca thibetana]: Zhao, 1997; blue monkeys: Payne, Lawes & Henzi, 2003; black howlers [Alouatta pigra]: Kitchen, 2004; ring-tailed lemurs: Nunn & Deaner, 2004; bonnet macaques [Macaca radiata]: Cooper, Aureli & Singh, 2004). Thus, collective action problems are solved in several species because the animals reaping the benefits are the ones providing the service (Kitchen & Beehner, 2007: 1564-1565).

 

In most primate species conflicts between groups are rare. Neighboring groups generally avoid each other. When groups spot one other, the group most eccentric in relation to its home range generally is the first to retreat, or, in case a dominance-subordination relationship exists among the groups (vide infra), the subordinate group retreats (Kawanaka, 1973; Eberhard & Candland, 1981; McKenna, 1982; Cheney, 1983, 1987).

Among primates, exclusive use of space is generally maintained by (a) site attachment and avoidance of the ranges of neighboring groups (mutual proximity-dependent avoidance); (b) site-dependent aggression and regular definition of the conventional location of boundaries; and (c) active defense of (exclusive access to) an area’s resources by advertisement and/or eviction of intruders (territoriality). The behavioral mechanisms regulating spacing and grouping may vary within a genus, and even within a species (Robinson, Wright & Kinzey, 1987). For example, in a Callicebus torquatus (yellow-handed titi monkey) population in Peru, exclusive use of space is maintained by mutual avoidance and restricting movements to familiar areas (Kinzey & Robinson, 1983). In contrast, Callicebus moloch (dusky titi) has been described as ‘territorial’ (Mason, 1966, 1968) in that, in addition to groups occupying exclusive areas, spacing between groups is maintained by site-dependent aggression: the probability that a group will attack, rather than avoid, another group depends on the site at which the encounter takes place. In this (and other) species, that probability is low at the center of the group’s own range, increases the closer the group is to the boundary, and then drops off rapidly as the boundary is crossed. The outcome of an aggressive encounter therefore varies with locality. Each group is more aggressive and therefore displaces other groups more easily when it is within its own exclusive area. Groups are most aggressive close to, but on their own side of, the boundary, a ‘doughnut’-shaped aggression field (Waser & Wiley, 1979) that results in the clear definition and reinforcement of the conventional location of the boundaries. Typically, chasing occurs, but physical contact is rare (Robinson, 1979, 1981; Robinson, Wright & Kinzey, 1987). The ordinal positions of dominance among groups are thus contingent upon their positions on a central-peripheral axis across their territories (Carpenter, 1974). Groups of ring-tailed lemurs, for example, win interactions in their ‘typical’ ranges, and tend to lose outside these areas, regardless of the strength of the opposing group (Pride, Felantsoa, Randriamboavonjy & Randriambelona, 2006).

Intergroup confrontations between dusky titi appear to be very frequent, conspicuous, regular, and predictable, occurring at stable boundary areas, and either limited or markedly affected by time of day (Klein, 1974).

 

When two or more groups of primates meet, the resulting behavior may range from complete fusion of the groups (no agonism), via threats and displays, fights and chases, to outright killing. Typical outcomes of primate group encounters in escalating order of antagonism (Eberhart & Candland, 1981) are:

 

 

Table 1: Outcomes of Primate Group Encounters (after Eberhard & Candland, 1981)

Description                 Characteristic

 

Complete Fusion         Groups coalesce permanently, with restruc­turing of social relations (e.g., rhesus monkeys: Bernstein, Gordon & Rose, 1974).

Fission-Fusion              Groups aggregate and interact, but subsequently diverge (e.g., mountain gor­illa: Schaller, 1963).

Indifference                 Groups in proximity, but appear indiffer­ent to, or ignore each other (e.g., Himalayan langurs: Sugiyama, 1976).

Mutual Avoidance       Coordination of range use such that groups seem to avoid contact (e.g., prosimians: Sussman & Richard, 1974; Owl monkeys: Wright, 1978).

Unilateral Avoidance  Avoidance or displacement of one group by another (e.g., baboons: Nash, 1976; rhesus: Gabow, 1973; Japanese macaques: Kawanaka, 1973).

Mild Threat                 Groups engage in gestural, locomotor, or vocal displays or threats (e.g., squirrel monkeys: Baldwin & Baldwin, 1976; bonobo: Nishida & Hiraiwa-Hasegawa, 1987).

Intensive Threat          High intensity threats involving chases, physical contact between animals is rare (e.g., Kloss’s gibbon: Tenaza, 1975);

Ritualized Contact      Aggressive physical contact between groups, but serious injury occurs rarely (e.g., black-and-white colobus: Schenkel & Schenkel-Hilliger, 1967).

Injurious Contact        Severe aggressive contact, with injury or death in both groups (e.g., cercopithecines: Struhsaker, 1969).

Killing                           Death of one or more animals in one group (e.g., squirrel monkeys: Candland et al., 1978).

 

 

As a rule, most primate agonistic group encounters are of the ‘ritualized contact’ type, in which injuries are rare and hardly serious, and fatalities virtually unknown (Eberhard & Candland, 1981; King, 1980). The few times when contact between groups results in a collective skirmish, it seems to be the result of incidental escalation of brawls between peripheral individuals in which others interfere, rather than a deliberately coordinated and concerted enterprise (van Hooff, 1990). Some non-territorial species have evolved loud calls (‘duetting calls’) that aid in the regulation of intergroup spacing and mutual avoidance. In contrast, when groups defend all or part of their home ranges, most intergroup interactions are characterized by aggression rather than by mutual avoidance. Friendly interactions (play, grooming, copulation) between members of different intraspecific groups do, however, occur in both territorial and nonterritorial species (Cheney, 1987: 272).

The agonistic repertoire in these encounters ranges from vocal and gestural displays, bluffing and intimidation, via threats, chases, ‘pitched battles’ to ‘deliberate’ killing. The level of agonism in macaques depends on the previous history of the troops, familiarity of the members, rank of the troops, and location of the encounter (Chapais, 1983; Ciani, 1986; Hausfater, 1972; Vessey, 1968).

 

3. Old World monkeys show a variety of types of spatial and social organization, but in most cases they use variants of the group home range system, with sometimes as much as 80 % overlap between groups (Hall, 1965). Communal defense of home range or territory against intraspecific intruders seems to be the most common manifestation of IAB in primates and carnivores. However, relatively few primate species maintain true territories in the sense of defended spatial exclusiveness (Crook, 1968; Rowell, 1972; Schuster, 1978; Scott, 1969; Vine, 1973). Rather, there is some overlapping of foraging ranges in most studied species of Cercopithecoidea and Ceboidea. According to Jolly (1972), the cercopithecoids (macaques and baboons) as a group are by far the most aggressive among the primates. They also seem to have the most thoroughly organized societies (Nagel & Kummer, 1975). The guenon-patas group, which ranges from forest to savanna, tends to have a low level of intragroup aggression in either place.

Troops of rhesus macaques (Southwick, 1962; Southwick, Beg & Siddiqi, 1965), Japanese macaques (Kawanaka, 1973), langurs (Sugiyama, Yoshiba & Parthasarathy, 1965; Ripley, 1967), and howlers (Southwick, 1962, 1963; Carpenter, 1965), avoid contact with each other when approaching the overlapping zones. When two howler troops approach each other, they will engage in a noisy howling battle, but Southwick (1963) reported that he never saw two howlers fight.

Adjacent social groups in the majority of colobines generally interact aggressively (Struhsaker & Leland, 1987: 91; Yeager & Kirkpatrick, 1998), with adult males usually the most aggressive and frequent participants. In most populations of Presbytis entellus, adult females (Ripley, 1967), as well as youngsters (Hrdy, 1977) may play prominent roles in intergroup fights. In at least one population of this species, however, neighboring groups were extremely tolerant of one another (Jay, 1965).

Poirier (1974) summarized the major characteristics of colobine aggression (genera Colobus and Presbytis). Much of colobine aggressive behavior, and certainly some of its most dramatic instances, are witnessed between rather than within troops. Among some colobines, extreme aggression is witnessed between males of bisexual and all-male groups. When members of the latter invade a bisexual troop, infanticide and group leadership change is common. Oates (1977) studied the black-and-white colobus of the Kibale Forest, West Uganda. Relationships between guereza groups were generally hostile, and in some habitats groups defended near-exclusive territories. While the Kibale groups had extensive range overlap, they still interacted aggressively in the majority of encounters. Poirier (1970) reported about the intergroup hostility in Nilgiri langurs (Presbytis [Trachypithecus] johnii). Adult male Nilgiri langurs regularly sought conditions for displaying against adult males of other troops. An adult male frequently deserted his troop to challenge another adult male. This is striking because Nilgiri langurs possess means of signaling troop locations and avoidance which could substitute for adult male aggression in the spacing of troops.

Home range size is influenced by both dietary requirements and the spatiotemporal distribution of food (Cheney, 1987; Clutton-Brock & Harvey, 1977). A group’s ability to patrol its range on a regular basis is correlated with the presence or absence of territoriality (Mitani & Rodman, 1979). This is in accordance with the ‘economic defendability’ model of territoriality (J.L. Brown, 1964, 1975).

 

4. Facilitating and/or aggravating conditions of IAB in primates include: extreme crowding and population density due to e.g., areal reduction, competition for food at artificial feeding sites and other rather ‘pathological’ conditions such as disruption caused by human interference. Probably the most fierce intertroop fighting observed in monkeys was that described by Southwick, Beg & Siddiqi (1965) among rhesus monkeys living in a temple in India where there was 90 % overlap between the ranges of three troops. Severe fighting was almost a daily occurrence and all the males bore scars and often sustained broken bones. While adult males usually started the fight, females and juveniles joined in.

 

5. In primate species which form all-male groups, defense by the leader of the bisexual group against the violent appropriation and abduction of females by the all-male group may result in prolonged and severe fighting with skirmishes and raids lasting for periods of 3-10 days (Bygott, 1979; Mohnot, 1971).

In capped langurs (Presbytis pileata [Trachypithecus pileatus]), intergroup aggression is not related to defense of food sources by either sex, but rather appears to involve attempts by males from outside the group to interact with group females. During intergroup encoun­ters, resident females bite and push females that approach an intruding male. Males appear to use intergroup encounters as a means of defending their own females while gaining access to those of other groups (Stanford, 1991).

Mate defense and the exploration of new breeding opportunities appear to be important functions of intergroup conflict also in moustached tamarins (Saguinus mystax) (Garber, Pruetz & Isaacson, 1993) and baboons (Papio spp.) (vide supra).

 

6. For some primate species agonistic intertroop or intercommunity encounters may be highly attractive (at least for some individuals, mostly young males) (chimpanzees: Bygott, 1979; Goodall, 1986; gibbons: Ellefson, 1968; mountain gorillas: Fossey, 1979; langurs: Ripley, 1967; Morrison & Menzel, 1972), and confrontations may be actively sought and provoked (which suggests intrinsic motivation: They seem to be ‘spoiling for a fight’). On the other hand, it has become increasingly clear lately that female involvement in IAB has been systematically underestimated. For example, among rhesus macaques at Cayo Santiago, “violent intergroup squabbles... were marked by sustained fighting in line formation... 2-20 animals faced off with individuals of an opposing group and reciprocally lunged, batted, and growled... participants in the line were most often adult females and 2- to 5-year-old males (i.e., juveniles and subadults)” (Hausfater, 1972). Adult females may be more involved than males and in a number of species are reported to constitute the central phalanx (Manson & Wrangham, 1991).

In many territorial primate species, females are frequently aggressive during intergroup interactions (e.g., ring-tailed lemurs, emperor [Saguinus imperator] and saddleback tamarins [Saguinus fuscicollis], vervets, redtail monkeys, blue monkeys, and Kloss’s gibbons [Hylobates klossii]). Female aggression is more variable, however, in species that only infrequent­ly defend ranges. In some of these, female aggression is common (e.g., macaque spp., red howlers, capuchins). In other species, however, males are the primary antagonists, perhaps because, as Cheney (1987) suggested, encounters more often concern mate, rather than food, defense (e.g., baboons, mountain gorillas).

The wild mountain gorilla (Gorilla gorilla berengei) is organized into age‑graded‑male troops. Dominance behavior is very low‑keyed and overt aggression nearly nonexistent (Schaller, 1963; 1965a,b; Fossey, 1971 et seq.). Groups respond in variable and unpredictable ways when they meet. Usually the encounters are peaceful, but mutual aggression and aversion, and sometimes severe violence, also occur on occasion. Fossey has stressed the impor­tance of the personal idiosyncracies of the dominant males (silverbacks), who control the movements of the group.

In primate species characterized by male dispersal (or female philopatry), female hostility toward other groups and cooperation in intergroup aggression is common, and may involve both resource defense against extragroup females, and, more or less collective, antagonism toward migrant, potentially infanticidal, males. Gang attacks, involving mostly females, have twice been reported to lead to deaths of males attempting to enter a group of red colobus (Manson & Wrangham, 1991). Cheney (1987: 278 ) noted that the hostility of the females toward intruder males often escalates and affects the whole group. Female antagonism toward extragroup females also occurs in some of the social carnivores.

In monogamous primate species, such as the gibbons (Hylobates spp.) and their close relative the siamang (Symphalangus syndactylus), females may be as aggressively participating in cooperative range or territory defense and other intergroup interactions as males. In these situations the animals tend to be most aggressive toward individuals of their own sex, perhaps because they represent potential mate competition (Cheney, 1987: 275). Gibbons maintain their family territories largely by vocalizations and bluffing behavior, which prevents a great deal of actual fighting. Despite this, Carpenter (1940) noted scars of healed wounds on many animals that indicate serious fighting does occur. Monogamy in the gibbons is enforced by hostility between the females, even between mother and daughter (Leighton, 1987; Cheney, 1987: 275).

In primate species characterized by female dispersion, in which the females transfer to new groups, on the other hand, females tend not to participate in aggressive intergroup interactions: e.g., in gorillas (Fossey, 1979; Harcourt, 1978), red colobus (Struhsaker & Leland, 1979), and hamadryas baboons (Abegglen, 1984). Among chimpanzee females attacks on females of other communities are rare, but not entirely absent (Goodall et al., 1979; Cheney, 1987).

Female involvement in intergroup aggression has been proposed to depend on whether resources that limit female reproduction are defensible through cooperative action. Accordingly, species in which females cooperate may be expected to be those with female philopatry. Female philopatry also appears to be associated with female participation in intergroup aggression in social carnivores such as spotted hyenas (Kruuk, 1972) and lions (Packer, Scheel & Pusey, 1990).

 

7. In a number of primate and carnivore species IAB is accompanied by infanticide and, occasionally, cannibalism. For the evolutionary rationale behind infanticide, as a male reproductive strategy, see Hausfater & Hrdy (1984) who also present evidence on other genera and taxa, and van Schaik & Janson (2000).

 

8. Personal idiosyncrasies and ‘character structure’ of (mostly male) leaders strongly influence group integration, the movements of the group within its home range, and dominance-subordinate relationships with other groups (Itani et al., 1963; Kawai, 1964; Fossey, 1971 et seq; Kawanaka, 1973). Careful control of trouble within the group and leading attacks on other groups was for instance characteristic of some leader males at Takasakiyama. Further­more, concerted action and scouting behavior of group males seem to indicate a “consciousness of belonging” (Kawanaka, 1973) in this species. In the Mahale Mountains area, inter-unit-group antagonism is based mainly on the antagonism among adult chimpanzee males who belong to different unit-groups (Nishida, 1979).

Furthermore, individuals may vary in their willingness to participate in intergroup aggression based on characteristics such as their age and experience (e.g., Lazaro-Perea, 2001; Kitchen, Cheney & Seyfarth, 2004; Majolo, Ventura & Koyama, 2005), fighting ability (e.g., Steenbeek, 1999), changing state (e.g., lactating: Lazaro-Perea, 2001) and temperament (Kitchen & Beehner, 2007: 1573).

 

9. When home ranges overlap extensively, the aggressive defense of a particular resource may be more costly than the simple avoidance of other groups. In such cases, intergroup competition is often mediated by the relative dominance of the groups involved. There is evidence for a positive correlation between a group’s size (and the number of adult males) and its ability to displace other groups (though occasionally more subtle factors – such as the history of past relations between the groups – are involved) (J.L. Brown, 1975). Observational studies support the expectation that larger groups should tend to win intergroup fights (Cheney, 1987; Packer, Scheel & Pusey, 1990; Harcourt & de Waal, 1992; Adams 1998; Gat 1999; Wrangham 1999; Boesch & Boesch-Achermann, 2000; Sugiura et al., 2000; Wilson, Britton & Franks, 2002; Thayer, 2004; Crofoot & Wrangham, 2009).

This results in a definite linear group dominance hierarchy revealed by approach-retreat encounters. Approach-retreat encounters (called Type C encounters by Deag [1973]) have, for example, been observed in most macaque species (Givens, 1975; Saito et al., 1998; Crofoot & Wrangham, 2009). Occasionally, groups expand their home ranges at the expense of their neighbors’, and in these cases the relative sizes of the groups – as well as the fighting ability of a particular individual – may determine success (Cheney, 1987: 271-272). Robinson (1988) demonstrated a linear dominance hierarchy among groups of wedge-capped capuchins (Cebus olivaceus). A group’s overall ability to dominate another may influence the behavior of its members  (Kitchen & Beehner, 2007: 1568).

 

10. With regard to the general function(s) of IAB in primates, Washburn & Hamburg (1972) succinctly epitomized: “Intergroup aggression either leads to one group’s having the resources of an area at its exclusive disposal, or at least creates a situation in which one is much more likely to obtain food in one area”.

Food is not the only resource nonhuman species fight over, however. Much more important is the struggle for differential reproduction. Nonhuman vertebrate males frequently come into open conflict over access to females, and/or control of resources useful in attracting females. Females, on the other hand, may cooperate in coalitions of kin to attack reproductive competitors, or the offspring of reproductive competitors (Wasser, 1983; Silk & Boyd, 1983); such situations typically involve harassment of subordinate females and infanticide, with little risk to the aggressors (See review by van der Dennen, 1992). The basic reasons for male-male intergroup aggression, rather than intergroup aggression by both sexes, probably include the different reproductive payoff curves for the two sexes in mammals generally (Low, 1993).

As Alexander (1989), and G. Johnson have emphasized: “Competition is often attributed to scarcity, but to understand fully the ubiquity in competition in a Darwinian world, it must be understood that competition normally occurs even in the absence of scarcity… there will be competition to acquire unequal proportions… In addition, certain reproductively relevant resources are inherently scarce” (G. Johnson, 1995: 293-4). In other words, in a Darwinian world, it is impossible not to compete.

 

Socio-ecology: Making Sense of It All

Four main reasons have been offered to explain why primates might live in groups (collective survival enterprises in Corning’s [1999] words). These are: (1) protection against predators, (2) defense of resources, (3) foraging efficiency, and (4) improved care-giving opportunities (Dunbar, 1988: 106). The costs of group living are mainly the result of competition between members of the same group for access to limited resources.

There are two important factors influencing group size in primates: predation risk promotes the formation of large groups while the costs of group living give rise to tensions that lead to the disintegration of large groups. The balance between these two countervailing forces yields the optimum group size and this will obviously be habitat-specific (Cf. Goodall’s centrifugal and centripetal forces in chimp communities). This optimum group size will, however, be subject to the constraint imposed by a third factor: the availability and dispersion of food in a given habitat (Dunbar, 1988: 131). Overall, the balance between competition and predation explains group size tendencies of different species reasonably well (van Schaik, 2002: 936).

Can we make sense of, and bring some order in, the apparent diversity of the intergroup behavioral patterns in the nonhuman primates and in the other species we have encountered? The most valiant attempt in that direction is the socio-ecological approach as developed by Wrangham (1980, 1987), van Schaik (1983 et seq.), van Schaik & van Hooff (1983), Cheney (1987), van Hooff (1988, 1990), Fashing (2001), Kitchen & Beehner (2007), among others, on the ultimate causes of primate sociality. Though differing in detail and emphasis, these authors consider powerful ecological selection pressures to have shaped the social structure of primates (and by implication other species). Other factors, such as phylogen­etic inertia (Thierry [2000, 2008; Thierry, Iwaniuk & Pellis, 2000] recently found evidence of phylogenetic inertia in the genus Macaca), are also acknowledged, but the emphasis is clearly on the physical and social environment.

The socio-ecological model underlying this reasoning is, highly simplified, the following:

 

1. Primates (organisms in general) are considered to behave as if they were maximizing their reproductive success (RS), and to compete for resources necessary to achieve this ‘aim’.

 

2. As scramble competition (also called ‘exploitation competition’) and contest competition (also called ‘exclusion competition’ or ‘interference competition’) can occur within social groups as well as between social groups, four main types of competition ought to be distinguished: Within-Group Scramble (WGS), Within-Group Contest (WGC), Between-Group Scramble (BGS), and Between-Group Contest (BGC). All four types of competition can be present simultaneously in one species. The main conditions giving rise to contest competition within as well as between groups are: (a) resources in short supply, and (b) the (economic) defensibility of access to those resources. The factors limiting the reproductive success of males and females tend to be different, however, due to the strong asymmetry in parental investment. Consequently, males and females compete for different resources, and the competitive and cooperative (alliances, coalitions, bonding) isosexual interactions, as well as male-female bonds, are expected to reflect these different interests. For example, in situations where males are not able to provide significant services to females (such as protection against sexual harassment or infanticide by other males), females are expected not to develop bonds with males, and in fact should actively attempt to keep them away or repel them from their groups.

 

3. Reproductive success of females is determined largely by the general factor ‘health’, good condition, or nutritional and energetic status, meaning the combined effects of access to vital (food) resources, safety from predators, absence of stress overload and infectious diseases, and absence of other factors conducive to a poor physical condition, and which may adversely affect fecundity.

 

4. Reproductive success of males, on the other hand, is determined to a very large extent by access to fertile females (the only ‘resources’ who can convert the males’ fitness potential into reproductive success). The social behavior among males largely reflects the distribution in space and time of females, and hence mating opportunities (van Schaik, 2002: 934-935).

These two consider­ations combined predict strikingly different reproductive strategies for the sexes. Females, in general, will maximize RS by maintaining a good condition for a long period of time. Males, on the other hand, maximize RS (a) by fertilizing many females and by investing in the maximization of the chances of fertilization (a polygamist strategy); or (b) by investing in long-term paternal care for a small number of consecutively raised offspring (a monogamist strategy). In the former case, males will have to face fierce competition by other males. In the latter, males will have to face possible ‘marital infidelity’ by their mates, and chronic challenges and threats to their paternity confidence. In fact, sexual fidelity “is a phenomenon that seems to occur only when males can impose it on females, either directly through their greater social power or brute force, or, indirectly through controlling the resources that females and their offspring require” (Strier, 2001: 92).

 

5. Predation pressure largely determines sociality versus solitariness, while distribution and monopolizability of food resources largely determine the competition regime. Predictable and defendable resources are conducive to contest competition within and between groups, while abundant, non-clumped, undefendable food resources are conducive to scramble competition (i.e., competition in terms of efficiency of exploitation). When kin-based alliances of females increase access to food patches, females are expected to remain in their natal groups and cooperate with kin, and to form hierarchies of nepotistic ‘matriarchal clan systems’. They are also expected, as the resident sex, to be hostile toward (females of) other groups, in proportion to the economic defensibility of the home ranges, and to participate in intergroup conflicts as ferociously as males or even more so. Because males may aid females in dominating other groups, aggression toward extragroup males is expected to be less severe than toward extragroup females. Similarly, monogamous species are predicted to aggressively defend home ranges.

 

6. The competition regime largely determines the distribution and organiz­ation of females (e.g., female-bonded societies with matrilines and complex hierarchies are common in frugivorous primates with within-group contest competition), and their attachment to the natal group, and, consequently, the migration of males. Males migrate (become the exogamous sex) when females are bonded in female kin-hierarchies and, therefore, discouraged from emigrating (because they will virtually always be worse off if they emigrate to another group). Once females emigrate as well, because they live in a situation of scramble competition and, therefore, are not forced into strong female bonds, males have the option to stay in their natal group and to develop long-lasting relationships based on familiarity and kinship. In those species characterized by female dispersal, females are expected to avoid agonistic intergroup interactions and not to participate in home range defense. The intergroup behavior of males, on the other hand, should primarily involve defense of females against extragroup males.

 

7. The distribution, organization, and reproductive competition of males is determined largely by the distribution, organization, and monopolizability of females. When BGC competition is important, group members are expected to form a large alliance in order to improve their competitive ability as a group. This generally implies a more relaxed and egalitarian WGC regime, otherwise subordinates might either refrain from taking risks in intergroup conflicts, or even defect to another group.

In addition to food, males are expected to compete above all over access to females. Whether this competition takes the form of scramble or contest competition is determined principally by the distribution in space and time of estrous females. If females live in compact groups, access to them can be monopolized, which results in female defense polygyny (either one-male groups if the females can be guarded or herded effectively, or else multi-male groups). If the home ranges in which the females live, or the resources to which they are attracted, can be defended effectively, this gives rise to resource defense polygyny. In these situations intrasexual selection will favor contest vigor and dimorphism in males. If monopolization of females is impossible (females actively resist being monopolized or choose a diversity of mating partners), males may form either monogamous bonds with a single female (most often in the form of exclusive consort relationships with fertile females), or engage in scramble competition polygyny, in which case natural selection favors sperm competition.

The male reproductive competition regime largely determines the ‘politics’ of males, the genesis of (opportunistic) coalitions and support strategies, cooperation in hunting and intergroup conflict if present, the sharing of prey, and the functional analogon to human ‘fraternal interest groups’ in chimpan­zees.

Fashing (2001) found evidence that male aggression toward extra-group rivals functions in resource defense. If males defend food resources to attract mates, it is possible that females ‘pay back’ contributing individuals in some manner (Kitchen & Beehner, 2007: 1561-1562).

 

Mate- and resource-defense strategies

Fashing (2001) found strong evidence consistent with both the direct male mate defense and indirect male mate defense via resource defense hypotheses in his study of male and female participation in agonistic intergroup encounters in Colobus guereza (Kakamega Forest, Kenya). Female guerezas were significantly less often involved in intergroup aggression than males and appeared to rely on males to engage in most of the resource defense. The occasional participation of females in intergroup aggression is difficult to explain, however, without invoking the resource defense hypothesis (Wrangham, 1980; Cheney, 1992).

A review of the most intensive studies of primate intergroup encounters suggests that direct male mate defense may occur in almost all primate species, while female resource defense appears to be most common in species with high levels of female philopatry.

Fashing (2001: 227-228) also presented a hypothesis that predicts when male primates are expected to adopt the resource defense strategy. First, food must be limiting and distributed in defensible patches. Second, groups must include relatively small numbers of females and/or females exhibiting estrous asynchrony so that the females are virtually reproductively monopolizable. Third, females are expected to choose to mate with males that defend resources, and to transfer to other groups once the male in their group begins to defend resources poorly. Fashing predicts that species that both meet the above predictions and include only one adult male per group are the most likely to feature males adopting the resource defense strategy. Species that may meet the above criteria include gibbons and many colobine monkeys.

In species in which males do not have priority of reproductive access to females in their groups, males are not expected to defend food resources during intergroup encounters. For example, in some cercopitechine species characterized by groups of many females and one male, including patas monkeys and at least two species of guenons (Cercopithecus ascanius and Cercopithecus mitis), adult males take no part in intergroup aggression, leaving the females to engage in range defense for themselves (Cords, 1987; Rowell, Wilson & Cords, 1991; Fashing, 2001: 228).

Although females are often victims or bystanders who do not participate at all during intergroup encounters, they can be equally or more involved than males (e.g., blue monkeys: Payne, Lawes & Henzi, 2003; vervets: Cheney, 1992; Hanuman langurs: Borries, 1993; ring-tailed lemurs: Sauther, Sussman & Gould, 1999; black and white colobus: Korstjens, Nijssen & Noë, 2005).

Given that nutrition limits female fitness, female involvement in intergroup encounters with or without male accompaniment should occur more frequently when food resources are “economically defendable” (J.L. Brown, 1964, 1975; Kitchen & Beehner, 2007: 1562).

Wrangham (1980) suggested that when resources are limiting and patchy in distribution, a group of female primates could cooperatively defend a clump against another group. Because cooperation would most likely necessitate bonding among females, Wrangham predicted that group-defense would occur more often in primate species where philopatry allows females to form social bonds with kin. Many empirical studies support this hypothesis. Participation by females is more likely (1) for female-philopatric species, (2) when at least loose bonds exist among female group members, (3) in areas with defendable resources, high between-group competition and low within-group competition, (4) when facing female rather than male competitors (particularly in sexually dimorphic and infanticidal species) and (5) when there is no option to freeload off male group-defense (Kitchen & Beehner, 2007: 1563-1564).

Crofoot & Wrangham (2009: 4-5) have recently criticized that the “tendency to treat feeding and mating competition between social groups as unrelated and, oftentimes, mutually exclusive phenomena, has created an artificial division between species where males compete over mates and species where females compete over food. Recent studies have demonstrated that males can defend food resources either directly, or as a byproduct of their mate defense (Fashing, 2001; Harris, 2005; Harris, 2006a), highlighting the flaws of this dichotomy and indicating that closer attention must be paid to the functional implications of intergroup aggression (Harris, 2007)”.

 

Phylogenetic Inertia

In the 1990s, many researchers set to work testing the predictions from socioecological models. This body of work confirmed many of the predictions derived from socioecological models (Altmann, 2000; Barton, Byrne & Whiten, 1996; Boinski, 1999; Isbell, Pruetz, Lewis & Young, 1999; Kappeler, 2000; Koenig, Beise, Chalise & Ganzhorn, 1998; Mitani, Gross-Louis & Manson, 1996; Mitchell, Boinski & van Schaik, 1991; Nunn, 1999; Pope, 2000; Sterck, Watts & van Schaik, 1997; see also Janson, 1992, 2000) (Silk, 2001: 12030-31)

But the analyses were unsatisfying because they could not account for much of the observed variation in social organization in primates. Moreover, while these studies revealed a number of robust associations among various ecological variables, they did not produce much insight about the causal processes underlying these correlations. Finally, these comparative studies did not consider the possibility that some of the similarities between species were the result of common phylogenetic history, not independent adaptations to ecological conditions (Silk, 2001: 12028).

The role of environment in shaping social organization was challenged by new comparative studies which took phylogeny explicitly into account (Di Fiore & Rendall, 1994). Selected pairwise comparisons of closely related primate species provide support for socioecological models, but these comparisons may not be representative. Most of the characteristics associated with WGC, including linear dominance hierarchies, nepotistic alliances, and female philopatry are found in almost all of the extant Cercopithecine species, even though they now occupy an extremely diverse range of habitats.

Thierry (2000, 2008; Thierry, Iwaniuk & Pellis, 2000) also found evidence of phylogenetic inertia in the genus Macaca. Despite sharing basic patterns of social organization, macaques display a broad range of interspecifc variation in their style of social relationships. Between-group comparisons showed in particular that conciliatory tendencies between nonkin are consistently below 12% in some species whereas they are around 50% or more in others. The use of phylogenetic analyses methods allows to assess how robust are such variations and address processes of evolutionary transformation. A first study showed that variations in the social style of macaques correlate with their phylogeny. By tracing the different characters of the social organization of macaques on their phylogenetic tree, it is possible to recognize their most ancient states and reconstruct their typical ancestral organization. A second study confirmed the occurrence of a strong phylogenetic signal for characters like conciliatory tendencies, proportions of explicit reconciliatory contacts, and kin bias in rates of reconciliation, meaning that such characters are linked to the evolutionary radiation of the macaque genus. Evolutionary change in one character leads to correlated change in other characters.

Di Fiore & Rendall’s and Thierry’s analyses indicate that there is considerable inertia in social evolution. The social systems categorized by socioecologists may represent different peaks in the adaptive landscape (Pope, 2000). If so, this seriously constrains the possible paths for change in social evolution (Silk, 2001: 12031). By limiting the changes possible to social organizations, interconnections between characters act as constraints that channel evolutionary processes and allow only a subset of organizations to arise (Thierry, 2008).

 

Explanations of Lethal Raiding in Chimpanzees

Goodall (1986: 526-533) herself explained the chimpanzee proto-warfare in terms of the idiosyncratic pattern of chimpanzee territoriality and preadaptations common in chimpanzees and early humans. In three important ways, she explains, chimpanzee behavior does not comply with classical territoriality:

(a) Both at Gombe and Mahale it is the relative size and the composition of the two neighboring parties that determine the outcome of an encounter, rather than the geographic location; (b) Chimpanzees have a large home range with considerable overlap between neighboring communities; and (c) It is perhaps in the violence of their hostility towards neighbors that chimpanzees, like hyenas and lions, differ most from the traditional territory owners of the animal kingdom. Their victims are not simply chased out of the owners’ territory if they are found trespassing; they are assaulted and left, perhaps to die. Moreover, chimpan­zees not only attack trespassers, but may make aggressive raids into the very heart of the core area of neighboring groups.

It is also of considerable interest to find that the chimpanzees show behaviors that bear strong resemblance to, and hence may be precursors to pseudospeciation in humans. First, their sense of group identity is strong; they clearly differenti­ate between ingroup and outgroup, between individuals who ‘belong to us’ and those who do not. This sense of group identity is, Goodall claims, far more sophisticated than mere xeno­phobia. The members of the Kahame chimpanzee community had, before they split, enjoyed close and friendly relations with their aggressors. By separating themselves, it is as though they forfeited their ‘right’ to be treated as group members – instead they were treated as strangers. Second, nongroup members may not only be violently attacked, but the patterns of attack may actually differ from those utilized in typical intracommunity aggression. The victims are treated more as though they were prey animals; they are ‘dechimpized’.

Finally, Goodall observed, chimpanzees appear to possess the cognitive sophistication which is a prerequisite for the genesis of cruelty: they are capable to some extent of imputing desires and feelings to others, and they are almost certainly capable of feelings akin to (human) sympathy and empathy.

 

Several hypotheses have been put forward to explain the occurrence of lethal raids in chimpanzees, both in terms of the proximate mechanisms and the long-term functions (e.g., Wrangham, 1979, 1999; Wilson & Wrangham, 2003). They can be reduced to two main hypotheses (Aureli et al., 2006). Lethal raids can be viewed as an extreme form of mate competition in which males kill rival males to defend access to their own females, and possibly to gain access to females from neighboring communities (the “male-only community hypothesis”: Wrangham, 1979; Manson & Wrangham, 1991; Boesch & Boesch-Achermann, 2000; Theoretically, according to this view, the ultimate benefit of intergroup aggression among chimpan­zees is expected to be increased access by aggressive males to reproductively valuable females, via either incorporation of neighbors or encroachment on the territory of neighboring males [Manson & Wrangham, 1991; cf. Low, 1993; van der Dennen, 1995]). Another possibility is that lethal raids are an extreme form of intercommunity feeding competition in which males defend or expand their territory, thus increasing the availability of food sources for themselves, the females of their community, and their offspring (the “bisexual community hypothesis” (implicit in the original ideas of Nishida and colleagues): Wrangham, 1999; Pusey, 2001; Pusey, Williams & Goodall, 1997; Williams, Oehlert & Pusey, 2004).

Since Wrangham formulated this idea of the male-only community hypothesis, evidence against it has accumu­lated. First, in addition to showing aggression to strange males, when males at the edges of the territory encounter stranger females who are not sexually receptive, they often attack them severely ‑ even lethally. Goodall described severe attacks between 1971 and 1982 on 20 different stranger females at Gombe, 15 of whom had infants or juveniles. In three attacks the males killed the females’ infants, but in most cases the aggression was di­rected primarily at the female. In 13 of 14 cases in which the female was well observed, she incurred serious wounds or left a great deal of blood. In no case were the males observed associating with these females after the attack (Pusey, 2001: 18; Wrangham, 2006). Second, females are sensitive to the position of the community border.

What then, is the purpose of a male territoriality whereby males repel not only males, but also females of other communities? Pusey, Williams & Goodall (1997) found evidence that the males are defending a feeding territory for all the community members. By expanding the community range and thereby excluding female as well as male feeding competitors, the males increased the repro­ductive rates of the resident females. If the community range remains large, the number of females may eventually increase as dispersing fe­males settle in the area, but females do not seem to be ‘captured’ in the course of the initial expansion as suggested by the male‑only community hypothesis (Pusey, 2001: 19).

 

The Imbalance-of-Power and Intergroup Dominance Hypotheses

Why should chimpanzees, but few other mammals, engage in lethal intergroup attacks? The imbalance-of-power hypothesis developed by Wrangham and colleagues (Manson & Wrangham 1991, Wrangham & Peterson 1996; Wrangham, 1999, 2001; Crofoot & Wrangham, 2009) attempts to answer this question by drawing attention to the costs, rather than the benefits, of aggression. A growing number of studies support the view that animals use aggression strategically, when the benefits are likely to outweigh the costs of aggression (Archer, 1988; Huntingford & Turner, 1987; Wilson, 2003; Wilson & Wrangham, 2003).

The imbalance-of-power hypothesis proposes that in chimpanzees fission-fusion social structure, combined with coalitionary bonds among males, creates opportunities for low-cost killing of rivals. Chimpanzees travel in parties of varying size and composition, which may result in large disparities in party size during intergroup encounters, even among communities that are similar in overall size. In such cases, members of the larger group can kill rivals at very low risk of injury to themselves. During observed lethal intergroup attacks, three or more attackers ganged up on a single victim, who was immobilized by some of the attackers while others beat and bit the victim. Attackers themselves rarely received injuries. The imbalance-of-power hypothesis thus argues that intergroup attacks are lethal not because of unusually high benefits to be obtained from killing, but because the costs of killing are low during gang attacks. The primary, and ultimate, benefit of intergroup killing is thought to be the reduction of the coalitionary strength of rival communities, or, in other words, the weakening of the territorial power of the neighboring communities (Wrangham, 2001: 265). By reducing the fighting strength of a given community, males increase their chances of success in future battles with that community. More frequent success in battles should result in territory expansion and thus more food for group members, including females and offspring (Wrangham, 1999: 11-12; Wilson & Wrangham, 2003: 381).

It pays for groups to achieve dominance over neighboring groups so that they can enlarge their territories. Also Wilson, Wallauer & Pusey’s (2004) research supports the view that intergroup violence is a persistent feature of chimpanzee societies and that the primary benefit attackers gain from them is reduced competition for resources.

Furthermore they direct their attacks almost entirely to males, the sex that alone defends the territory. Thus while female strangers are often found alone and may be chased and beaten, more than 90% of adult victims of lethal inter-community aggression are male. Even infants killed by adults are mostly male. Those who die, therefore, are mainly the present and future defenders of the territory (Wrangham, 2006).

In the event of a successful attack there is no immediate pay-off other than the satisfaction the aggressors experience from the act itself. The implication is that natural selection has favored in chimpanzees a tendency to relish the prospect and performance of such brutality (Wrangham, 2006).

The idea that the low cost of lethal aggression elicits lethal raiding is central to the imbalance-of-power hypothesis. The imbalance-of-power hypothesis also predicts that the aggressors will be members of the philopatric sex, whether females (as in spotted hyenas, Crocuta crocuta) or males (as in chimpanzees) (Wrangham, 1999; Crofoot & Wrangham, 2009: 22).

 

Boesch et al. (2008) described 485 intergroup encounters involving four communities of chimpanzees of known composition from 1982 to 2005 in the Taï National Park, Côte d’Ivoire. They presented new evidence of lethal violence in this population, but confirmed that fatal violence is less common than documented for other chimpanzee populations.

Taï females have been recorded as being active participants of some of the intergroup encounters led by males (Boesch & Boesch Achermann, 2000), and confirmed by Boesch et al. (2008).

Most striking is the variation in the rate of intergroup killings, with Ngogo chimpanzees having the highest rate at two deaths per year of observation, whereas at Taï none have been observed in two of the three groups despite decades of observations. This difference cannot be attributed, according to the authors, to study duration, since some groups with shorter observation times have high death rates. Nor could it be attributed to the number of neighboring groups. Finally, the number of males in the community does not seem to explain the differences in intergroup death rates.

The intergroup death rate will likely be influenced by the number of individuals present during encounters. This includes support by other group members once encounters have started. Support can change the balance of power between opponents and influence encounter outcomes. Chimpanzee communities seem to vary in the extent to which supporting individuals join intergroup encounters.

These new observations on intergroup encounters in Taï chimpanzees confirm earlier suggestions that, in all known chimpanzee populations with enough observations, intergroup encounters are mostly aggressive and occur regularly throughout the period of study (Boesch & Boesch-Achermann, 2000; Boesch et al., 2008). Boesch et al.’s initial observations made with the North Group have been confirmed with two additional communities in Taï forest. These data as well as recent data coming from chimpanzees in other populations (Ngogo: Watts, Mitani & Sherrow, 2002; Watts, Muller, Amsler, Mbabazi & Mitani, 2006; Budongo: Newton-Fisher, 1999; Reynolds, 2005; Gombe: Wilson, Wallauer & Pusey, 2004; Loango: Boesch et al., 2007) confirm the general occurrence of intergroup competition in chimpanzees.

Direct observations of lethal aggression during intergroup conflicts in Taï chimpanzees show that this extreme behavior is general and not only restricted to East African chimpanzees (Wrangham, 1999; Wrangham, Wilson & Muller, 2006). Similar intergroup attacks by coalitions of many males who inflict numerous injuries on their victims, sometimes including removal of testes and penis, have been observed throughout the range of the chimpanzees in Africa. This strengthens the claim that this behavior is natural in chimpanzees, contrary to claims that humans induce it (Clark, 2002; Hart & Sussman, 2005; Power, 1991).

The observations by Boesch et al. (2008) show that supports regularly change the balance of power and thus create unpredictability. Second, small communities do not refrain from attacking larger communities (for example, when the Middle Group members attacked the much larger South Group or when the South Group members attacked the larger East Group), even when in very small parties. The authors’ general impression was that chimpanzees can take large risks when potential benefits are large or when failure to do so could inflict larger costs (see Grinnell, Packer & Pusey, 1995. for examples in male lions). Males in communities with a relatively small number of sexually active females might take more risks to attack groups with more females to try and improve their reproductive success, even if those groups have many more males (Boesch et al., 2008) [[reinforcing the view that intergroup violence in chimpanzees is a coalitionary high-risk/high-gain reproductive strategy, as proposed by van der Dennen, 1995]].

Recently, Mitani, Watts & Amsler (2010) found evidence that chimpanzees attack neighbors to expand their territories and to gain access to more food. They presented data collected over 10 years from an unusually large chimpanzee community at Ngogo, Kibale National Park, Uganda. During this time, they observed the Ngogo chimpanzees kill or fatally wound 18 individuals of other groups, and they inferred three additional cases of lethal intergroup aggression based on circumstantial evidence. The researchers concluded that a causal link between lethal intergroup aggression and territorial expansion can be made now that the Ngogo chimpanzees use the area once occupied by some of their victims.

Also Wilson, Mjungu & Pusey’s (2011) conclusion on the Kasekela (Gombe) population pointed in that direction: “In the past, Kasekela chimpanzees directed most of their territorial effort toward the food-rich north, but gained most of their female immigrants from the south, suggesting that competition for food, rather than females, is the more common motivator for intergroup aggression in this population”.


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