The Evolutionary Roots of Lethal Conflict
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NEWS & VIEWS RESEARCH ANIMAL BEHAVIOUR (Pan troglodytes) study sites, and 92 years of research at 4 bonobo (Pan paniscus) study sites. The authors assembled information on all instances of lethal aggression that have been The evolutionary roots observed directly by researchers, inferred from the nature of the victim’s injuries or suspected on the basis of the circumstances of their of lethal conflict deaths or disappearances. Coalitional killings were documented at 15 of the 18 chimpanzee A comprehensive analysis of lethal coalitionary aggression in chimpanzees study sites, but there was only one suspected convincingly demonstrates that such aggression is an adaptive behaviour, not killing among bonobos. one that has emerged in response to human impacts. See Letter p.414 The authors then tested how the frequency of killings by chimpanzees was affected by several variables linked to human impact, JOAN B. SILK victims gravely wounded or dead (Fig. 1). including provisioning and habitat distur- The opposing view is that lethal aggression bance, and a second set of variables related to n 2013, there were 33 armed state-level is a non-adaptive response to anthropogenic the intensity of resource competition, includ- conflicts around the world1. Many of these influences, particularly artificial provision- ing the number of males and population had persisted for decades, killed thousands ing. In early primate field studies, researchers density. The statistical model that best fits the Iof people and thwarted international peace- often used food to lure animals out of the for- data includes variables linked to the intensity keeping efforts. War is certainly a contempo- est, to facilitate close-range observation and to of resource competition, rather than those rary fixture, but has it always been one? There enhance habituation. At the Gombe National linked to human impact. Specifically, the is vigorous disagreement over the answer to Park in Tanzania, where in 1962 primatolo- authors’ modelling shows that killings occur this question. Some argue that warfare has gist Jane Goodall began providing bananas to at higher rates in communities that have more been a pervasive feature throughout human chimpanzees who visited her camp, this prac- males and higher population densities. history and has had important effects on tice had the desired effect. However, the chim- These results should finally put an end to the human nature2, whereas others contend that panzees began spending more and more time idea that lethal aggression in chimpanzees is war is rare in foraging groups3, the kinds of in the camp and rates of aggression among a non-adaptive by-product of anthropogenic societies that we lived in for most of our evo- them increased. Provisioning was curtailed, influences — but they will probably not be lutionary history. Debates about the origins and eventually terminated altogether. There enough to convince everyone. Perceptions of and prevalence of human warfare are echoed were no reports of lethal aggression at Gombe the behaviour of non-human primates, par- in the question of whether lethal coalition- before provisioning began, leading some ticularly chimpanzees, are often distorted by ary aggression in chimpanzees has evolved researchers to conclude that these killings were ideology and anthropomorphism, which pro- through natural selection or whether it is a the consequence of human intervention7. Sub- duce a predisposition to believe that morally non-adaptive consequence of human distur- sequent reports of killings at other sites, where desirable features, such as empathy and altru- bance. In this issue, Wilson et al.4 (page 414) chimpanzees had never been provisioned, have ism, have deep evolutionary roots, whereas argue persuasively on the side of adaptation. been attributed to other forms of human inter- undesirable features, such as group-level vio- Many species of non-human primates have vention, including habitat loss8. lence and sexual coercion, do not. This reflects hostile relationships with members of neigh- Wilson et al. have comprehensively tested a naive form of biological determinism. Selec- bouring groups, and some species collectively these two hypotheses by analysing 426 com- tive pressures alter traits as organisms move defend the boundaries of their territories. bined years of research at 18 chimpanzee into new environments and confront new But intergroup encounters rarely lead to seri- ous injuries or deaths, perhaps because the risks of escalated aggression usually do not outweigh the benefits of killing opponents. Lethal coalitionary attacks on individuals from neighbouring communities have been docu- ANDREW BERNARD mented only in chimpanzees. The first report of such killings was published 35 years ago5, but the debate about their adaptive significance continues. One point of view is that natural selection has favoured the evolution of lethal coalition- ary intergroup aggression in chimpanzees as a means to enhance access to valuable resources, such as food and mates. Intergroup aggression might be more deadly in chimpanzees than in most other species because chimpanzees can exploit the imbalances of power that arise from ‘fission–fusion’ social organization6. Chimpanzees often fragment into tempo- rary parties that travel and forage indepen- dently within their community’s home range. When parties of males encounter single individuals from other communities, they sometimes launch brutal assaults that leave Figure 1 | A male chimpanzee with fresh wounds following an inter-group attack. 18 SEPTEMBER 2014 | VOL 513 | NATURE | 321 © 2014 Macmillan Publishers Limited. All rights reserved RESEARCH NEWS & VIEWS challenges and opportunities. The data tell imbalance-of-power hypothesis relies on the University, Tempe, Arizona 85287-2402, USA. us that there are some ecological and demo- odds being in the aggressors’ favour, males e-mail: [email protected] graphic circumstances in which the benefits of forgo opportunities to mate and forage while 1. Themnér, L. & Wallensteen, P. J. Peace Res. 51, lethal aggression exceed the costs for chimpan- they are on patrol, and run at least some risk 541–554 (2014). zees, nothing more. Humans are not destined of being injured in attacks. Do males that join 2. Choi, J.-K. & Bowles, S. Science 318, 636–640 (2007). to be warlike because chimpanzees sometimes patrols and lead attacks gain more benefits 3. Fry, D. P. & Söderberg, P. Science 341, 270–273 kill their neighbours. than those that remain in the security of their (2013). 4. Wilson, M. L. et al. Nature 513, 414–417 (2014). For those who are persuaded by Wilson and own community’s territory? What forces cur- 5. Goodall, J. et al. in The Great Apes (eds Hamburg, D. A. colleagues’ evidence, a more interesting set tail free-riding? The answers to these ques- & McCown, E. R.) 13–53 (Benjamin/Cummings, of questions emerges. For example, how do tions will provide interesting insight into 1979). 6. Crofoot, M. C. & Wrangham, R. W. in Mind the chimpanzees overcome the collective-action the selective forces that favour group-level Gap: Tracing the Origins of Human Universals (eds problem? By eliminating rival males and cooperation in species without language, Kappeler, P. M. & Silk, J. B.) 171–195 (Springer, 2010). infants sired by males from other communi- social institutions and systems for sanctioning 7. Power, M. The Egalitarians — Human and ties, chimpanzees gain access to new territo- free-riders. ■ Chimpanzee: An Anthropological View of Social Organization (Cambridge Univ. Press, 2005). ries and mating partners. But these benefits 8. Ferguson, R. B. in Origins of Altruism and flow to the group as a whole, which creates Joan B. Silk is in the School of Human Cooperation (eds Sussman, R. W. & Cloninger, C. R.) opportunities for free-riding. Although the Evolution & Social Change, Arizona State 249–270 (Springer, 2011). ASTROPHYSICS Seth et al. ‘weighed’ the black hole by determining its gravitational influence on nearby stars orbiting it5,6. To explain the observed stellar velocities and the distribu- Giant black hole in tion of light within M60-UCD1, they had to invoke the presence of a central black hole with a mass 21 million times that of our Sun. a stripped galaxy A black hole of that size would be expected to reside in a host galaxy with a mass of about An oversized, supermassive black hole has been discovered at the centre of a 7 billion solar masses. However, Seth et al. densely packed conglomeration of stars. The finding suggests that the system is estimate that M60-UCD1 has a stellar mass of the stripped nucleus of a once-larger galaxy. See Letter p.398 only 120 million solar masses. Although the discovery of an enormous black hole in such a small galaxy is surpris- AMY E. REINES Seth and colleagues present the first clear case ing, recent work has uncovered a substantial that an individual ultra-compact dwarf is a number of black holes in other low-mass dwarf upermassive black holes, which have stripped-galaxy nucleus, because star clusters galaxies7. However, M60-UCD1 is clearly a masses millions or even billions of times do not host supermassive black holes. different beast from those — it is far more that of our Sun, reside at the centre compact and has a much more massive Sof almost every massive galaxy, includ- black hole. The small black holes in other ing our Milky Way1. There seems to be low-mass dwarf galaxies are probably some connection between the evolution similar to the first ‘seeds’ of supermassive of galaxies and that of these black holes, black holes8. Over cosmic time, such seeds although the nature of the relationship is grow by swallowing gas and coalesc- not well understood. What we do know is ing with other black holes during galaxy ; OPTICAL: NASA/STSCI that, in general, bigger galaxies harbour mergers. With a mass 200 times that of AL. ET bigger black holes at their centres, and the smallest nuclear black holes known, these black holes are typically about 0.5% M60-UCD1’s black hole seems to have of the total mass of a spheroidal galaxy’s already grown considerably.