Pacing through a hushed African forest, a chimpanzee stares uneasily into the twilight. He is skirting along the fringe of his group’s dwindling territory, a no-man’s land in an ongoing war. Adrenaline focuses the senses: he can hear the steady snapping of twigs somewhere in the undergrowth. Is it an enemy patrol? Should he strike? Should he retreat? Make the wrong move and he risks his own death and the demise of his group. Behind his restless eyes, one of the most formidable machines on the planet (a 7-billion-neurone supercomputer) makes some swift calculations.
What drove the evolution of powerful brains? In our recent Nature Communications article, we propose a novel possibility: the idea that threats and opportunities from conspecific outsiders favoured the evolution of animal intelligence.
If you want to catch up on evolutionary biology, the second floor of the glass-panelled Life Sciences Building in Bristol is a good place to start. It’s a revolving door of biologists disappearing to and re-emerging from distant field sites, laden with hard-won data on mongooses, dolphins, birds, fish, insects, and even (once) a group of bewildered human babies wearing GoPros. In 2018, Ben appeared among this menagerie of zoologists – bringing with him a fascination for animal cognition, gleaned from countless hours of enlisting wild Australian magpies to solve logic puzzles during his PhD. As a professor at Bristol, Andy had been running a large research project on interactions between groups, in which Patrick is a postdoc in social evolution theory. Somewhere (which may have been the Highbury Vaults pub), a plan was hatched: to bring together our three main backgrounds (cognition, inter-group interactions, and social evolution theory) to think about how they might be connected in the evolution of intelligence.
Despite decades of study, the driving forces behind animal intelligence remain hotly debated. Broadly, two camps have emerged. One faction holds that brain size is driven by ecological factors. The other contends that social life is crucial (the so-called ‘social intelligence hypothesis’1,2). These views are not mutually exclusive, and both may play a role in cognitive evolution. And yet, each hypothesis has only managed to explain a relatively small proportion of the immense variation in brain size between species. We do not yet have a convincing explanation for why some species have more powerful brains than others.
The ‘social intelligence hypothesis’ holds that the challenges of competing against (and cooperating with) group-members led to the extreme brain power found in many primates. Famously, many proponents have invoked ‘Machiavellian intelligence’, the House of Cards-style cunning needed to navigate social politics. However, the social intelligence hypothesis has traditionally looked inwards, focusing on interactions within social groups. This leaves a major dimension of animal social life unexplored: interactions with outsiders.
Life in a world of threat, bluff, and invasion from conspecific outsiders is a daily reality for many animals, and can ultimately result in war between rival factions. In recent years, interest in the role of outsiders in shaping animal social evolution has been growing – biologists have meticulously quantified the remarkable effects of competition between chimpanzee groups3, caught female vervet monkeys using underhand tactics to prod males into fighting rival groups4, and revealed how threats from outsiders have major knock-on effects to how dwarf mongoose groups behave5. Research into outsider pressure is expanding, and we argue that it may have much to offer the study of cognitive evolution. Complementing the ‘Machiavellian’ world of within-group politics, we make the case that it may be illuminating to consider the ‘Napoleonic’ intelligence needed to navigate interactions with outsiders.
Napoleonic intelligence comes in diverse forms. It lies in the ability to react tactically to outsider movements, to interpret cues of rival presence, to think quickly during skirmishes, to inveigle oneself into neighbouring groups for mating opportunities, to update mental maps of shifting territories, to fine-tune investment in territorial defence on several fronts, to evaluate cues of group identity, to calibrate behaviour towards outsiders depending on who’s watching, to act in a coordinated fashion in intergroup fights, and so on. This endless world of outsider interaction has remained largely unstudied in the social origins of complex cognition, which has been restricted almost completely to within-group behaviour. The product of many caffeine-powered corridor conversations, inter-continental Zoom debates, and strange diagrams scrawled on office whiteboards, our paper makes the case that these and other outsider interactions generate strong pressures favouring the evolution of several aspects of intelligence, and we sketch a roadmap for testing these ideas empirically.
The possibility that outsiders have shaped intelligence has been suggested before, in particular by two of the pioneers of social evolution – R. D. Alexander6 and W. D. Hamilton7. However, both of these brainy primates were thinking only of our own species. It is time to expand their insights beyond human brain evolution. What do today’s big-brained chimps, dolphins, and birds have in common with Machiavelli and Napoleon? We suspect that their ancestors possessed the intelligence to triumph in one of the highest-stakes games of all: outmanoeuvring outsiders.
The paper (‘Interactions with conspecific outsiders as drivers of cognitive evolution’ by Benjamin J. Ashton, Patrick Kennedy and Andrew N. Radford) is available in Nature Communications here. The beautiful drawings of dwarf mongooses in Figure 1 of the paper were produced by wildlife artist Martin Aveling.
Dunbar, R. I. M. & Shultz, S. Why are there so many explanations for primate brain evolution? Philos. Trans. R. Soc. B Biol. Sci. 372, (2017).
Ashton, B. J., Thornton, A. & Ridley, A. R. An intraspecific appraisal of the social intelligence hypothesis. Philos. Trans. R. Soc. B Biol. Sci. 373, 20170288 (2018).
Lemoine, S. et al. Between-group competition impacts reproductive success in wild chimpanzees. Curr. Biol. 30, 312-318.e3 (2020).
Arseneau-Robar, T. J. M. et al. Female monkeys use both the carrot and the stick to promote male participation in intergroup fights. Proc. R. Soc. B Biol. Sci. 283, 20161817 (2016).
Morris-Drake, A. et al. Experimental field evidence that out-group threats influence within-group behavior. Behav. Ecol. 30, 1425–1435 (2019).
Alexander, R. D. Evolution of the Human Psyche. in The Human Revolution (eds. Mellars, P. & Stringer, C.) 455–513 (Edinburgh University Press, Edinburgh, 1984).
Hamilton, W. D. Innate social aptitudes of man: an approach from evolutionary genetics. in ASA Studies 4: Biosocial Anthropology (ed. Fox, R.) 133–153 (Malaby Press, 1975).