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Dunbar RIM, Shultz S. Four errors and a fallacy: pitfalls for the unwary in comparative brain analyses. Biol Rev Camb Philos Soc 2023; 98:1278-1309. [PMID: 37001905 DOI: 10.1111/brv.12953] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/09/2022] [Revised: 03/17/2023] [Accepted: 03/17/2023] [Indexed: 04/03/2023]
Abstract
Comparative analyses are the backbone of evolutionary analysis. However, their record in producing a consensus has not always been good. This is especially true of attempts to understand the factors responsible for the evolution of large brains, which have been embroiled in an increasingly polarised debate over the past three decades. We argue that most of these disputes arise from a number of conceptual errors and associated logical fallacies that are the result of a failure to adopt a biological systems-based approach to hypothesis-testing. We identify four principal classes of error: a failure to heed Tinbergen's Four Questions when testing biological hypotheses, misapplying Dobzhansky's Dictum when testing hypotheses of evolutionary adaptation, poorly chosen behavioural proxies for underlying hypotheses, and the use of inappropriate statistical methods. In the interests of progress, we urge a more careful and considered approach to comparative analyses, and the adoption of a broader, rather than a narrower, taxonomic perspective.
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Affiliation(s)
- Robin I M Dunbar
- Department of Experimental Psychology, Anna Watts Building, University of Oxford, Oxford, OX2 6GG, UK
| | - Susanne Shultz
- Department of Earth and Environmental Sciences, Michael Smith Building, University of Manchester, Manchester, M13 9PT, UK
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2
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Kifle Z, Bekele A. Effect of human disturbance on the group size and population structure of geladas (Theropithecus gelada) in and surrounding area of Borena Sayint National Park, Ethiopia. Glob Ecol Conserv 2022. [DOI: 10.1016/j.gecco.2022.e02307] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
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Dunbar RIM. Laughter and its role in the evolution of human social bonding. Philos Trans R Soc Lond B Biol Sci 2022; 377:20210176. [PMID: 36126664 PMCID: PMC9489299 DOI: 10.1098/rstb.2021.0176] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2021] [Accepted: 01/26/2022] [Indexed: 01/19/2023] Open
Abstract
In anthropoid primates, social grooming is the principal mechanism (mediated by the central nervous system endorphin system) that underpins social bonding. However, the time available for social grooming is limited, and this imposes an upper limit on the size of group that can be bonded in this way. I suggest that, when hominins needed to increase the size of their groups beyond the limit that could be bonded by grooming, they co-opted laughter (a modified version of the play vocalization found widely among the catarrhine primates) as a form of chorusing to fill the gap. I show, first, that human laughter both upregulates the brain's endorphin system and increases the sense of bonding between those who laugh together. I then use a reverse engineering approach to model group sizes and grooming time requirements for fossil hominin species to search for pinch points where a phase shift in bonding mechanisms might have occurred. The results suggest that the most likely time for the origin of human-like laughter is the appearance of the genus Homo ca 2.5 Ma. This article is part of the theme issue 'Cracking the laugh code: laughter through the lens of biology, psychology and neuroscience'.
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Affiliation(s)
- R. I. M. Dunbar
- Department of Experimental Psychology, University of Oxford, Radcliffe Observatory Quarter, Oxford OX2 6GG, UK
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Dunbar RIM. Managing the stresses of group-living in the transition to village life. EVOLUTIONARY HUMAN SCIENCES 2022; 4:e40. [PMID: 37588930 PMCID: PMC10426039 DOI: 10.1017/ehs.2022.39] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2022] [Revised: 07/16/2022] [Accepted: 08/16/2022] [Indexed: 11/07/2022] Open
Abstract
Group living is stressful for all mammals, and these stresses limit the size of their social groups. Humans live in very large groups by mammal standards, so how have they solved this problem? I use homicide rates as an index of within-community stress for humans living in small-scale ethnographic societies, and show that the frequency of homicide increases linearly with living-group size in hunter-gatherers. This is not, however, the case for cultivators living in permanent settlements, where there appears to be a 'glass ceiling' below which homicide rates oscillate. This glass ceiling correlates with the adoption of social institutions that allow tensions to be managed. The results suggest (a) that the transition to a settled lifestyle in the Neolithic may have been more challenging than is usually assumed and (b) that the increases in settlement size that followed the first villages necessitated the introduction of a series of social institutions designed to manage within-community discord.
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Affiliation(s)
- R. I. M. Dunbar
- Department of Experimental Psychology, University of Oxford, Radcliffe Observatory Quarter, Oxford OX2 6GG, UK
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Dunbar RIM. Female Dispersion Is Necessary, but Not Sufficient, for Pairbonded Monogamy in Mammals. Front Ecol Evol 2022. [DOI: 10.3389/fevo.2022.905298] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Explanations for the evolution of social monogamy in mammals typically emphasise one of two possibilities: females are overdispersed (such that males cannot defend access to more than one female at a time) or males provide a service to the female. However, the first claim has never been formally tested. I test it directly at three levels using population-level data from primates and ungulates. First, I show that the females of monogamous genera do not have territories that are significantly larger, either absolutely or relatively, than those of polygynous genera. Second, using two indices of territorial defendability, I show that, given their typical day journey lengths, males of most monogamous species could easily defend an area large enough to allow them to monopolise as many as 5–10 females if they ranged solitarily. Finally, I use a model of male mate searching strategies to show that the opportunity cost incurred by pairbonded males is typically 5–10 times the reproductive success they actually obtain by being obligately monogamous. This suggests that the selection pressure dissuading them from pursuing a roving male strategy must be very considerable. At present, the evidence is undecided as to whether mitigating predation or infanticide risk is the primary function, but estimates of their impacts suggest that both are in fact plausible.
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Dunbar RIM, Shultz S. The Infertility Trap: The Fertility Costs of Group-Living in Mammalian Social Evolution. Front Ecol Evol 2021. [DOI: 10.3389/fevo.2021.634664] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Mammal social groups vary considerably in size from single individuals to very large herds. In some taxa, these groups are extremely stable, with at least some individuals being members of the same group throughout their lives; in other taxa, groups are unstable, with membership changing by the day. We argue that this variability in grouping patterns reflects a tradeoff between group size as a solution to environmental demands and the costs created by stress-induced infertility (creating an infertility trap). These costs are so steep that, all else equal, they will limit group size in mammals to ∼15 individuals. A species will only be able to live in larger groups if it evolves strategies that mitigate these costs. We suggest that mammals have opted for one of two solutions. One option (fission-fusion herding) is low cost but high risk; the other (bonded social groups) is risk-averse, but costly in terms of cognitive requirements.
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Dunbar RIM, Shultz S. Social complexity and the fractal structure of group size in primate social evolution. Biol Rev Camb Philos Soc 2021; 96:1889-1906. [PMID: 33945202 DOI: 10.1111/brv.12730] [Citation(s) in RCA: 22] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2020] [Revised: 04/17/2021] [Indexed: 12/25/2022]
Abstract
Compared to most other mammals and birds, anthropoid primates have unusually complex societies characterised by bonded social groups. Among primates, this effect is encapsulated in the social brain hypothesis: the robust correlation between various indices of social complexity (social group size, grooming clique size, tactical behaviour, coalition formation) and brain size. Hitherto, this has always been interpreted as a simple, unitary relationship. Using data for five different indices of brain volume from four independent brain databases, we show that the distribution of group size plotted against brain size is best described as a set of four distinct, very narrowly defined grades which are unrelated to phylogeny. The allocation of genera to these grades is highly consistent across the different data sets and brain indices. We show that these grades correspond to the progressive evolution of bonded social groups. In addition, we show, for those species that live in multilevel social systems, that the typical sizes of the different grouping levels in each case coincide with different grades. This suggests that the grades correspond to demographic attractors that are especially stable. Using five different cognitive indices, we show that the grades correlate with increasing social cognitive skills, suggesting that the cognitive demands of managing group cohesion increase progressively across grades. We argue that the grades themselves represent glass ceilings on animals' capacity to maintain social and spatial coherence during foraging and that, in order to evolve more highly bonded groups, species have to be able to invest in costly forms of cognition.
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Affiliation(s)
- Robin I M Dunbar
- Department of Experimental Psychology, Radcliffe Observatory Quarter, University of Oxford, Oxford, OX2 1GG, UK
| | - Susanne Shultz
- Department of Earth and Environmental Sciences, Michael Smith Building, University of Manchester, Manchester, M13 9PT, UK
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Dunbar RIM. Structure and function in human and primate social networks: implications for diffusion, network stability and health. Proc Math Phys Eng Sci 2020; 476:20200446. [PMID: 32922160 PMCID: PMC7482201 DOI: 10.1098/rspa.2020.0446] [Citation(s) in RCA: 18] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2020] [Accepted: 07/10/2020] [Indexed: 12/12/2022] Open
Abstract
The human social world is orders of magnitude smaller than our highly urbanized world might lead us to suppose. In addition, human social networks have a very distinct fractal structure similar to that observed in other primates. In part, this reflects a cognitive constraint, and in part a time constraint, on the capacity for interaction. Structured networks of this kind have a significant effect on the rates of transmission of both disease and information. Because the cognitive mechanism underpinning network structure is based on trust, internal and external threats that undermine trust or constrain interaction inevitably result in the fragmentation and restructuring of networks. In contexts where network sizes are smaller, this is likely to have significant impacts on psychological and physical health risks.
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Affiliation(s)
- R. I. M. Dunbar
- Department of Experimental Psychology, University of Oxford, New Radcliffe Building, Radcliffe Observatory Quarter, Oxford OX1 6GG, UK
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9
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Dunbar RIM. Fertility as a constraint on group size in African great Apes. Biol J Linn Soc Lond 2019. [DOI: 10.1093/biolinnean/blz172] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022]
Abstract
Abstract
Gorillas and chimpanzees live in social groups of very different size and structure. Here I test the hypothesis that this difference might reflect the way fertility maps onto group demography as it does in other Catarrhines. For both genera, birth rates and the number of surviving offspring per female are quadratic (or ∩-shaped) functions of the number of adult females in the group, and this is independent of environmental effects. The rate at which fertility declines ultimately imposes a constraint on the size of social groups that can be maintained in both taxa. The differences in group size between the two genera seem to reflect a contrast in the way females buffer themselves against this cost. Gorillas do this by using males as bodyguards, whereas chimpanzees exploit fission–fusion sociality to do so. The latter allows chimpanzees to live in much larger groups without paying a fertility cost (albeit at a cognitive cost).
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Affiliation(s)
- R I M Dunbar
- Department of Experimental Psychology, University of Oxford, Radcliffe Observatory Quarter, Oxford, UK
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10
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Goodfellow CK, Whitney T, Christie DM, Sicotte P, Wikberg EC, Ting N. Divergence in gut microbial communities mirrors a social group fission event in a black-and-white colobus monkey (Colobus vellerosus). Am J Primatol 2019; 81:e22966. [PMID: 30920682 DOI: 10.1002/ajp.22966] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/14/2018] [Revised: 02/11/2019] [Accepted: 02/16/2019] [Indexed: 12/16/2022]
Abstract
Host behavior and social factors have increasingly been implicated in structuring the composition of gut microbial communities. In social animals, distinct microbial communities characterize different social groups across a variety of taxa, although little longitudinal research has been conducted that demonstrates how this divergence occurs. Our study addresses this question by characterizing the gut microbial composition of an African Old World monkey, the black-and-white colobus (Colobus vellerosus), before and after a social group fission event. Gut microbial taxonomic composition of these monkeys was profiled using the V-4 hypervariable region of the bacterial 16S ribosomal RNA gene, and pairwise-relatedness values were calculated for all individuals using 17 short tandem repeat loci and partial pedigree information. The two social groups in this study were found to harbor distinct microbial signatures after the fission event from which they emerged, while these communities were not divergent in the same individuals before this event. Three genera were found to differ in abundance between the two new social groups: Parabacteroides, Coprococcus, and Porphyromonadaceae. Additionally, although this fission happened partially along lines of relatedness, relatedness did not structure the differences that we found. Taken together, this study suggests that distinct gut microbial profiles can emerge in social groups in <1 year and recommends further work into more finely mapping the timescales, causes, and potentially adaptive effects of this recurring trend toward distinct group microbial signatures.
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Affiliation(s)
- Claire K Goodfellow
- Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon.,Department of Biology, University of Oregon, Eugene, Oregon
| | - Tabor Whitney
- Department of Biology, University of Oregon, Eugene, Oregon
| | - Diana M Christie
- Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon.,Department of Anthropology, University of Oregon, Eugene, Oregon
| | - Pascale Sicotte
- Department of Anthropology and Archaeology, University of Calgary, Calgary, Canada
| | - Eva C Wikberg
- Department of Anthropology, University of Texas at San Antonio, San Antonio, Texas
| | - Nelson Ting
- Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon.,Department of Anthropology, University of Oregon, Eugene, Oregon
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Dunbar RIM, Cheyne SM, Lan D, Korstjens A, Lehmann J, Cowlishaw G. Environment and time as constraints on the biogeographical distribution of gibbons. Am J Primatol 2019; 81:e22940. [DOI: 10.1002/ajp.22940] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/19/2018] [Revised: 11/14/2018] [Accepted: 11/19/2018] [Indexed: 11/10/2022]
Affiliation(s)
| | - Susan M. Cheyne
- Borneo Nature Foundation; Palangka Raya; Indonesia & Oxford Brookes University; Oxford UK
| | - Daoying Lan
- Guangdong Institute of Applied Biological Resources; Guangzhou China
| | - Amanda Korstjens
- Department of Life and Environmental Sciences; Bournemouth University; Poole UK
| | - Julia Lehmann
- Department of Life Science; University of Roehampton; London UK
| | - Guy Cowlishaw
- Institute of Zoology; Zoological Society of London; London UK
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12
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Dunbar RIM, Mac Carron P. Does a trade‐off between fertility and predation risk explain social evolution in baboons? J Zool (1987) 2018. [DOI: 10.1111/jzo.12644] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- R. I. M. Dunbar
- Department of Experimental Psychology University of Oxford Oxford UK
- Department of Computer Science Aalto University Espoo Finland
| | - P. Mac Carron
- Department of Experimental Psychology University of Oxford Oxford UK
- Maynooth University Maynooth Co. Kildare Ireland
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Abstract
We examine community longevity as a function of group size in three historical, small scale agricultural samples. Community sizes of 50, 150 and 500 are disproportionately more common than other sizes; they also have greater longevity. These values mirror the natural layerings in hunter-gatherer societies and contemporary personal networks. In addition, a religious ideology seems to play an important role in allowing larger communities to maintain greater cohesion for longer than a strictly secular ideology does. The differences in optimal community size may reflect the demands of different ecologies, economies and social contexts, but, as yet, we have no explanation as to why these numbers seem to function socially so much more effectively than other values.
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Affiliation(s)
- Robin I M Dunbar
- Department of Experimental Psychology, University of Oxford, Oxford, UK.,Department of Computer Sciences, Aalto University, Espoo, Finland
| | - Richard Sosis
- Department of Anthropology, University of Connecticut, Storrs, CT 06269-2176, USA
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