Baboons (Papio anubis) living in larger social groups have bigger brains.
Open accessAbstract
Abstract The evolutionary origin of Primates' exceptionally large brains is still highly debated. Two competing explanations have received much support: the ecological hypothesis and the social brain hypothesis (SBH). We tested the validity of the SBH in (n = 82) baboons (Papio anubis) belonging to the same research centre but housed in groups with size ranging from 2 to 63 individuals. We found that baboons living in larger social groups had larger brains. This effect was driven mainly by white matter volume and to a lesser extent by grey matter volume but not by the cerebrospinal fluid. In comparison, the size of the enclosure, an ecological variable, had no such effect. In contrast to the current re-emphasis on potential ecological drivers of primate brain evolution, the present study provides renewed support for the social brain hypothesis and suggests that the social brain plastically responds to group size. Many factors may well influence brain size, yet accumulating evidence suggests demonstrates that the complexity of social life might be is an important determinant of brain size in primates.
AI snapshot
Written from the paper’s full text.
TL;DR
In captive olive baboons, larger social group size was associated with larger overall brain volume, while enclosure size was not. The effect was driven mainly by white matter volume.
Key findings
- Across 82 captive olive baboons (28 males, 2–26 years; group size 2–63; brain volume 117–189 cm³; enclosure size 9–304 m²), brain volume showed a significant positive relationship with social group size (β = 0.33, SE = 0.11, t = 2.90, p = 0.0048) but not enclosure size (β = -0.015, SE = 0.018, t = -0.82, p = 0.41).
- Follow-up analyses were consistent: using yearly veterinary records since 2008 for 24 baboons, group size predicted brain volume (β = 0.28, SE = 0.12, t = 2.39, p = 0.019) and enclosure size did not (β = -0.004, SE = 0.02, t = -0.22, p = 0.83); in a subsample of 20 baboons with variable enclosure sizes (99–304 m²) but homogeneous group sizes (15–25), group size predicted brain size (β = 0.38, SE = 0.16, t = 2.83, p = 0.030) but enclosure size did not (β = -0.07, SE = 0.07, t = -0.10, p = 0.34).
- Social group size influenced white matter volume (β = 0.14, SE = 0.045, t = 3.04, p = 0.0032), had a marginal tendency for grey matter (β = 0.098, SE = 0.055, t = 1.77, p = 0.080), and did not affect CSF (β = 0.029, SE = 0.022, t = 1.32, p = 0.19).
- Initial comparison: baboons in small “Loge” enclosures had smaller overall brain volume than those in larger “Parc” enclosures (one-sided t-test, t(80) = -1.71, p = 0.046).
Methods
- Captive olive baboons (Papio anubis, N=82 including 28 males, 2–26 years old) at a primate station; structural MRI collected from 84 baboons (Aug 2013–Jan 2015) with a 3T Bruker MEDSPEC 30/80 ADVANCE, T1-weighted MPRAGE; two individuals removed because only approximate birth date was available.
- Brain volumes (whole brain, grey matter, white matter, CSF) extracted via BrainVISA 4.4.0 Morphologist after denoising, skull stripping, and N4 intensity correction. Linear models tested group size and enclosure size separately on brain volumes, with age and sex as nuisance variables; analyses in R 3.5.1, p<0.05.
- Group/enclosure histories reconstructed from computerized daily records introduced in 2011 for the two years preceding MRI; robustness checks used yearly veterinary records since 2008 (24 baboons) and a subsample of 20 baboons with variable enclosure sizes (99–304 m²) but homogeneous group sizes (15–25).
Limitations
- The paper states that without further developmental data it cannot conclude the direction of the changes: brain size may increase when individuals live in larger groups through socialisation and/or decrease for individuals living in smaller groups.
- The paper states the grey matter effect was only marginally significant and that supplementary analyses (e.g., deformation-, voxel-, or surface-based morphometry, or ROI GM quantification) and comparisons with other primates would be needed to assess its strength and reliability.
- Evident from the text: the design is observational/correlational in captive animals, and some methodological information was omitted to preserve anonymity; the authors argue social transfers were rare (9 low-ranking females moved for social reasons among an average colony of 270 over 5 years, a rate of 0.6% a year), but group assignment was not freely chosen by the baboons.
Model-written summary. Check it against the paper before relying on it.
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Claims the paper makes
0 of 21 cited sources checked · 0 supported · 0 flagged
- Findingown data
Brain volume displays a significant positive relationship with social group size in captive olive baboons.
“We found that brain volume displays a significant positive relationship with social group size”
- Also stated in: § J o u r n a l P r e -p r o o f · plain
“The results show that brain volume and WM volume increase with social group size but not with enclosure area.”
Quote in the paperSubstantial claim - Also stated in: § J o u r n a l P r e -p r o o f · plain
- Interpretationown data
Some social factors related to group size are critical determinants of brain size in primates.
“However, our results provide clear evidence that some social factors related to group size are critical determinants of brain size in primates.”
Quote in the paperSubstantial claim - Interpretationreasoning
The social brain plastically responds to changes in social group size and social complexity.
“This effect shows that the social brain plastically responds to changes in social group size and social complexity.”
Quote in the paperNot a substantial claim - Interpretationreasoning
The results support the social brain hypothesis more broadly.
“Our results therefore support the SBH more broadly”
Quote in the paperSubstantial claim - Interpretationown data
White matter (WM) volume was the main driver of the effect of social group size on brain volume.
“Finally, our results show that WM was the main driver of the effect of social group size on brain volume.”
Quote in the paperSubstantial claim
- Premisecitation
In humans, social network size significantly correlates with grey matter density of regions constituting the social brain, including the amygdala and posterior superior temporal sulcus (STS).
“In humans, studies have shown that social network size significantly correlates with grey matter density of regions constituting the social brain”
Quote in the paperSubstantial claim0 of 3 sources checked - Interpretationreasoning
Enclosure size is an excellent non-social ecological control variable for our study in captivity.
“Enclosure size is thus an excellent non-social ecological control variable for our study in captivity.”
Quote in the paperSubstantial claim - Interpretationreasoning
The relationship between social group size and brain volumes in this study is most likely due to plastic brain changes in response to social factors related to group size.
“Thus, in our study the relationship between social group size and brain volumes is most likely due to plastic brain changes in response to social factors related to group size.”
Quote in the paperSubstantial claim - Premisecitation
Home range size has an influence on brain structures in primates.
“because home range size has been shown to have an influence on brain structures in primates (Clutton-Brock & Harvey, 1980;Sawaguchi, 1990)”
Quote in the paperSubstantial claim0 of 2 sources checked- cites Clutton-Brock & Harvey, 1980 source
Not checked in the free run.
- cites Sawaguchi, 1990
Not checked in the free run.
- Interpretationown data
The results provide novel evidence in favour of the social brain hypothesis (SBH) by demonstrating the effect of social group size on brain volume in a large sample of olive baboons.
“Our results therefore provide novel evidence in favour of the SBH by demonstrating the effect of social group size on brain volume in a large sample of olive baboons.”
Quote in the paperSubstantial claim - Interpretationown data
Monkeys living in larger social groups tend to have a larger brain.
“To conclude, our results show that monkeys living in larger social groups tend to have a larger brain.”
Quote in the paperSubstantial claim - Premisecitation
Since ecology, social life and group size are tightly linked in nature, teasing apart the different factors is difficult.
“Since ecology, social life and group size are tightly linked in nature, the teasing apart of the different factors is difficult (Dunbar & Shultz, 2017;Healy & Rowe, 2007).”
Quote in the paperSubstantial claim0 of 2 sources checked- cites Dunbar & Shultz, 2017
Not checked in the free run.
- cites Healy & Rowe, 2007 source
Not checked in the free run.
- May combine two claims. 2
- Classified as probably not a substantial claim. 8
May combine two claims.
Findingown dataHome range and social group size are tightly linked in nature and substantially correlated in our sample (rτ = 0.60, z = 7.82, p<0.001).
“Furthermore, home range and social group size are tightly linked in nature and substantially correlated in our sample (r τ = 0.60, z = 7.82, p<0.001).”
Quote in the paperSubstantial claimProbably not a substantial claim: may not pass S1 (asserted), S3 (load-bearing).
PremisecitationThe ecological hypothesis posits that higher cognitive abilities were demanded by novel types of foraging.
“However, the ecological hypothesis posits that higher cognitive abilities were demanded by novel types of foraging (DeCasien, Williams, & Higham, 2017;González-Forero & Gardner, 2018;Louail, Gilissen, Prat, Garcia, & Bouret, 2019).”
Quote in the paperNot a substantial claim0 of 3 sources checked- cites DeCasien, Williams, & Higham, 2017
Not checked in the free run.
- cites González-Forero & Gardner, 2018
Not checked in the free run.
- cites Louail, Gilissen, Prat, Garcia, & Bouret, 2019
Not checked in the free run.
Classified as probably not a substantial claim. May combine two claims.
PremisecitationThe SBH is supported by a positive relationship between brain size and social group size across primate species, suggesting a constraint imposed by the brain volume on the size of social groups.
“The SBH is supported by a positive relationship between brain size and social group size across primate species, suggesting a constraint imposed by the brain volume on the size of social groups (Dunbar, 1992(Dunbar, , 2009;;Sawaguchi & Kudo, 1990).”
Quote in the paperSubstantial claim0 of 3 sources checkedProbably not a substantial claim: may not pass S3 (load-bearing).
PremisecitationIn rhesus macaques, individual dominance status correlates with density in some of these areas (n=25).
“and Noonan et al. (2014) have found that individual dominance status correlates with the density in some of these areas (n = 25).”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Noonan et al., 2014 source
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing).
PremisecitationBoth hypotheses assume that energetically costly increases in relative brain size were driven by selection for higher cognitive abilities.
“Both hypotheses assume that energetically costly increases in relative brain size were driven by selection for higher cognitive abilities (Aiello & Wheeler, 1995;Isler & van Schaik, 2006).”
Quote in the paperNot a substantial claim0 of 2 sources checkedClassified as probably not a substantial claim.
PremisecitationThe social brain hypothesis proposes that the increase in brain size has been driven by the cognitive demands of primate's complex social life.
“In contrast, the social brain hypothesis (SBH), originating from the Machiavellian intelligence hypothesis, proposes that the increase in brain size has been driven by the cognitive demands of primate's complex social life (Byrne & Whiten, 1988;Dunbar, 1998;Humphrey, 1976; see Whiten, 2018 for an historical account).”
Quote in the paperSubstantial claim0 of 3 sources checked- cites Byrne & Whiten, 1988
Not checked in the free run.
- cites Dunbar, 1998 source
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- cites Humphrey, 1976
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing).
PremisecitationIn rhesus macaques, social network size correlates with grey matter densities in some regions of the social brain (n=23).
“Furthermore, in rhesus macaques (Macaca mulatta), Sallet et al. (2011) have shown that social network size correlates with the grey matter densities in some regions of the social brain (n = 23)”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Sallet et al., 2011 source
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing).
PremisecitationThe SBH has been tested within primate species rather than across species.
“More recently, the SBH has been tested within primate species rather than across species (Dunbar, 2012).”
Quote in the paperNot a substantial claim0 of 1 sources checked- cites Dunbar, 2012 source
Not checked in the free run.
Probably not a substantial claim: may not pass S3 (load-bearing).
Study design and conductown dataTo test the robustness of our results we performed two follow-up analyses that showed remarkably consistent results.
“To test the robustness of our results we performed two follow-up analyses that showed remarkably consistent results (see ESM for further details).”
Quote in the paperNot a substantial claim
The paper relies on this without support.
Most comparative studies supporting the ecological or social hypothesis are correlational and were done across species.
“However, most comparative studies supporting the ecological or the social hypothesis are correlational and were done across species.”
Claims other papers make about this one
Each is checked against this paper’s own text: does it say what the citing paper says it does?
In primates, greater cognitive challenges associated with forming bonded social groups in large societies appear to have selected for increased brain size.
“In primates, greater cognitive challenges associated with forming bonded social groups in large societies, among other influences (DeCasien et al., 2017;González-Forero and Gardner, 2018;DeCasien and Higham, 2019), appear to have selected for increased brain size (Dunbar, 1998;Dunbar andShultz, 2007, 2017;Meguerditchian et al., 2021).”
From When and Why Did Human Brains Decrease in Size? A New Change-Point Analysis and Insights From Brain Evolution in Ants (2021) · cites this paper as Meguerditchian et al., 2021
Statistics recomputed
Every reported test result, recomputed from its test statistic and degrees of freedom. A result shows as not adding up only once an independent check has confirmed it.
- Not confirmedt(80) = -1.71reported p = .046 · recomputed p = .091
“t(80)=-1.71,p = .046”
- Consistentz = 7.82reported p < .001 · recomputed p < .001
“z = 7.82, p < .001”
Self-check
Percentages, sums and reported differences re-computed from the paper’s own numbers.
Each contradiction between two passages listed here was confirmed by an independent check.
Group-size coefficient and SE do not match the reported t statistic
MinorFor the 20-individual subsample, dividing the reported coefficient by its SE gives 2.375, not 2.83. The gap is too large to be explained by rounding the printed values.
Reported 2.83 · recomputed 2.375
“social group size ( = 0.38, SE = 0.16, t = 2.83, p = 0.030)”
Figure 2a is assigned conflicting results
MinorThe Results cite Figure 2a for the white-matter result with a coefficient of 0.14, but elsewhere cite Figure 2a for a brain-volume result with a coefficient of 0.33.
“We found that social group size influenced WM volume overall (Figure 2a; = 0.14, SE = 0.045, t = 3.04, p = 0.0032)”
“Figure: ( Figure 2a; = 0.33, SE = 0.11, t = 2.90, p = 0.0048)”
Enclosure-size coefficient and SE do not match the reported t statistic
MinorThe reported coefficient divided by its SE is −1, not −0.10. The reported p value of 0.34 is also more consistent with a t statistic near −1.
Reported -0.1 · recomputed -1
“enclosure size ( = -0.07,SE = 0.07, t = -0.10,p = 0.34)”
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- Authors
- Meguerditchian, A., Marie, D., Margiotoudi, K., Roth, M., Nazarian, B., Anton, J.-L., & Claidière, N.
- Affiliations
- Centre National de la Recherche Scientifique (FR)
- Corresponding author
- Claidière, Nicolas
- Venue
- Evolution and Human Behavior
- Publisher
- Elsevier BV
- Published
- 2021-01
- Volume
- 42
- Issue
- 1
- Pages
- 30-34
- ISSN
- 1090-5138 (ISSN-L)
- OpenAlex
- W3041399997
- Semantic Scholar
- 65151a805c3454b41d0708413bde8678d72e5bc2
- Cited by
- 41
- References
- 43
- Citations by year
- 2026: 7 · 2025: 7 · 2024: 4 · 2023: 8 · 2022: 6 · 2021: 5 · 2020: 4
- FWCI
- 6.29
- Type
- journal article
- Language
- en
- Open access
- bronze
- doi.org
- Licence
- Locations
- Evolution and Human Behavior (acceptedVersion)
- Archive ouverte UNIGE (University of Geneva) (publishedVersion, other-oa)
- Archive ouverte UNIGE (University of Geneva) (submittedVersion, cc-by)
- HAL (submittedVersion, other-oa)
- Funders
- Agence Nationale de la Recherche (ANR-16-CONV-0002, ANR-11-IDEX-0001-02, ANR-11-LABX-0036)
- European Commission
- European Research Council (716931)
- Topics
- Social Sciences › Psychology › Social Psychology › Primate Behavior and Ecology
- Social Sciences › Psychology › Experimental and Cognitive Psychology › Evolutionary Psychology and Human Behavior
- Life Sciences › Agricultural and Biological Sciences › Ecology, Evolution, Behavior and Systematics › Animal Behavior and Reproduction
- Keywords
- social brain hypothesis, primate brain evolution, brain size, group size, baboons, Papio anubis, white matter volume, gray matter volume, cerebrospinal fluid, enclosure size, social complexity
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