Evolution in the Social Brain

Abstract

The evolution of unusually large brains in some groups of animals, notably primates, has long been a puzzle. Although early explanations tended to emphasize the brain's role in sensory or technical competence (foraging skills, innovations, and way-finding), the balance of evidence now clearly favors the suggestion that it was the computational demands of living in large, complex societies that selected for large brains. However, recent analyses suggest that it may have been the particular demands of the more intense forms of pairbonding that was the critical factor that triggered this evolutionary development. This may explain why primate sociality seems to be so different from that found in most other birds and mammals: Primate sociality is based on bonded relationships of a kind that are found only in pairbonds in other taxa.

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REVIEW
Evolution in the Social Brain
R. I. M. Dunbar* and Susanne Shultz
The evolution of unusually large brains in some groups of animals, notably primates, has long been
a puzzle. Although early explanations tended to emphasize the brain’s role in sensory or technical
competence (foraging skills, innovations, and way-finding), the balance of evidence now clearly
favors the suggestion that it was the computational demands of living in large, complex societies
that selected for large brains. However, recent analyses suggest that it may have been the
particular demands of the more intense forms of pairbonding that was the critical factor that
triggered this evolutionary development. This may explain why primate sociality seems to be so
different from that found in most other birds and mammals: Primate sociality is based on bonded
relationships of a kind that are found only in pairbonds in other taxa.
T
he brain is one of the most expensive
organs in the body , second only to the
heart: The brain’s running costs are about
8 to 10 times as high, per unit mass, as those of
skeletal muscle (1, 2). Although the brain’s
ability to control t he body’s functions is obvi-
ously us eful, it entails something of an evolu-
tionary puzzle. The neurobiologist Harry Jerison
first pointed this out during the 1970s (3), when
he drew a distinction between the component of
the brain required to meet the body’s physical
needs and the component that was left over,
which could attend to tasks of a more cogni-
tively complex nature. This second component
of the brain has been increasing over evolution-
ary time across the birds and mammals, but fish
and reptiles continue to thrive with brains of
very modest size. Although it is easy to un-
derstand why brains in general have evolved, it
is not so obvious why the brains of birds and
mammals have grown substantially larger than
the minimum size required to stay alive.
T raditional explanations for the evolution of
large brains in primates focused either on eco-
logical problem solving or on developmental con-
straints. Early studies identified physiological
and life-history traits—including large body size,
metabolic rates, and prolonged development—
that were associated with large brains (4, 5).
Some argued that this correlation was due to
the more efficient metabolism of larger-bodied
animals, which allowed more energy to be de-
voted to fetal brain growth and thereby made
the evolution of larger brains possible (6, 7). All
else being equal, big brains are a useful if un-
intended by-product of efficient energy use. In
addition to this theory, some evidence supported
ecological problem-solving as a possible ex-
planation: Among primates, for example, large-
brained species have larger home ranges (per-
haps requiring more sophisticated mental maps),
and frugivores have larger brains than folivores
(fruits are much less predictable in their location
and availability than leaves) (8).
On closer examination, most of the energetic
explanations that have been offered identify
constraints on brain evolution rather than selec-
tion pressures. In biology, constraints are inevi-
table, and crucial for understanding evolutionary
trajectories, but they do not constitute functional
explanations—that is, just because a species can
afford to evolve a larger brain does not mean
that it must do so. Proponents of developmen-
tal explanations seem to have forgotten that
evolutionary processes involve costs as well as
benefits. Because evolution is an economical
process and does not often produce needless
organs or capacities, especially if they are ex-
pensive to maintain, it follows that some propor-
tionately beneficial advantage must have driven
brain evolution against the steep selection gra-
dient created by the high costs of brain tissue. In
this respect, most of the ecological hypotheses
proposed to date also fail. None can explain
why primates (which have especially large brains
for body mass, even by mammal standards) need
brains that are so much larger than, say, squir-
rels, to cope with what are essentially the same
foraging decisions.
As an alternative, Byrne and Whiten pro-
posed the Machiavellian Intelligence hypothesis
(9) in the late 1980s: They argued that what dif-
ferentiates primates from all other species (and,
hence, what might account for their especially
large brains) was the complexity of their social
lives. Unfortunately, the term “Machiavellian”
was widely interpreted as implying deceit, ma-
nipulation, and connivance—traits that most
people were reluctant to attribute to any species
other than humans. In fact, although these are
potential aspects of social complexity, they
did not lie at the heart of Byrne and Whiten’s
proposal. Instead, the proposal emphasized the
complex social environments in which pri-
mates lived. The less contentious label social
brain hypothesis (SBH) (10, 11) has thus been
adopted.
Although initially criticized for being con-
ceptually vague, the SBH eventually began to
receive increasing quantitative
support. A series of studies dem-
onstrated that, among primates
at least, relative brain size [usu-
ally indexed as relative size of
the neocortex, the area that has
disproportionately expanded in
primates (12)] correlates with
many indices of social complex-
ity, including social group size
(Fig. 1) (13), number of females
in the group (14), grooming
clique size (15), the frequency
of coalitions (16), male mating
strategies (17), the prevalence
of social play (18), the frequen-
cy of tactical deception (19),
and the frequency of social learn-
ing (20).
A weakness of most analyses,
however , is that they invariably
test a single hypothesis without
ensuring that the same predic-
tions could not also arise from
other equally plausible explanations. Although
some attempts have been made to discri mi-
nate between ecological and social theories
(13, 21), and these have largely suppo rted the
social hypothesis, there has been little effort to
develop an explanatory framework that inte-
grates the many social, ecological, and life-
history correlates of brain size that have been
identified. As a result, constraint-type expla-
nations (e.g., correlations with life history)
Social Cognition
British Academy Centenary Research Project, School of
Biological Sciences, University of Liverpool, Liverpool L69
7ZB, UK.
*To whom correspondence should be addressed. E-mail:
rimd@liverpool.ac.uk
Fig. 1. In anthropoid primates, mean social group size increases
with relative neocortex volume (indexed as the ratio of neocortex
volume to the volume of the rest of the brain). Solid circles,
monkeys; open circles, apes. Regression lines are reduced major
axis fits. [Redrawn from (47)]
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have continued to be emphasized as though
they were alternative explanations for evolu-
tionary function.
Social Brain, Social Complexity
The broad interpretation of the social brain
hypothesis is that individuals living in stable
social groups face cognitive demands that in-
dividuals living alone (or in unstable aggre-
gations) do not. To maintain group cohesion,
individuals must be able to meet their own re-
quirements, as well as coordinate their behavior
with other individuals in the group. They must
also be able to defuse the direct and indirect
conflicts that are generated by foraging in the
same space.
Appreciating that the problem to be solved
lies at the level of the group (i.e., the need to
maintain group coherence through time) and not
just at the level of individual foraging strategies
might allow us to reconcile the apparent conflict
between the ecological and social hypotheses.
One example of this apparent conflict is the
suggestion that flexibility of foraging skills might
be more important than social skills. The evidence
that brain size correlates with technical innova-
tion and the acquisition of new food sources
through social learning (or cultural transmission)
in both birds (22) and primates (20) supports
this claim. However , in the final analysis, all of
these hypotheses (social and ecological alike)
are at root ecological: They allow animals to sur-
vive and reproduce more effectively . The SBH
proposes that ecological problems are solved
socially and that the need for mechanisms that
enhance social cohesion drives brain size evolu-
tion. In contrast, the more conventional ecolog-
ical hypotheses assume that animals solve these
same ecological problems individually by trial
and error learning and do not rely on any form of
social advantage.
For primates at least, sociality is specifically
driven by the need to minimize predation risk
(23–25). However, we have shown that two dif-
ferent kinds of predators (chimpanzees and
felids) from five different ecological commu-
nities on two continents differentially select small-
brained prey species (relative to their availability
in the population) when we control for other
traits (including group size) (26). Predation thus
acts directly and indirectly (by means of group
size) on brain evolution. Nonetheless, whatever
its advantages, group living incurs substantial
costs, both in terms of ecological competition
and, for females, reproductive suppression
(23, 24). Hence, behavioral flexibility within a
social situation may be essential for individuals
to make the most of sociality. For anthropoid
primates, this behavioral flexibility is in part
reflected in the use of intense social bonds (often,
but not always, serviced by social grooming) to
prevent groups from disintegrating under these
pressures (15).
The net consequence of these kinds of pres-
sures is that species that evolve larger brains
ultimately have higher fitness. Jerison (3)him-
self pointed out that, in the Paleolithic record,
increases in brain size among carnivores and
their prey species (mainly ungulates) seem to
track each other closely over time, with un-
gulate brain size leading. These findings are
interesting in themselves and also mesh well
with findings that brain size can be associated
with other types of ecological flexibility—for
example, that brain size is a predictor of both
extinction risk and invasion success in birds
(27, 28).
To tease out the relationship between the
nexus of factors that correlate with brain size,
we have recently undertaken path analyses of
primate and bird data to identify causes, con-
sequences, and constraints in brain evolution
(16). These analys es demonstrate not only
that energetics (i.e., ecology) and life history
impose constraints on brain size (such that
these constraints require solutions if a species
is to evolve a substantially larger brain) but
also that the key selection pressure promoting
the evolution of large brains is explicitly social
(Fig. 2).
Brain Evolution in Birds and Mammals
Although the SBH was originally conceived
for primates, the same principle could apply
more widely, and several attempts have been
made to extend the hypothesis to nonprimate
taxa, including ungulates (29, 30), carnivores
(31), bats (32),andevenbirds(33), albeit with
somewhat mixed results. Indeed, several studies
have argued that sexual selection rather than
sociality might be a more important factor
driving brain evolution (32, 34). Yet evidence
shows that the correlation is the reverse of
what one might expect (polygamous species
actually have the smallest brains), making
sexual selection an unlikely suggestion, al-
though it may influence some components of
the brain [such as the limbic system in male
primates (35, 36)].
Although it is possible that the SBH applies
exclusively to primates, biologists are usually
reluctant to argue for special cases. Fortunately,
the recent availability of more powerful statisti-
cal tools has allowed us to resolve this enigma.
First, we have shown that there is a strong co-
evolutionary relationship between relative brain
size and the evolution of sociality from an
asocial (or less social) state in primates, un-
gulates, and carnivores (31). Second, for four
orders of mammals (primates, bats, artiodactyl
ungulates, and carnivores) and 135 species of
birds representing a wide cross-section of avian
orders, we have shown that, in all taxa except
anthropoid primates, the relationship between
brain size and sociality is qualitative and not
Fig. 2. Path analysis of correlates of brain size in primates. The best model for group size included
just three variables (neocortex size, activity, and range size). Factors that are more remote in the
path diagram provide a significantly poorer fit, suggesting that they act as constraints rather than
driving variables. BMR, basal metabolic rate. [Reproduced with permission from (16)]
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quantitative: In each case, large relative brain
size is associated explicitly with pairbonded
(i.e., social) monogamy (Fig. 3).
These findings suggest that it may have
been the cognitive demands of pairbonding
that triggered the initial evolution of large brains
across the vertebrates. More important, pair-
bonding is the issue, not biparental care. This is
obvious in the case of ungulates: Biparental care
does not occur at all in this taxon, yet ungulates
that mate monogamously have substantially
larger brains than those that mate polygamously
(Fig. 3).
How Complex Can Pairbonds Be?
The important issue in the present context is the
marked contrast between anthropoid primates
and all other mammalian and avian taxa (in-
cluding, incidentally, prosimian primates): Only
anthropoid primates exhibit a correlation be-
tween social group size and relative brain (or
neocortex) size. This quantitative relationship
is extremely robust; no matter how we analyze
the data (with or without phylogenetic cor-
rection, using raw volumes, or residuals or
ratios against any number of alternative body
or brain baselines) or which brain data set we
use (histological or magnetic resonance imag-
ing derived, for whole brain, neocortex, or just
the frontal lobes), the same quantitative rela-
tionship always emerges. This suggests that, at
some early point in their evolutionary history ,
anthropoid primates used the kinds of cogni-
tive skills used for pairbonded relationships
by vertebrates to create relationships between
individuals who are not reproductive partners.
In other words, in primates, individuals of the
same sex as well as members of the opposite
sex could form just as intense and focused a
relationship as do reproductive mates in non-
primates. Given that the number of possible
relationships is limited only by the number of
animals in the group, primates naturally ex-
hibit a positive correlation between group size
and brain size. This would explain why, as
primatologists have argued for decades, the
nature of primate sociality seems to be qual-
itatively different from that found in most other
mammals and birds. The reason is that the
everyday relationships of anthropoid primates
involve a form of “bo ndedness” that is only found
elsewhere in reproductive pairbonds.
This suggestion merely adds to the puzzle
of social bonding. What is it about social
bonds that is cognitively so demanding? There
seem to be two obvious possibilities in the case
of reproductive pairbonds. One is that lifelong
monogamy is a risky commitment; to avoid
the risk of bearing a disproportionate share of
the costs of reproduction, individuals must be
especially careful in choosing good-quality
(i.e., fertile) mates who will be reproductively
loyal and play their full role in the processes
of rearing. The other possibility is that a work-
ing reproductive relationship that involves sub-
stantial postnatal parental investment requires
very close coordination and behavioral syn-
chrony; if successful rearing requires both
partners to invest time and energy in the rear-
ing process, then the pair needs to regulate its
activities so that each has enough time for
feeding and rest. That will usually necessitate
some degree of activity synchronization—in
some cases, to ensure the pair do not drift
apart as a result of different activity schedules,
and in other cases, to ensure that rearing or
vigilance duties are time-shared appropriately
(37). Which of these two has been the key
driver for brain evolution, or whether both
have been equally important, remains to be
determined.
It has become apparent that we lack ade-
quate language with which to describe rela-
tionships, yet bondedness is precisely what
primate sociality is all about. Intuitively, we
know what we mean by bondedness because
we experience it ourselves, and we recognize
it when it happens. The problem, perhaps, is
that bondedness is an explicitly emotional ex-
perience and language is a notoriously poor
medium for describing our inner, emotional
experiences. Because relationships do not
have a natural objective cognitive dimension
that we can easily express in language, com-
paring the bondedness of different species is
difficult (this may also explain why ethologists
have invariably ducked the problem completely,
preferring observable descriptions of behavior
to grappling with what is going on inside the
animal).
Part of the problem here is that social rela-
tionships have been seen as mere epiphenomena
spawned by the issues of real biological interest,
namely mate choice and parental investment.
The social learning version of SBH (20, 22)in-
herits a sense of that assessment: Sociality is of
interest only in so far as it provides a context
in which animals can acquire foraging informa-
tion that has immediate benefits for them in
terms of individual fitness. However, this misses
the point of primate sociality—indeed, the nature
of sociality , and especially pairbonding—in all
higher vertebrates. In these intensely social spe-
cies, social relationships are not so much an
emergent property of mating and parenting strat-
egies as the means to achieving those strategies.
A group of this kind is an implicit social con-
tract: To form a group that provides a benefit
of cooperation (for example, reducing preda-
tion risk), members are necessarily obliged
to trade off short-term losses in immediate
benefits in the expectation of greater gains in
the long term through cooperation. Fitness
payoffs are determined not by an individual’s
immediate “here-and-now” personal fitness,
but by the extent to which the group can gen-
erate longer-term payoffs for the individual. In
effect, we are dealing explicitly with multi-
level selection and the long-overlooked topic
of niche construction (38). Once we understand
this, the reasons why animals should invest in
relationships become clear. Relationships pro-
vide the key to fitness benefits at the group
level, and the “trickle-down” benefits are reaped
by the individual (39). An individual will be
prepared to invest in social strategies that create
groups if, by doing so, it gains higher net
Fig. 3. Mean (±SE) of residual brain volume (controlling for body size and phylogeny) in species
with pairbonded (purple bars) versus all other mating systems (gray bars) in birds and four orders
of mammals. The differences are significant in all cases except primates.
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fitness (i.e., at the end of its lifetime) than
pursuing more individualistic strategies.
What Microneurobiology Has to Tell Us
There has, of course, been growing interest in
recent years in some of the neurobiological
correlates of social bonding. Particular interest
has focused the role of oxytocin (and its male
equivalent, vasopressin) in pairbonded species
(40), but other neuropeptides have also been
identified as playing an important role in so-
cial bonding [e.g., endorphins (41)]. In addition,
a parallel interest has been developing in the role
of several specific neuronal assemblages, includ-
ing mirror neurons (42) and so-called spindle
cells in the anterior cingulate cortex (43), as well
as in specific genes such as GLUD2 [a retro-
gene, derived from glutamate dehydrogenase,
which is responsible for clearing the by-products
of neuron activity (44)] and the abnormal spindle-
like microcephaly-associated (ASPM) gene and
microcephalin, which are implicated in brain
growth (45).
Each of these has been seen by their respective
protagonists as the holy grail for understanding
both social cognition generally, and, in particular,
for explaining the differences between humans,
apes, and monkeys (43, 46). There is no question
that these are individually important and novel
discoveries, and they undoubtedly all play a
role in the nature of sociality. However , there is
a great deal more to how and why humans are
different from other apes, or why apes are dif-
ferent from monkeys. We will need better studies
of cognition and behavior to answer these ques-
tions. More important, perhaps, is one key point:
Species differences in a handful of very small
neuronal components do not explain the apparent
need for massive species differences in total brain
size. Most of these studies fall into the same
trap as the developmental explanations for brain
size did in the 1980s: They mistake mechanistic
constraints for evolutionary function. It is un-
clear why this point continues to be ignored, but
we will still have a lot of explaining to do about
volumetric differences in brains.
References and Notes
1. L. C. Aiello, P. Wheeler, Curr. Anthrop. 36, 199 (1995).
2. J. A. Kaufman, Curr. Anthropol. 44, 705 (2003).
3. H. J. Jerison, Evolution of the Brain and Intelligence
(Academic Press, London, 1973).
4. E. Armstrong, Science 220, 1302 (1983).
5. P. H. Harvey, T. H. Clutton-Brock, Evolution Int. J.
Org. Evolution 39, 559 (1985).
6. R. D. Martin, Nature 293, 57 (1981).
7. M. A. Hofman, Q. Rev. Biol. 58, 495 (1983).
8. T. H. Clutton-Brock, P. H. Harvey, J. Zool. 190, 309
(1980).
9. R. W. Byrne, A. Whiten, Eds., Machiavellian Intelligence
(Oxford Univ. Press, Oxford, 1988).
10. R. Barton, R. I. M. Dunbar, in Machiavellian Intelligence II,
A.Whiten,R.Byrne,Eds.(CambridgeUniv.Press,
Cambridge, 1997), pp. 240–263.
11. R. I. M. Dunbar, Evol. Anthrop. 6, 178 (1998).
12. B. L. Finlay, R. B. Darlington, Science 268, 1578
(1995).
13. R. I. M. Dunbar, J. Hum. Evol. 22, 469 (1992).
14. P. Lindenfors, Biol. Lett. 1, 407 (2005).
15. H. Kudo, R. I. M. Dunbar, Anim. Behav. 62, 711
(2001).
16. R. I. M. Dunbar, S. Shultz, Phil. Trans. R. Soc. London
Ser. B 362, 649 (2007).
17. B. P. Pawlowski, C. B. Lowen, R. I. M. Dunbar, Behaviour
135, 357 (1998).
18. K. Lewis, Folia Primat. 71, 417 (2000).
19. R. W. Byrne, N. Corp, Proc. R. Soc. London 271, 1693
(2004).
20. S. M. Reader, K. N. Laland, Proc. Natl. Acad. Sci. U.S.A.
99, 4436 (2002).
21. R. O. Deaner, C. L. Nunn, C. P. van Schaik, Brain Behav.
Evol. 55, 44 (2000).
22. L. Lefebvre, S. M. Reader, D. Sol, Brain Behav. Evol. 63,
233 (2004).
23. C. P. van Schaik, Behaviour 87, 120 (1983).
24. R. I. M. Dunbar, Primate Social Systems (Chapman &
Hall, London, 1988).
25. S. Shultz, R. Noë, S. McGraw, R. I. M. Dunbar, Proc. R.
Soc. London Ser. B 271, 725 (2004).
26. S. Shultz, R. I. M. Dunbar, Biol. Lett. 2, 505 (2006).
27. S. Shultz, R. Bradbury, K. Evans, R. Gregory, T. Blackburn,
Proc. R. Soc. London Ser. B 272, 2305 (2005).
28. D. Sol, R. P. Duncan, T. M. Blackburn, P. Cassey, L. Lefebrve,
Proc. Natl. Acad. Sci. U.S.A. 102, 5460 (2005).
29. F. J. Perez-Barberia, I. J. Gordon, Oecologia 145, 41 (2005).
30. S. Shultz, R. I. M. Dunbar, Proc. R. Soc. London Ser. B
273, 207 (2006).
31. F. J. Pérez-Barbería, S. Shultz, R. I. M. Dunbar, Evolution,
in press.
32. S. Pitnick, K. E. Jones, G. S. Wilkinson, Proc. R. Soc.
London Ser. B 273, 719 (2006).
33. G. Beauchamp, E. Fernandez-Juricic, Evol. Ecol. Res. 6,
833 (2004).
34. M. Schillaci, PloS ONE 1, e62 (2007).
35. E. B. Keverne, F. L. Martel, C. M. Nevison, Proc. R. Soc.
London Ser. B 262, 689 (1996).
36. P. Lindenfors, C. L. Nunn, R. A. Barton, BMC Biol. 5,20
(2007).
37. R.I.M.Dunbar,E.P.Dunbar,Anim. Behav. 28, 219 (1980).
38. F. J. Odling-Smee, K. N. Laland, M. W. Feldman,
Niche Construction: The Neglected Process in Evolution
(Princeton Univ. Press, Princeton, NJ, 2003).
39. J. B. Silk, Science 317, 1347 (2007).
40. L. J. Young, Z. X. Wang, Nat. Neurosci. 7, 1048 (2004).
41. E. B. Keverne, N. D. Martinez, B. Tuite,
Psychoneuroendocrinology 14, 155 (1989).
42. G. Rizzolatti, Anat. Embryol. (Berl.) 210, 419 (2005).
43. E. A. Nimchinsky et al., Proc. Natl. Acad. Sci. U.S.A. 96,
5268 (1999).
44. F. Burki, H. Kaessmann, Nat. Genet. 36, 1061 (2004).
45. N. Mekel-Bobrov et al., Science 309, 1720 (2005).
46. J. Bradbury, PLoS Biol. 3, e50 (2005).
47. L. Barrett, J. Lycett, R. Dunbar, Human Evolutionary
Psychology (Palgrave-Macmillan, Basingstoke, UK, 2002).
10.1126/science.1145463
REVIEW
Social Components of Fitness in
Primate Groups
Joan B. Silk
There is much interest in the evolutionary forces that favored the evolution of large brains in the
primate order. The social brain hypothesis posits that selection has favored larger brains and more
complex cognitive capacities as a means to cope with the challenges of social life. The hypothesis is
supported by evidence that shows that group size is linked to various measures of brain size. But it
has not been clear how cognitive complexity confers fitness advantages on individuals. Research in
the field and laboratory shows that sophisticated social cognition underlies social behavior in
primate groups. Moreover, a growing body of evidence suggests that the quality of social
relationships has measurable fitness consequences for individuals.
L
ife in primate groups rivals the best tele-
vision soap opera―the weak are often
exploited by the powerful; strong alli-
ances and lasting bonds are formed; dynasties
are established, but are occasionally toppled;
and not all of your favorite characters survive
the season. Ecological constraints generate the
dramatic tension, and natural selection crafts the
plot. The complicated storylines reflect the fact
that primates have evolved large brains, sophis-
ticated social cognition, and complex social
relationships (Fig. 1). There has been consider -
able discussion of the selective pressures that
favor the evolution of large brains in social
species (1–4), but it has has not been clear how
large brains, social cognition, and social rela-
tionships are translated into fitness advantages
for individuals. New evidence indicates that the
competitive success and reproductive perform-
ance of individuals in primate groups is affected
by the nature and quality of the relationships
that they form. These data enable us to tie to-
gether what we have learned from comparative
analyses of brain morphology, experimental
studies of social cognition, and naturalistic
observations of the structure of social relation-
ships in primate groups.
What the Social Brain Knows
The capacity to develop complex social relation-
ships may be an important benefit derived from
having a “social brain.” According to the social
Department of Anthropology, University of California, Los
Angeles, CA 90095, USA. E-mail: jsilk@anthro.ucla.edu
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