DUNBAR: CO-EVOLUTION OF NEOCORTEX SIZE, GROUP SIZE
AND LANGUAGE IN HUMANS
First comments by Orsolya Thuma
Introduction
Dunbar's article aims at examining the relationship between
the brain size, the average groupe size and the grooming
behavior of primate species to draw conclusions about
the evolution of languge.
> Primates are, above all, social animals. {...} mean
> group size is directly related to relative
> neocortical volume in nonhuman primates.
So Dunbar first establishes the starting point of his
argument. We, human beings, and the other species of
primates are characteristically social animals. What
is really interesting is the observation that there
is a close relationship between brain size and group
size, at least in primates.
> ...These analyses suggest that although the size of
> the group in which animals live in a given habitat
> is a function of habitat-specific ecologically-
> determined costs and benefits (see for example
> Dunbar 1988, 1992b), there is a species-specific
> upper limit to group size which is set by purely
> cognitive constraints: animals cannot maintain the
> cohesion and integrity of groups larger than a size
> set by the information- processing capacity of
> their neocortex.
Certainly based on etological observations the group
size always conforms to the a number of interrelated
ecological and species specific factors, such as the
need for defence against predators, availability of
food resources. But the author hypothesizes that it
is not the environmental or some biological factors
that set a limit to group size, but the cognitive
capacities of the individuals to remember and keep
up "personal" relationships. The predicted and
hypothesized cognitive capacity with its limits
is one of the key points in Dunbar's argument.
It also means that we, human beings, are also
constrained in our capacity to form integrated
groups. What are the specially cognitive limitations?
What does this upper limit refers to?
> The group size identified by this relationship
> appears to refer to the maximum number of
> individuals with whom an animal can maintain social
> relationships by personal contact.
It is known that primates have a quite complex
knowledge about all the members in their group.
Dunbar says that the total number of relationships
an animal can maintain is limited somehow in the mind.
What about the complexity of the relationship
between all the individuals in the group and
the different interrelated levels of groupings,
e.g. family, dominance hierarchy, friendships?
How do they load the cognitive capacity?
> It is important to appreciate that the causal
> relationship between group size and neocortex size
> depends on the explanatory perspective (or level)
> adopted. In evolutionary terms, the size of a
> species' neocortex is set by the range of group
> size required by the habitat(s) in which it
> typically lives. However, seen in proximate terms
> from an individual animal's point of view, current
> neocortex size sets a limit on the number of
> relationships that it can maintain through time,
> and hence limits the maximum size of its group.
Earlier Dunbar claimed a correlation between
brain size and group size. Now he says that
it is a causal relationship. However, a correlation
should not be interpreted as a causal relationship
without further study into the question. The two
correlated features might have a common cause,
for example.
> ...although the evolution of neocortex size is
> driven by the ecological factors that select for
> group size, we can use the relationship in reverse
> to predict group sizes for living species.
Even if the reader has some reservations about the
causal link between brain size and group size, the
correlation permits us to predict any of the two factors
knowing the value of the other factor.
> It is generally accepted that the cohesion of
> primate groups is maintained through time by social
> grooming.
Grooming in the case of primates means fiddling in the
fur of the other animal, looking for insects, so it
has both a cleaning and a socal function.
> Social grooming is used both to establish and to
> service those friendships and coalitions that give
> primate groups their unique structure. As might be
> anticipated, the amount of time devoted to social
> grooming correlates well with group size, notably
> among the catarrhine primates.
So the bigger a group, the more time is spent on
grooming. There is a correlation between group size
and grooming time, but we still do not know if it is
a causal relationship. If time is a limiting factor,
why do we need to refer to cognitive factors?
> However, the relationship between group size and
> time devoted to grooming appears to be a
> consequence of the intensity with which a small
> number of key "friendships" (the primary network)
> is serviced rather than to the total number of
> individuals in the group.
Is this remark not in contrast with the earlier
claim that is in only the total number of relationships
that matter? Within the larger group smaller coalitions
or friendships can be identified where the individuals
groom relatively more than in other realtionships.
What explains then the observation that the correlation
between group size and grooming is linear?
And now, let's turn to the predictions and
observations about our species. Dunbar takes
the measures of human brain size to predict
the limits of group size for h.s. sapiens.
> Equation (1) yields a predicted group size for
> humans of 147.8. {...}the 95% confidence limits
> around this prediction (...) are moderately wide
> (100.2- 231.1).
So by extrapolating the data, the size of the
human brain predicts an average group size of 148
individuals, meaning that this amount of people can
form a group where all members know each other individually
and personally. This is just an average value. Because of
the large variance, actual group size may vary between
100 and 230.
Now Dunbar has to prove somehow that this predicted
group size is observable in real natural human groups.
> Given that our brain size has its origins in the
> later stages of human evolution some 250,000 years
> ago (Martin 1983, Aiello & Dean 1990), we may
> assume that our current brain size reflects the
> kinds of groups then prevalent and not those now
> found among technologically advanced cultures.
> {...} The closest we can get to this is to examine
> those modern humans whose way of life is thought to
> be most similar to that of our late Pleistocene
> ancestors. These are generally presumed to be the
> hunter-gatherers.
Since Dunbar intends to talk about evolutionary processes,
it is important to reconstruct the group behaviour of
our evolutionary ancestors. One possibilty would be to
look for paleontological data, an other is to look for
data of the so-called primitive peoples, who allegedlylive
in a natural way.
Dunbar chooses to examines anthropological data about
tribal groups and he is able to identify 3 levels of grouping:
> .. small living groups of 30-50 individuals
> (commonly measured as overnight camps, but often
> referred to as bands in some of the hunter-gatherer
> literature), a large population unit (the tribe or
> in some cases sub-tribe) that typically numbers
> between 500 and 2500 individuals and an
> intermediate level of grouping (either a more
> permanent village or a culturally defined clan or
> lineage group) that typically contains 100-200
> people.... The average size of the intermediate
> level groups for those societies for which accurate
> census data are available is 148.4.
Beside the hunter-gatherers group forming behaviour,
he presents observations from other areas (e.g. Neolithic
villages in Mesopotamia, farming communities,
academic communities, basic units of armies,
modern companies) that also support his prediction.
So the observational data seem to support his hypothesis
about a special group size in our species, namely for
groups where members phisically know each other.
But does he have evidence that these groups are really
based on personal knowledge of all the members? And what
is really special about this level of grouping among
smaller and larger units beside the fact that it smashes
with the number he predicted?
After showing evidence of the existence of his identified
grouping level he regards the effect of group size on
grooming behaviour.
> Irrespective of precisely how grooming functions to
> integrate large primate groups, we can use the
> relationship between group size and grooming time
> to predict the grooming time required to maintain
> cohesion in groups of the size predicted for modern
> humans.
Based on observational data, Dunbar concludes that:
> The group size predicted for modern humans by <equation
would require as much as 42% of the <total time budget
to be devoted to social <grooming.{...} A group of 200,
for instance, would <have to devote 56.6% of its day
to social grooming.
The time needed for the same level of maintaining group
cohesion is significantly bigger than in the non-human
primates, which spend only 20% of their day on grooming.
If humans would go on grooming each other in the primate
fashion, they would not have enough time for other
essential life functions.
The problem is how to maintain a large group and group
cohesion at the same time if there is not enough time
for the traditional primate grooming activities?
> The only option will thus be a more efficient use
> of the time available for social bonding. In this
> context, the main problem with grooming as a
> bonding mechanism is that it is highly inflexible:
it is all but impossible to do anything else while
> grooming or being groomed.
Grooming requires the animals to sit calmly in physical
contact and use their hands, so it is not possible to
persue other activities at the same time, which would
be a good was to save time.
> In addition, grooming is an essentially dyadic
> activity: only one other individual can be groomed
> at a time.
Another solutions for time saving would be to groom
several individuals at the same time, but grooming does
not allow this either.
> Modern humans do, however, possess a form of social
> communication that overcomes both of these
> limitations very effectively: not only can speech
> be combined with almost every other activity (we
> can forage and talk at the same time), but it can
> also be used to address several different
> individuals simultaneously. Thus, language
> introduces major savings by allowing an individual
> to do two different things at once.
Thus, Dunbar's conclusion is that it is language
that makes it possible for us to overcome the
limitations of physical grooming and as a result
permitting larger groups to maintain.
Language is a symbolic means of communication,
we can talk about things that are not present,
talk about other people to follow what is happening
to them even when we cannot see them. And speech
is transmitted auditorily, it does not require
physical contact between the interacting individuals.
This permits the speakers to persue other activities,
thus to save time.
> My suggestion, then, is that language evolved as a
> "cheap" form of social grooming, so enabling the
> ancestral humans to maintain the cohesion of the
> unusually large groups demanded by the particular
> conditions they faced at the time.
Finally, Dunbar concludes that the time saving nature
of linguistic grooming was the main selective pressure
on the evolution of language.
What does he mean by "cheap" form?
Does language really serve a grooming function?
He declares that:
> The acquisition and exchange of information about
> social relationships is clearly a fundamental part
> of human conversation. I suggest that it implies
> that this was the function for which it evolved.
This statement is in contrast with traditional ideas
of language evolution, which originate language from
the need to communicate factual information and to control
common actions, such as hunting.
He shows some observational data to illustrate how language
is used as a grooming activity.
> Table 4 summarises data on the content of
> conversations in a university refectory.
> Approximately 38% of conversation content was
> devoted to personal relationships (either of those
> present or of third parties) and a further 24%
> involved discussion of personal experiences of a
> more general kind, both topics being clearly
> related to social knowledge. Considering the
> potential importance of academic and other
> intellectual topics of conversation in a university
> environment, these are remarkably high values.
The observation that we spend most of our talking
time with chatting about ourselves and other
people is probably not really surprising. What
is interesting in the refered research is that
highly intellectual communities behave in the same
way as the groups we know from our ordinary life
as far as the content of talking is concerned.
However, to put all the pieces of Dunbar's arguments
we should know if there was any evolutionary
advantage of larger groups in our ancestors' life?
> Just why early humans should have found it
> necessary to evolve such large groups remains
> uncertain, however, and there is little that can
> usefully be said to clarify this point at present.
The lack of explanation here somewhat weakens the
argument for the proposed selective mechanism in
language evolution. But it certainly does not
invalidate it.
END
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