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Understanding Primate Brain Evolution

Analysis of Brain data for primates

Jona Somaraju

on 29 August 2012

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Transcript of Understanding Primate Brain Evolution

Understanding Primate Brain Evolution Abstract 1. Introduction 2. The social brain in mamalian perspective 3. Primate Brain evolution revisted a. Univariate relationships b. Minimum adeqaute models c. Path analysis 4. The Nature of Social Complexity 5. How to evolve a big brain 6. Conclusions *Analysis of the comparative brain data for primates
*Develop a model that presents the coevolution
of primate brain and sociality within ecological
and life-history framework
* shows that body size, basal metabolic rate, and life history
act as constraints on brain evolution
*In order to evolve a large neocortex, a species must first evolve
a large brain to support that neocortex
* in turn requires adjustment in diets for energy
* in turn requires adjustment for life history for adequte amount
of time for the brain growth and "software programming"
* The social brain hypothesis is not about the realtionship between
brain/neocortex size and group; but it is about social complexity Social hypothesis main claim Since primates had unusually complex
social life, primates needed a comparably large brain to cope with the computations involved 1. The focus has been mainly on bivariates correlations
between brain size and either sociality; little attempt has
been made to evaluate whether the relationship between
group size and brain size is acutally confound of these
ecological variables
2. What it is about sociality that creates the cognitive load
that is so demanding of neural computational power?
Is it solely based in terms of groups size?
Is it explicitly about the complexity of social relationships? Issues that arose from findings How are these issues addressed? Some points about the social brain hypothesis 1. New analyses which incorporate a number of ecological and demographic variables

2. Consider in more detail the nature of the socio-cognitive demands * It is an ecological hypothesis
*The claim is that one or more ecological problems (survival, foraging, and rearing offspring) are more effectively solved socially than by an individuals unaided efforts Discussion Points: A) Is an animals solutions to the problems of successful survival and reproduction social?
Is an animals solutions to the problems of successful survival a product of individual problem solving? B) What is the prinicpal selection pressure
for the evolution of socio-cognitive skills?
Social bonding?
Feeding? Social brain hypothesis holds
in other non-primate mammals:
Carnivores & Ungulates There is a strong correlation between relative brain size and sociality in carnivores, ungulates, and primates Supports the social brain hypothesis and weakens alternative hypothesis Lagged evolution in: Primates
No consistent pathways
Switching back and forth
Showed strong correlation Life- History Characteristics *onset of reproduction
*constrains the investment individuals can make in non-reproductive age classes External mortality rates: An evaluation of the ecological and social characteristics associate with primate brain size and social complexity using a three-step analysis 1. Univariate analyses
2. General linear models to test alternative models
3. Build and test a path model *One measure of neocortex explained more variation than either total brain size or relative cerebellum size

*Life-history measures were also associated consistently with the various indices of brain volume

*Species with large brains have, on average, higher metabolic rates, larger bodies, longer life spans and longer juvenile periods

*Life-history traits can permit species to support larger brains but overall brain itself is not tied to life-history characteristics Overall, the relationship between 'slow' life-history characteristics and higher than predicted metabolic rates indicates that a set of life-history characteristics are necessary to support the development of a large brain Ecological stand point:
* Diet may be a metabolic constraint rather than being cognitively demanding

*The relationship between ecological variables and the various indices of brain size may be an artefact of the fact that both ecology and brain size are strongly associated with group size Discussion Points:
A) In order to support the energetic needs of all group members, individuals must solve ecological problems in order to maintain their large groups

B) It is the behavioral flexibilty required to feed groups members that drives cognitive evolution rather than the ecological Constructed a global model Restricted brain size to two measures:
Brain/ Body Residuals
Neocortex/ Rest-of-brain Residuals *Brain size was best explained by a combination of
Neocortex size and longevity
*Neocortex size was best explained by total brain size, group size, and longevity
*Home range was by body size, group size, and day range
*Day range by diet and home range Least stable model for group size:
* Neocortex size and home range
* Home range and activity
* Neocortex size and activity Two Conclusions:
*If an individual can exceed their basic metabolic rate (BMR) then they can invest extra available energy into evolving and maintaining expensive brain design

*BMR does not have a strong influence on the way gross brain volume is allocated to different brain design Model is based on assumptions Figure 2 shows that the factors closely associated with group size on the diagram are better predictors than the factors that are more deply embedded in the diagram Discussion point:
Does figure two, which assumes that the three most promiate factors in the path model are the best predictors is better than a model that considers succcessively more remote sets of independent variables that only influence group size directly? Used two models to compare between alternative models
A) Change in Bayesian information (BIC)

B) Likelihood ratio test (LRT) Path diagram is the best because
*lower BIC
*alternative models are less good at predicting group size

This means that relative neocortex size, activity pattern, and home range size are the most strongly associated with group size Predation imposes both direct selection on brain size and indirect selection via the buffering effect of large group size Two pathways limit maximum group size:

The intersection of these two limiting factors can be used to describe
the maximum group size obtainable by any population

When the ecological or cognitive limits are pushed passed their limits
then a fission-fusion social structure is adapted

High energy diet is necessary to support a large brain,
but a large brain is not vital for managing a high quality diet The social brain hypothesis is a complexity of social relationships

Ex: the size of grooming cliques, tactical deception, and all play are correlated with neocortex ratio

Compare relative neocortex volumes in species

An important highlight of cognitive evolution regarding social context Discussion Point: Does growing up in a social environment be as important as having a brain of the right size? Discussion Point:
Have developmental constraints forced a significant degree of uniformity on brain structure?

Has there been a mosaic evolution whereby some brain units have enlarged more rapidly than others?

Have brain units enlarged propotionally as total brain volume enlarges
do so disportiontately as a function of specific selection pressures? The Cerebellum is unrelated to social group size when total brain and neocortex volumes are partialled out

The neocortex and the frontal lobe have increased disproportionately during the course of primate brain evolution

When brain evolution occurs it mainly involves adding more frontal cortex rather than increasing all brain units porportinally

Increasing brain size correlates with increasing social skills Brain volume correlates with sociality, which also emerges as a strong relationship with social group size

The social brain hypothesis has a quantitative form: a direct correlation with social group size
In non-primates: a correlation with sociality, but not social group size

Brain evolution involves different traits: life history, development, diet, and other ecological and behavior

Major phase shifts in constraint variables may be necessary for a species to increase in group size in response to ecological changes

Cohesion of groups have be limited by their cognitive abilities, but thye may also be constrained by time budgeting issues

Brian size increases allowing for increasing sophisticted social behavior Thanks for your time!

Shultz, Susan. "Unvertanding Primate Evolution." Understandin Primate Evolution (2007): 649-58. Web.
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