Primate Morphological Traits
Allometry of Brain and Body Size
Us and Them
• No single trait differentiates primates from other animals
2 Features that Stand Out1. Larger brains, with variation within2. Greater dexterity and mobility than other
animals, with variation within
Brains• Neocortex is larger– Cognitive abilities– Reasoning– Consciousness– 50-80% of total brain volume
Neocortex in blue, from: http://www.nibb.ac.jp/brish/Gallery/cortexE.html
Why develop a bigger brain?
• Typically, morphological traits are a function of ecological adaptation, tied to the:– Need for food– Need to mate to reproduce– Need to diminish predation and other threats
“Social Brain Hypothesis” Robin Dunbar, 1988
• Larger brains correlate with more social primates
• Larger brains = larger social groups
http://www.nature.com/neuro/journal/v5/n3/fig_tab/nn0302-190_F1.html
Colored areas: frontal cortices
Competition or Cooperation
• Allies in social groups ensure access to resources and protection…
but…• Neocortex size is also correlated with tactical
deception
Competition or Cooperation• Tactical deception: acts or behaviors that
deliberately mislead others; a form of Machiavellian intelligence– Machiavellian or
Social Intelligence Hypothesis(read particularly the last few paragraphs)
– Machiavelli defined
Rhesus macaques; from: http://scienceblogs.com/zooillogix/Langurs%20Fighting.jpg
Competition or Cooperation• Others argue in favor of expedience– Primates have the ability to adjust competitive
and cooperative behaviors as needed
• Larger neocortex: cooperation stimulates reward centers located in this part of the brain
Bonobos; from: http://www.primates.com/bonobos/wild-bonobos.jpg
Memory• A larger neocortex has ecological advantages• Memory: remembering where food is
located, how to extract food
Allometry• 2 traits can be related in 2 ways:– Isometrically: 2 variables increase or decrease in
direct proportion to one another– Allometrically: 2 variables increase or decrease at
different rates• e.g., hominid brain size relative to body size beginning
around 2-3 mya
Body Allometry: Diet
• Basal Metabolic Rate: BMR– Rate at which energy is used to maintain bodily
functions at rest
Allometry: Body and Brain• BMR (and brain size):
allometrically related to body weight– Greater weight = lower metabolism
= eating more lower energy foods (e.g., gorillas are folivores, top photo)
– Lesser weight = higher metabolism = eating more high energy foods (e.g., chimps favor fruit (bottom photo), nuts)
Allometry: Body and Brain
• Gorillas: larger bodied, smaller brain• Chimpanzees: smaller bodied, larger brain• Brains: require high amounts of energy to run– 2% of body weight– 20% of energy to run
Why bigger or smaller brains?
• Metabolic rates might constrain brain sizeOr
• Skills needed to find high energy foods might result in selection for bigger brains
Jarman/Bell Principle
• Originally explained antelope behavior, but applies to primates…
• Relationship between body size, metabolic rate, and food quality
Nutrient requirement
Nutrient requirementBody weight
Large animal Large (abundant foods)
Small(poor quality foods)
Small animal Small (rare foods) Large(high quality foods)
Life Histories• Larger brains relate to extended life histories• Social consequences?– Long infant dependency allows for more time to
develop socially
Far right: collared lemursNear right: macaques
Life Histories
• Presocial: animals well-developed at birth• Altricial: animals under-developed at birth;
long dependency (see Table 2.1 page 43)
Jarman/Bell Principle and Sexual Dimorphism
• Body size and physical differences between the sexes
• Within the same species, when compared to females, larger males:– Require more time to develop– Have a greater dietary intake– Spend different amounts of time eating– May eat different types of foods
The End
Next topic: More primate morphological traits
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