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