1. Primate Morphological Traits
Allometry of Brain and Body Size

2. Us and Them
No single trait differentiates primates from other animals

3. 2 Features that Stand Out
Larger brains, with variation within
Greater dexterity and mobility than other animals, with variation within

4. Brains Neocortex is larger Cognitive abilities Reasoning Consciousness
50-80% of total brain volume
Neocortex in blue, from:

5. 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

6. “Social Brain Hypothesis” Robin Dunbar, 1988
Larger brains correlate with more social primates
Larger brains = larger social groups
Colored areas: frontal cortices

7. Competition or Cooperation
Allies in social groups ensure access to resources and protection…
but…
Neocortex size is also correlated with tactical deception

8. 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:

9. 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:

10. Memory A larger neocortex has ecological advantages
Memory: remembering where food is located, how to extract food

11. 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

12. Body Allometry: Diet Basal Metabolic Rate: BMR
Rate at which energy is used to maintain bodily functions at rest

13. 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)

14. 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

15. Why bigger or smaller brains?
Metabolic rates might constrain brain size Or
Skills needed to find high energy foods might result in selection for bigger brains

16. Jarman/Bell Principle
Originally explained antelope behavior, but applies to primates…
Relationship between body size, metabolic rate, and food quality
Nutrient requirement
Body weight
Large animal
Large (abundant foods)
Small
(poor quality foods)
Small animal
Small (rare foods)
Large
(high quality foods)

17. Life Histories Larger brains relate to extended life histories
Social consequences?
Long infant dependency allows for more time to develop socially
Far right: collared lemurs
Near right: macaques

18. Life Histories Presocial: animals well-developed at birth
Altricial: animals under-developed at birth; long dependency (see Table 2.1 page 43)

19. 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

20. Next topic: More primate morphological traits
The End
Next topic: More primate morphological traits