lecture 12, chapter 7. primate sociality and the evolution of cooperation
DESCRIPTION
Notes on Primate Sociality and Cooperation for an Introductory Anthropology course.TRANSCRIPT
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Primate Sociality
& The Evolution
of Cooperation
Todays Game Plan
Primate sociality and cooperative living The evolution of cooperation
Kin Selection Reciprocal Altruism
Primates are social Humans are social animals. Why?
For this, it helps to look at why our primate relatives are social animals as well.
Benefits of being social
More eyes to see predators detection
Benefits of being social
Less chance of being singled out by a predator dilution
Gelada herds can number over 800 individuals the largest in the world.
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Benefits of being social
More to attack defense
Benefits of being social
Mates usually easy to find More individuals to look for food More individuals to defend food
Costs of being social
Larger groups more visible to predators Larger groups need more food, so more
competition for food. Increased competition for mates
Costs of being social
Social hierarchies produce subordinates
Male dominance rank
Stre
ss h
orm
one
(cor
tisol
)
Costs of being social
Increase in disease transmission
Primate sociality
When to be social? When the benefits outweigh the costs.
Social animals are those that either
Rely on food that is hard to find but easy to share. Rely on active (detection, defense) anti-predator
strategies.
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Cooperative Living
Social animals not only live in close proximity, but live together cooperatively.
Examples: Grooming Alliance formation Alarm calls and predator defense
The evolution of cooperation
Why are these two grooming? How do alliances form?
These actions are costly to the participants, so why would they evolve?
Altruism
Altruism: Taking a cost to benefit another individual.
This is difficult to explain from an evolutionary perspective.
Cooperative living
-- + Self
--
+ Other
Cooperative living
-- + Self
--
+ Other
+ + -- +
-- -- + --
Cooperative living
-- + Self
--
+ Other
+ + Mutualism
-- + Altruistic
-- -- Spiteful
+ -- Selfish
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Gene-level selection
How do we explain altruism? The genes are ultimately what is passed on to
offspring so it is the genes that must benefit. This is not the same at the level of
individual population species
Primate social groups
Primate societies are kin-based In principle, individuals could aggregate any way, including
randomlybut they dont. We see vastly more cooperation/altruism/investment among
family members than among non-family members.
Kin selection and inclusive fitness theory
Kin Selection
One of two explanations for apparent altruism (the other is reciprocal altruism).
Kin selection is also referred to as Hamiltons rule.
W. D. Hamilton
Observed that genes have 2 routes to the next generation: Direct: Producing more offspring by
meeting the challenges of the environment.
Indirect: Aiming the benefits of altruism at individuals that carry the same genes you do.
Inclusive fitness: The combined fitness benefits of direct and indirect reproduction.
Hamiltons Rule
A formal model of kin selection Total advantage must take into account costs,
benefits, and the probability the same gene is in the other individual.
rb > c r: 0-1; coefficient of relatedness; the probability that two people
share a gene by virtue of their sharing a common ancestor. b: benefit to other c: cost to individual
Coefficient of relatedness
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Coefficient of relatedness
50%
? ?
Coefficient of relatedness
How related are Bart, Lisa and Maggie to each other?
Same as asking how likely they are to share any one particular allele.
Coefficient of relatedness
Marge has Allele X. What is the probability it got passed to both Lisa and Bart?
To Lisa Yes No
Yes To Bart
No
YY NY
YN NN
Coefficient of relatedness
There is a 25% chance that an allele from Marge makes it into both Lisa and Bart.
The same logic works for an allele from Homer.
Therefore, Lisa and Bart (and Maggie) share 50% of their genes with each other.
25% + 25% = 50%
Coefficient of relatedness
Some common values for r: .5 (50%): full siblings, parents .25 (25%): half-siblings, g-parents, full aunt/uncle .125 (12.5%): cousins, half aunt/uncle
Example of Hamiltons Rule
A and B are siblings Cost to A: 3 fitness units Benefit to B: 7 fitness units
rb > c
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Example of Hamiltons Rule
A and B are siblings Cost to A: 3 fitness units Benefit to B: 7 fitness units
rb > c .5x7 ? 3
Example of Hamiltons Rule
A and B are siblings Cost to A: 3 fitness units Benefit to B: 7 fitness units
rb > c
3.5 > 3 Beneficial for A to help.
Kin selection
Acts of support in wild baboons
Kin selection
Time spent grooming in rhesus monkeys
Inclusive fitness
Inclusive fitness: It is the genes that benefit, not necessarily the individual.
Parent-offspring conflict Example: Weaning conflict
A and B are siblings, their mother Cost to A: 3 fitness units Benefit to B: 2 fitness units
Mother: Cr < Br 3x.5 ? 2x.5 1.5 > 1 Adaptive, wants A to share, but A
does not want to share.
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Reciprocal Altruism
Robert Trivers The other evolutionary
explanation for apparent altruism.
You pay a cost now to benefit me, I'll pay a cost later to benefit you and we'll both be better off in the long run.
Reciprocal Altruism
Me You + - - + - + + -
Both parties have a net benefit from the interaction.
Time 1
Time 2
Time 3
Time 4
Reciprocal Altruism
Reciprocal altruism can only work if these requirements are met: Regular interaction between individuals Recognition of individuals Memory for others' past actions Discrimination against non-reciprocators
Reciprocal Altruism
Case Study: Vampire Bats
Basic ecology Live in caves with hundreds of others Hunt exclusively If meal doesn't happen, severely impairs ability to get
meal next night Die within three days of not eating Can vomit portions of food at will
Reciprocal Altruism
Bats must choose whether or not to help another that did not eat.
Costs: Loose a portion of their nutrients. Benefits: Other might help when they don't get to
eat.
Observed behavior: Vampire bats give food to those that have given food them in the past.
Reciprocal Altruism
A primate example Vervet monkeys
repond more strongly to a distress call if they had previously groomed the caller.
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Cooperation and Strategies
Cooperation can evolve under reciprocal altruism only if 'cheaters' are not assisted.
Must discriminate against those that will not repay the benefit.
How can you tell who this will be?
Cooperation and Strategies
A rule of thumb is to withhold benefits from those that have failed to provide previously and continue the relationship with those that have cooperated previously.
This is a strategy called Tit-for-Tat.
Cooperation and Strategies
Options are 'Cooperate' and 'Defect'
Tit-for-Tat Strategy Always cooperate the first turn. Do what the other player did in the previous turn.
Cooperation and Strategies
Tit-for-Tat withholds cooperation from those that do not provide benefits.
This protects from excessive losses.
Tit-for-Tat continues cooperation with those that provide benefits.
Cooperating first allows Tit-for-Tat to play well with itself.
Tit-for-Tat also 'forgives.
Cooperation and Strategies
There are other strategies for how to play the game.
Always Defect Always Cooperate Grim Trigger
Reciprocal Altruism
Tit-for-Tat does better than Always Defect Always Defect looses out on cooperative exchanges
when it becomes common Tit-for-Tat does better than Always Cooperate
Always Cooperate plays well with Tit-for-Tat but is vulnerable to Always Defect
Tit-for-Tat does better than Grim Trigger Grim Trigger does not forgive so looses out on any
cooperation potential after first defection