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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