Cooperation and Reciprocal Altruism

• Definitions

• Iterated prisoner’s dilemma

• Examples– Food sharing– Alliance formation– Egg trading– Predator inspection– Social grooming

Evolution of cooperation• Mutualism

• Kin selection

• By-product mutualism– as a consequence of behaving

selfishly, the donor inadvertantly benefits the recipient.

– Example: cleaner wrasse selfishly consume ectoparasites of larger fish. The large fish cooperates by not eating the cleaner fish.

– Cleaner wrasse have mimics that cheat!

• Reciprocal altruism

Reciprocal Altruism

• the trading of altruistic acts in which the benefit is larger than the cost so that over time participants enjoy a net gain.– Delay between donation cost and receipt of benefit separates

mutualism from reciprocal altruism.

– Delay allows for the possibility of cheating, thus cheaters must be detected and excluded

– A sufficient number of interactions must occur to provide a net benefit to participants. Note that in many instances, the net benefit will increase with the number of exchanges. Thus, a large number of interactions favors reciprocity.

The prisoner’s dilemma

• PD is defined by T > R > P > S and R > (T + S) / 2

• ESS for single round of the game: always defect!

Two suspected criminals are jailed separately and encouraged to provide evidence that the other was involved in the crime

The Iterated Prisoner’s Dilemma

• Iterating this game allows for cheating - the key distinction between mutualism and reciprocity

• Iteration permits complicated strategies, e.g. one player can perform CDCDCCCD while another might do CCCCCCCC, etc.

• TFT (cooperate on the first move and thereafter mimic your opponent) is the best strategy because– Outscored all other strategies in computer tournament (Axelrod)– Is an ESS if the probability of future encounter, w, meets these criteria:– w > (T - R)/(T - P) and w > (T - R)/(R - S)– Obtain these inequalities by applying 1, w, w2, w3,... to successive future

payoffs and noting that w + w2 + w3 +... = 1/(1 - w)

Beyond tit-for-tat

• Once TFT evolves, can other strategies invade?

• Subsequent work indicates that other trajectories may occur, e.g. TFT-> Generous TFT-> Pavlov-> cooperation (Nowak & Sigmund)

– If mistakes are made, Generous-tit-for-tat does better than TFT (GTFT cooperates after opponent cooperates but also after opponent defects with some probability)

– Pavlov - win-stay, lose-shift does better than TFT because it corrects occasional mistakes and exploits unconditional cooperators.

Vampire bat food sharing

Costs and benefits of food sharing

Survival gain of food sharing

Chimpanzee food sharing

Cotton-top tamarin food sharing

Actor

Conditions A1,B1 Conditions A2,B2

Conditions A3,B3 Conditions A4,B4

food

A

Btool

Test chamber

partitionopening for subject

A= with barrier; B= no barrier

.

Session0.0

1.0

1 2 3 4Frequency of Pulls

A

BPulls for Altruist Pulls for Defector

Alliance formation

• Baboons

• Vervet monkeys

• Bottlenose dolphins

Egg-trading in polychaetes and bass

Predator inspection in fish

Predator inspection - mirror expt

Fish with parallel mirror approachedcloser than fish with oblique mirror

But, same result is seen inThe absence of any predator!Suggests that fish tend to school.

Social grooming in antelopeFemales Males

Implications for human behavior• Friendship formation

– non-kin directed altruism

– gift exchange ceremonies

• Emotion evolution– Gratitude

– guilt and reparative altruism

• Justice– moralistic aggression

– revenge

• Reciprocal network size– cartel formation

– dialects