Cooperation and Reciprocal Altruism

1
Cooperation and Reciprocal Altruism

• Definitions
• Iterated prisoners dilemma
• Examples
• Food sharing
• Alliance formation
• Egg trading
• Predator inspection
• Social grooming

2
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

3
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.

4
The prisoners dilemma
Two suspected criminals are jailed separately and
encouraged to provide evidence that the other was
involved in the crime

• PD is defined by T gt R gt P gt S and R gt (T S) /
  2
• ESS for single round of the game always defect!

5
The Iterated Prisoners 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 gt (T - R)/(T - P) and w gt (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)

6
Beyond tit-for-tat

• Once TFT evolves, can other strategies invade?
• Subsequent work indicates that other trajectories
  may occur, e.g. TFT-gt Generous TFT-gt Pavlov-gt
  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.

7
Vampire bat food sharing
8
Costs and benefits of food sharing
9
Survival gain of food sharing
10
Chimpanzee food sharing
11
Cotton-top tamarin food sharing
12
Alliance formation

• Baboons
• Vervet monkeys
• Bottlenose dolphins

13
Egg-trading in polychaetes and bass
14
Predator inspection in fish
15
Predator inspection - mirror expt
But, same result is seen in The absence of any
predator! Suggests that fish tend to school.
Fish with parallel mirror approached closer than
fish with oblique mirror
16
Social grooming in antelope
Females
Males
17
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