C. By-product mutualism: as a consequence of behaving selfishly, the donor inadvertantly benefits the recipient. The benefit sometimes may not occur instantaneously, as in mutualism, but may occur after a delay in time. Many examples, e.g. cleaner fish, oxpeckers removing ectoparasites.

D. Reciprocal altruism: the trading of altruistic acts in which benefit is larger than cost so that over a period of time both participants enjoy a net gain.

1. Delay between receipt of benefit and cost donation separates mutualism from reciprocal altruism.
2. Delay allows for the possibility of cheating, thus cheaters must be detectable and excluded
3. Sufficient numbers of interactions must occur to provide net benefit to participants. Note that in many instances, net benefit will increase with number of reciprocal exchanges received in a lifetime. Thus, a large number of interactions will favor reciprocity.

II. How can reciprocity increase when initially rare?

A. Occur in kin groups and be facilitated by kin selection
B. Be directed preferentially to reciprocators
C. By drift

III. TIT FOR TAT as a model of reciprocity

A. Based on the Prisoner's Dilemma game: two suspected criminals are jailed separately and encouraged to provide evidence that the other was involved in the crime.


1. The payoff matrix for one iteration of this game is

Individual 2 responses:

cooperate defect

Individual 1 actions: cooperate R=3 S=1
defect T=4 P=2

2. To be a PD, T > R > P > S and R > (T + S) / 2
3. Always defect is best strategy in a finite round game

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

1. Iteration permits complicated strategies, e.g. CDCDCCCD, TFT, etc.
2. TFT (cooperate on the first move and thereafter mimic your opponent) is the best
strategy because

a. Outscored all other strategies in computer tournament (Axelrod)
b. Is an ESS if the probability of future encounter, w, meets these criteria:

w > (T - R)/(T - P) and w > (T - R)/(R - S) (Axelrod & Hamilton)

Obtain these inequalities by applying 1, w, w2, w3,... to successive future payoffs and noting that w + w2 + w3 +... = 1/(1 - w)

C. Once TFT evolves, can other strategies invade? New work indicates that the following trajectory may occur: TFT->GTFT->Pavlov->cooperation (Nowak & Sigmund)

1. 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)
2. Pavlov - win-stay, lose-shift does better than TFT because it corrects occasional mistakes and exploits unconditional cooperators.

IV. Probable examples of reciprocity

A. Social grooming in antelope

B. Food sharing

1. vampire bats (Wilkinson)
2. chimpanzees (Frans de Waal)

C. Egg-trading in fish (Fischer) and polychaetes (Sella)

D. Predator inspection in fish? (Milinski, Dugatkin)

Milinski and Dugatkin used a mirror to simulate a cooperating partner (parallel mirror) or a defecting partner (oblique mirror) fish.

The fish with the parallel mirror approached the predator more closely, as expected if TFT operates. But, Masters and Waite showed that the same outcome occurs in the absence of a predator implying that the response is due to a schooling tendency of the fish, not cooperative predator inspection.

E. Dear neighbors - territorial birds often do not defend borders from neighbors as aggressively as they will from foreigners as judged by playback studies

1. tree swallows
2. hooded warblers

F. Alliance formation

1. baboons (Packer, Cheney, Noe)
2. vervet monkeys (Seyfarth & Cheney)
3. bottlenose dolphins (Connor)

V. Potential implications for human behaviors

A. Friendship formation, non-kin directed altruism, gift exchange ceremonies
B. Emotion evolution: gratitude, guilt and reparative altruism
C. Justice, moralistic aggression, and revenge
D. Reciprocal network size: cartel formation, dialects

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