Reciprocal altruism

Reciprocal altruism
Diagram showing reciprocal altruism

In evolutionary biology, reciprocal altruism is a behaviour whereby an organism acts in a manner that temporarily reduces its fitness while increasing another organism's fitness, with the expectation that the other organism will act in a similar manner at a later time. The concept was initially developed by Robert Trivers to explain the evolution of cooperation as instances of mutually altruistic acts. The concept is close to the strategy of "tit for tat" used in game theory.



The concept of “reciprocal altruism”, as introduced by Trivers, suggests that altruism, defined as an act of helping someone else although incurring some cost for this act, could have evolved since it might be beneficial to incur this cost if there is a chance of being in a reverse situation where the person whom I helped before may perform an altruistic act towards me.[1] Putting this into the form of a strategy in a repeated prisoner’s dilemma would mean to cooperate unconditionally in the first period and behave cooperatively (altruistically) as long as the other agent does as well.[1] If chances of meeting another reciprocal altruist are high enough or the game is repeated for a long enough amount of time, this form of altruism can evolve within a population. This is very close to the notion of "tit for tat" introduced by Anatol Rapoport, although there still seems a slight distinction in that "tit for tat" cooperates in the first period and from thereon always replicates an opponent’s previous action, whereas “reciprocal altruists” stop cooperation in the first instance of non-cooperation by an opponent and stay non-cooperative from thereon. This distinction leads to the fact that in contrast to reciprocal altruism, tit for tat may be able to restore cooperation under certain conditions despite cooperation having broken down.

Stephens shows a set of necessary and jointly sufficient conditions “… for an instance of reciprocal altruism:[2]

  1. the behaviour must reduce a donor's fitness relative to a selfish alternative;
  2. the fitness of the recipient must be elevated relative to non-recipients;
  3. the performance of the behaviour must not depend on the receipt of an immediate benefit;
  4. conditions 1, 2, and 3 must apply to both individuals engaging in reciprocal helping.”

There are two additional conditions necessary “…for reciprocal altruism to evolve:

  1. A mechanism for detecting 'cheaters' must exist.
  2. A large (indefinite) number of opportunities to exchange aid must exist.”

The first two conditions are necessary for altruism as such, while the third is distinguishing reciprocal altruism from simple mutualism and the fourth makes the interaction reciprocal. Condition number five is required as otherwise non-altruists may always exploit altruistic behaviour without any consequences and therefore evolution of reciprocal altruism would not be possible. However, it is pointed out that this “conditioning device” does not need to be conscious. Condition number six is required to avoid cooperation breakdown through backwards induction—a possibility suggested by game theoretical models.[2]


The following examples could be understood as altruism. However, showing reciprocal altruism in an unambiguous way requires more evidence as will be shown later.

Cleaner fish

The first example is that of the cleaner fish. Aside from the mutuality and the clear advantageous symbiosis of the cleaner and the host, which cannot be interpreted in terms of altruism, the host displays some additional behaviour that meets the criteria for altruism: The host allows the cleaner free entrance and exit and does not eat the cleaner, even after the cleaning is done.[3][4][5][6] The host signals the cleaner it is about to depart the cleaner's locality, even when the cleaner is not in its body. The host may chase off possible dangers to the cleaner.[6]

The following evidence supports the hypothesis:

The cleaning by cleaners is essential for the host. In the absence of cleaners the hosts leave the locality or suffer from injuries done by ecto-parasites.[7] Difficulty and danger in finding a cleaner. Hosts leave their element to get cleaned.[6] Others wait no longer than 30 seconds before searching for cleaners elsewhere.[3]

A key element in the establishment of reciprocal altruism is that the same two individuals interact repeatedly. This is conditioned by site specifics of either individuals (spatial condition) and by a large enough lifespan of the species (temporal condition). Surprisingly there is sufficient evidence that individual cleaners and hosts indeed interact repeatedly.[5][7][8]

This example meets the criteria that are described in Robert Trivers’ model. However, some important elements, which are essential to establish reciprocity, are not yet shown: The criterion that an individual doesn’t expect an immediate payment. In the cleaner-host system the benefit for a cleaner is always immediate. The criterion that failing of one individual to act altruistically will cause the other one to avoid future altruistic acts. This will be very hard to show since such failure will mean death of the cleaner.

If Randall’s claim that the host may chase off possible dangers to the cleaner will be proved right, an experiment might be constructed in which reciprocity could be demonstrated.[5]

Warning calls in birds

Warning calls, although exposing a bird and putting it in danger, are frequently given by birds. An explanation in terms of altruistic behaviour is given by Trivers:

It has been shown that predators learn specific localities and specialize individually on prey types and hunting techniques.[9][10][11][12] It is therefore disadvantageous for a bird to have a predator eat a conspecific, because the experienced predator may then be more likely to eat him. Alarming another bird by giving a warning call tends to prevent predators from specializing on the caller’s species and locality. In this way, birds in areas in which warning calls are given will be at a selective advantage relative to birds in areas free from warning calls.

Nevertheless, this presentation lacks important elements of reciprocity. It is very hard to detect cheaters. Also, there is no evidence that a bird refrains from giving calls when another bird is not reciprocating. And there is no evidence that individuals interact repeatedly.

Another explanation for warning calls is that these are not warning calls at all: A bird, once it has detected a bird of prey, calls to signal to the bird of prey that it was detected, and that there is no use trying to attack the calling bird. Two facts support this hypothesis:

  • The call frequencies match the hearing range of the predator bird.
  • Calling birds are less attacked—predator birds attack calling birds less frequently than other birds.

Vampire bats

Vampire bats also display reciprocal altruism, as described by Wilkinson.[13][14] The bats feed each other by regurgitating blood. To qualify for reciprocal altruism, the benefit to the receiver would have to be larger than the cost to the donor. This seems to hold as these bats usually die if they do not find a blood meal two nights in a row. Also, the requirement that individuals who have behaved altruistically in the past are helped by others in the future is confirmed by the data.[13] However, the consistency of the reciprocal behaviour, namely that a previously non-altruistic bat is refused help when it requires it, has not been demonstrated. Therefore, the bats do not seem to qualify yet as an example for reciprocal altruism. However, a closer look at the data shows that - except for a single interaction - all instances of feeding happened between individuals of the same group, who are on average cousins.[13] Thus, it seems much more probable that this example is a case of kin selection than reciprocal altruism.

Known emotional dispositions as a complex regulating system for reciprocal altruism

The human altruistic system is a sensitive and unstable one.[1] Therefore, the tendency to give, to cheat, and the response to other’s acts of giving and cheating must be regulated by a complex psychology in each individual. Individuals differ in the degree of these tendencies and responses. According to Trivers the following emotional dispositions and their evolution can be understood in terms of regulation of altruism.[1]

  • Friendship and emotions of liking and disliking.
  • Moralistic aggression. A protection mechanism from cheaters acts to regulate the advantage of cheaters in selection against the altruists. The moralistic altruist may want to educate or even punish a cheater.
  • Gratitude and sympathy. A fine regulation of altruism can be associated with gratitude and sympathy in terms of cost/benefit and the level in which the beneficiary will reciprocate.
  • Guilt and repetitive altruism. Prevents the cheater from cheating again. The cheater shows his regret in order to save him from paying too dearly for his acts.
  • Subtle cheating. A stable evolutionary equilibrium could include a low percentage of mimics in controversial support of adaptive sociopathy.
  • Trust and suspicion. These are regulators for cheating and subtle cheating.
  • Partnerships. Altruism with the purpose of creating friendships.

However, it is to be noted that there have been few theoretical and experimental research that assess the importance of reciprocal partner choice; it is known that theoretical models of the evolution of altruism by reciprocal partner choice are rare, mostly due to difficulties of 'payoffs' between multiple individuals. [15]

See also


  1. ^ a b c d Trivers, R.L. (1971). "The evolution of reciprocal altruism". Quarterly Review of Biology 46: 35–57. doi:10.1086/406755. 
  2. ^ a b Stephens, C. (1996). "Modeling Reciprocal Altruism". British Journal for the Philosophy of Science 47 (4): 533–551. doi:10.1093/bjps/47.4.533. 
  3. ^ a b Eibi-Eibesfeldt, T (2010). "Über Symbiosen, Parasitismus und andere besondere zwischenartliche Beziehungen tropischer Meeresfische1". Zeitschrift für Tierpsychologie 12 (2): 203–219. doi:10.1111/j.1439-0310.1955.tb01523.x. 
  4. ^ Heidiger, H. 1968. Putzer-fische im aquarium. Natur und Museum, 98:89-96
  5. ^ a b c Randall, J. E. (1958). "A review of the Labrid fish genus Labriodes with descriptions of two new species and notes on ecology". Pacific Science 12: 327–347. 
  6. ^ a b c Randall, J. E. (1962). "Fish service stations". Sea Frontiers 8: 40–47. 
  7. ^ a b Feder, H. M. 1996. "Cleaning symbioses in the marine environment". In S. M. Henry (ed.), Symbiosis, Vol. 1, p. 327-380. Academic Press, N.Y.
  8. ^ Limbaugh, C.; Pederson, H.; Chase, F. (1961). "Shrimps that clean fishes". Bull. Mar. Sci. Gulf Caribb 11 (1): 237–257. 
  9. ^ Murie, A. 1944. The Wolves of Mount McKinley. Fauna of National Parks, Faunal Series # 5; Wash., D.C.
  10. ^ Southern, H. N. (1954). "Tawny owls and their prey". Ibis 96: 384–410. doi:10.1111/j.1474-919X.1954.tb02332.x. 
  11. ^ Tinbergen, N. (1968). "On war and peace in animals and man". Science 160 (3835): 1411–1418. doi:10.1126/science.160.3835.1411. PMID 5690147. 
  12. ^ Owen, D. F. (1963). "Similar polymorphismas in an insect and a land snail". Nature 198 (4876): 201–203. doi:10.1038/198201b0. 
  13. ^ a b c Wilkinson, G. (1984). "Reciprocal Food Sharing in the Vampire Bat". Nature 308 (5955): 181–184. Bibcode 1984Natur.308..181W. doi:10.1038/308181a0. 
  14. ^ Wilkinson, G. (1988). "Reciprocal Altruism in Bats and Other Mammals". Ethology and Sociobiology 9 (2–4): 85–100. doi:10.1016/0162-3095(88)90015-5. 
  15. ^ Schino G., Aureli F. (2010). A few misunderstandings about reciprocal altruism. Commun Integr Biol 3(6): 561–563. doi: 10.4161/cib.3.6.12977

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