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In biology, altruism refers to behaviour by an individual that increases the fitness of another individual while decreasing the fitness of the actor. Altruism in this sense is different from the philosophical concept of altruism, in which an action would only be called "altruistic" if it was done with the conscious intention of helping another. But in the behavioural sense, there is no such requirement. It is the consequences of an action for reproductive fitness that determine whether the action is considered altruistic, not the intentions, if any, with which the action is performed.
Altruistic behaviours appear most obviously in kin relationships, such as in parenting, but may also be evident amongst wider social groups, such as in social insects. They allow an individual to increase the success of its genes by helping relatives that share those genes.
- 1 Overview
- 2 Implications in evolutionary theory
- 3 Reciprocity mechanisms
- 4 Biological market theory
- 5 Examples in vertebrates
- 6 Examples in invertebrates
- 7 Examples in protists
- 8 See also
- 9 References
- 10 Further reading
- 11 External links
In the science of ethology (the study of behavior), and more generally in the study of social evolution, on occasion, some animals do behave in ways that reduce their individual fitness but increase the fitness of other individuals in the population; this is a functional definition of altruism. Research in evolutionary theory has been applied to social behaviour, including altruism. Cases of animals helping individuals to whom they are closely related can be explained by kin selection, and are not considered true altruism. Beyond the physical exertions that in some species mothers and in some species fathers undertake to protect their young, extreme examples of sacrifice may occur. One example is matriphagy (the consumption of the mother by her offspring) in the spider Stegodyphus; another example is a male spider allowing a female fertilized by him to eat him. Hamilton's rule describes the benefit of such altruism in terms of Wright's coefficient of relationship to the beneficiary and the benefit granted to the beneficiary minus the cost to the sacrificer. Should this sum be greater than zero a fitness gain will result from the sacrifice.
When apparent altruism is not between kin, it may be based on reciprocity. A monkey will present its back to another monkey, who will pick out parasites; after a time the roles will be reversed. Such reciprocity will pay off, in evolutionary terms, as long as the costs of helping are less than the benefits of being helped and as long as animals will not gain in the long run by "cheating" – that is to say, by receiving favours without returning them. This is elaborated on in evolutionary game theory and specifically the prisoner's dilemma as social theory.
Implications in evolutionary theory
The existence of altruism in nature is at first sight puzzling, because altruistic behaviour reduces the likelihood that an individual will reproduce. The idea that group selection might explain the evolution of altruism was first broached by Darwin himself in The Descent of Man, and Selection in Relation to Sex, (1871). The concept of group selection has a chequered and controversial history in evolutionary biology but the uncritical ‘good of the species’ tradition came to an abrupt halt in the 1960s, due largely to the work of George C. Williams and John Maynard Smith. In the 1960s and 1970s the rival theory of kin selection emerged, due originally to W. D. Hamilton. Kin selection is an instance of inclusive fitness, which combines the number of offspring produced with the number an individual can produce by supporting others, such as siblings. This theory showed how altruistic behaviour could evolve without the need for group-level selection, and quickly gained prominence among biologists interested in the evolution of social behaviour.
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Recent developments in game theory have provided some explanations for apparent altruism, as have traditional evolutionary analyses. Among the proposed mechanisms are:
- Behavioural manipulation (for example, by certain parasites that can alter the behavior of the host)
- Bounded rationality (for example, Herbert A. Simon)
- Kin selection including eusociality (see also "The Selfish Gene")
- Memes (by influencing behavior to favor their own spread; see religion as a meme)
- Reciprocal altruism, mutual aid
- Sexual selection, in particular, the Handicap principle
Social behavior and altruism share many similarities to the interactions between the many parts (cells, genes) of an organism, but are distinguished by the ability of each individual to reproduce indefinitely without an absolute requirement for its neighbors.
Altruist theories in evolutionary biology were contested by Amotz Zahavi, the inventor of the signalling theory and its correlative, the handicap principle, based mainly on his observations of the Arabian babbler, a bird commonly known for its surprising (alleged) altruistic behaviours.
Researchers in Switzerland have developed an algorithm based on Hamilton's rule of kin selection. The algorithm shows how altruism in a swarm of entities can, over time, evolve and result in more effective swarm behaviour.
Altruism in animals describes a range of behaviors performed by animals that may be to their own disadvantage but which benefit others.  The costs and benefits are measured in terms of reproductive fitness, or expected number of offspring. So by behaving altruistically, an organism reduces the number of offspring it is likely to produce itself, but boosts the likelihood that other organisms are to produce offspring. There are other forms of altruism in nature other than risk-taking behavior, such as reciprocal altruism. This biological notion of altruism is not identical to the everyday human concept. For humans, an action would only be called ‘altruistic’ if it was done with the conscious intention of helping another. Yet in the biological sense there is no such requirement. Instead, until we can communicate directly with other species, an accurate theory to describe altruistic acts between species is Biological Market Theory. Humans and other animals exchange benefits in several ways, known technically as reciprocity mechanism. No matter what the mechanism, the common thread is that benefits find their way back to the original giver.
Also known as the "buddy-system", mutual affection between two parties prompts similar behavior in both directions without need to track of daily give-and-take, so long as the overall relationship remains satisfactory. This is one of the most common mechanism of reciprocity in nature, this kind is present in humans, primates, and many other mammals.
Also known as, "If you're nice, I'll be nice too". This mechanism of reciprocity is similar to the heuristic of the golden rule, "Treat others how you would like to be treated". Parties mirror one another's attitudes, exchanging favors on the spot. Instant attitudinal reciprocity occurs among monkeys, and people often rely on it with strangers and acquaintances.
Also known as, "what have you done for me lately?" Individuals keep track of the benefits they exchange with particular partners, which helps them decide to whom to return favors. This mechanism is typical of chimpanzees and very common among human relationships.  Yet some opposing experimental research suggests that calculated or contingent reciprocity does not spontaneously arise in laboratory experimental settings, despite patterns of behavior
Biological market theory
Biological market theory is an extension of the idea of reciprocal altruism, as a mechanism to explain altruistic acts between unrelated individuals in a more flexible system of exchanging commodities. The term 'biological market' was first used by Ronald Noe and Hammerstein in 1994 to refer to all the interactions between organisms in which different organisms function as 'traders' that exchange goods and services such as food and water, grooming, warning calls, shelter, etc. Biological market theory consists of five formal characteristics which present a basis for altruism.
1. Commodities are exchanged between individuals that differ in the degree of control over those commodities
2. Trading partners are chosen from a number of potential partners.
3. There is competition among the members of the chosen class to be the most attractive partner. This competition by 'outbidding' causes an increase in the value of the commodity offered.
4. Supply and demand determine the bartering value of commodities exchanged.
5. Commodities on offer can be advertised. As in commercial advertisements there is a potential for false information. 
The applicability of biological market theory with its emphasis on partner choice is evident in the interactions between the cleaner wrasse and its "client" reef fish. Cleaners have small territories, which the majority of reef fish species actively visit to invite inspection of their surface, gills, and mouth. Clients benefit from the removal of parasites while cleaners benefit from the access to a food source. Some particularly choosy client species have large home ranges that cover several cleaning stations, whereas other clients have small ranges and have access to one cleaning station only (resident clients). Field observations, field manipulations, and laboratory experiments revealed that whether or not a client has choice options influences several aspects of both cleaner and client behaviour. Cleaners give choosy clients priority of access. Choosy clients switch partners if cheated by a cleaner by taking a bite of out of the cleaner, whereas resident clients punish cheats. Cleaners and resident clients, but not choosy clients, build up relationships before normal cleaning interactions take place. Cleaners are particularly cooperative if choosy clients are bystanders of an interaction but less so when resident clients are bystanders.
Researchers tested whether wild white-handed gibbon males from Khao Yai National Park, Thailand, increased their grooming activity when the female partner was fertile. Adult females and males of our study population are codominant (in terms of aggression), they live in pairs or small multi male groups and mate promiscuously. They found that males groomed females more than vice versa and more grooming was exchanged when females were cycling than during pregnancy or lactation. The number of copulations/day was elevated when females were cycling, and females copulated more frequently with males on days when they received more grooming. When males increased their grooming efforts, females also increased their grooming of males, perhaps to equalize give and take. Although grooming might be reciprocated because of intrinsic benefits of receiving grooming, males also interchange grooming as a commodity for sexual opportunities during a female’s fertile period.[clarification needed]
Examples in vertebrates
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- Wolves and wild dogs bring meat back to members of the pack not present at the kill.
- Mongooses support elderly, sick, or injured animals
- Meerkats often have one standing guard to warn whilst the rest feed in case of predator attack.
- Raccoons inform conspecifics about feeding grounds by droppings left on commonly shared latrines. A similar information system has been observed to be used by common ravens.
- Male baboons threaten predators and cover the rear as the troop retreats.
- Gibbons and chimpanzees with food will, in response to a gesture, share their food with others of the group. Chimpanzees will help humans and conspecifics without any reward in return.
- Bonobos have been observed aiding injured or handicapped bonobos.
- Vampire bats commonly regurgitate blood to share with unlucky or sick roost mates that have been unable to find a meal, often forming a buddy system.
- Vervet monkeys give alarm calls to warn fellow monkeys of the presence of predators, even though in doing so they attract attention to themselves, increasing their personal chance of being attacked.
- Lemurs of all ages and of both sexes will take care of infants unrelated to them.
- Dolphins support sick or injured animals, swimming under them for hours at a time and pushing them to the surface so they can breathe.
- Walruses have been seen adopting orphans who lost their parents to predators.
- African buffalo will rescue a member of the herd captured by predators.
- In numerous bird species, a breeding pair receives support in raising its young from other "helper" birds, including help with the feeding of its fledglings. Some will even go as far as protecting an unrelated bird's young from predators 
Examples in invertebrates
- Some termites and ants release a sticky secretion by fatally rupturing a specialized gland. This autothysis altruistically aids the colony at the expense of the individual insect. For example, defending against invading ants by creating a tar baby effect. This can be attributed to the fact that ants share their genes with the entire colony, and so this behaviour is evolutionarily beneficial (not necessarily for the individual ant but for the continuation of its specific genetic make-up).
Examples in protists
An interesting example of altruism is found in the cellular slime moulds, such as Dictyostelium mucoroides. These protists live as individual amoebae until starved, at which point they aggregate and form a multicellular fruiting body in which some cells sacrifice themselves to promote the survival of other cells in the fruiting body.
- Cheating (biology)
- Evolutionarily stable strategy
- Gene-centered view of evolution
- Evolution of morality
- Evolutionary ethics
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- Okasha, S. (2008). "Biological altruism". The Stanford Encyclopedia of Philosophy. Retrieved July 20, 2013.
- Nicholas B. Davies; John R. Krebs; Stuart A. West (9 April 2012). "11". An Introduction to Behavioural Ecology. John Wiley & Sons. pp. 307–333. ISBN 978-1-4443-3949-9.
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- Hamilton, W.D., (1964). The genetical evolution of social behaviour, I and II. Journal of Theoretical Biology, 7: 1-16, 17-32
- Herbert Gintis (September 2000). "Strong Reciprocity and Human Sociality". Journal of Theoretical Biology 206 (2): 169–179. doi:10.1006/jtbi.2000.2111. PMID 10966755.
- Altruism helps swarming robots fly better genevalunch.com, 4 May 2011.
- Waibel M, Floreano1 D and Keller L (2011) "A quantitative test of Hamilton's rule for the evolution of altruism" PLoS Biology, 9(5): e1000615. doi:10.1371/journal.pbio.1000615
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- Bshary, R., & Noë, R. (2003). Biological markets: The ubiquitous influence of partner choice on the dynamics of cleaner fish-client reef fish interactions. In P. Hammerstein (Ed.) , Genetic and cultural evolution of cooperation (pp. 167-184). Cambridge, MA US: MIT Press.
- Hohmann, Ulf; Bartussek, Ingo; Böer, Bernhard (2001). Der Waschbär (in German). Reutlingen, Germany: Oertel+Spörer. ISBN 978-3-88627-301-0.
- Human-like Altruism Shown In Chimpanzees
- Brosnan, S., Silk, J. B., Henrich, J., Mareno, M., Lambeth, S. P., & Schapiro, S. J. (2009) "Chimpanzees (Pan troglodytes) do not develop contingent reciprocity in an experimental task" Animal Cognition, 12(4), 587–597. doi:10.1007/s10071-009-0218-z
- October 7, 2005, Hour Two:
- de Waal, Frans (1996). Good Natured. Harvard University Press. pp. 20–21. ISBN 0-674-35660-8.
- Perry, Julie (April 19, 2002). "Reciprocal Altruism in Vampire Bats". Retrieved October 10, 2009.
- Cheney, D. L. & Seyfarth, R. M. (1990). How monkeys see the world: Inside the mind of another species. University of Chicago Press. ISBN 978-0-226-10246-7.
- Davidson College, biology department (2001) Bottlenose Dolphins - Altruism, article retrieved March 11, 2009.
- "Walrus: Odobenidae - Behavior And Reproduction". Retrieved 2008-08-12.
- Brown, David (August 17, 2007). "Birds' Cooperative Breeding Sheds Light on Altruism". The Washington Post. Retrieved April 23, 2010.
- Fackelmann, Kathy A. (1989). "Avian altruism: African birds sacrifice self-interest to help their kin - white-fronted bee eaters". Science News.
- Bordereau, C., Robert, A., Van Tuyen V. & A. Peppuy (1997). "Suicidal defensive behavior by frontal gland dehiscence in Globitermes sulphureus Haviland soldiers (Isoptera)". Insectes Sociaux 44 (3): 289–297. doi:10.1007/s000400050049.
- "Altruism". Reference.com. Retrieved July 20, 2013.
- Stoel, Amanda (2012) "The meme of altruism and degrees of personhood" Journal of Personal Cyberconsciousness, 7(1): 27–36.
- Biological Altruism
- International Union for the Study of Social Insects
- Quick Guide: Kin Selection (Current Biology)
- Quick Guide: Altruism (Current Biology)
- "Mutt-ernal Instincts" An article about dogs caring for other species' young (cats, tigers, etc.).