Eusociality
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[[Image:Meat eater ant nest swarming03.jpg|thumb|250px|Meat Eater ant colony swarming]]'''Eusociality''' ([[Ancient Greek language|Greek]] ''eu'': "good" + "social") is a term used for the highest level of social organization in a hierarchical classification. The term "eusocial" was introduced in 1966 by Suzanne Batra<ref>Batra, S. W. T. 1966: Nests and social behavior of halictine bees of India (Hymenoptera: Halictidae). — Indian J. Entomol 28 375-393.</ref> and given a more definitive meaning by [[E. O. Wilson]].<ref>Wilson, E. O. 1971: The insect societies. — Belknap Press of Harvard University Press. Cambridge. Massachusetts.</ref> It was originally defined to include those organisms (originally, only [[invertebrate]]s) that had certain features:<ref>[[Charles D. Michener|Michener, C. D.]], Annu. Rev. Entomol, 1969, 14, 299-342.</ref><ref>Gadagkar, Raghavendra (1993) And now... eusocial thrips!. Current Science 64(4):pp. 215-216 [http://eprints.iisc.ernet.in/archive/00009227/ PDF]</ref>
# Reproductive division of labor (with or without sterile castes)
# Overlapping generations
# Cooperative care of young
The lower levels of social organization, [[presociality]], were classified using different terms, including '''presocial''', '''[[subsocial]]''', '''semisocial''', '''parasocial''' and '''quasisocial'''.
==Examples==
The most familiar examples are insects such as [[ant]]s, [[bee]]s, and [[wasp]]s (order [[Hymenoptera]]), as well as [[termite]]s (order Isoptera), all with reproductive queens and more or less [[infertility|sterile]] workers and/or soldiers. The only mammalian examples are the [[Naked Mole Rat|naked mole rat]] and the [[Damaraland Mole Rat|Damaraland mole rat]].<ref>Burda, H. Honeycutt, R. L, Begall, S., Locker-Grutjen, O & Scharff A. (2000) Are naked and common mole-rats eusocial and if so, why? Behavioral ecology and sociobiology 47(5):293-303 [http://cat.inist.fr/?aModele=afficheN&cpsidt=1456956 Abstract]</ref>
Eusociality with biologically sterile individuals represents the most extreme form of [[kin altruism]]. The analysis of eusociality played a key role in the development of theories in [[sociobiology]].
The phenomenon of reproductive specialization is found in various organisms. It generally involves the production of sterile members of the species, which carry out specialized tasks, effectively caring for the reproductive members. It most commonly manifests in the appearance of individuals within a group whose behavior (and sometimes anatomy) is modified for group defense, including self-sacrifice ("[[altruism]]").
==Definition debates==
Subsequent to Wilson's original definition, other authors have sought to expand or narrow the definition of eusociality, focusing on the nature and degree of the division of labor, which was not originally specified. A narrower definition specifies the requirement for irreversibly distinct behavioral groups or castes (with respect to sterility and/or other features), and such a definition excludes all social [[vertebrate]]s (including mole rats), none of which have irreversible castes.<ref>Crespi, B.J. and Yanega, D. (1995) The definition of eusociality. Behav. Ecol. 6, 109–115</ref> A broader definition allows for any temporary division of labor or non-random distribution of reproductive success to constitute eusociality, and some have accordingly argued that even humans may be considered eusocial.<ref>Kevin R. Foster & Francis L.W. Ratnieks 2005 A new eusocial vertebrate? TRENDS in Ecology and Evolution 20(7):363-364 [http://www.people.fas.harvard.edu/~kfoster/FosterRatnieksTREE2005.pdf PDF]</ref> Others believe that the hierarchical classification may not serve much purpose.<ref>James T. Costa & Terrence D. Fitzgerald 2005 Social terminology revisited: Where are we ten years later? Ann. Zool. Fennici 42:559-564 [http://www.sekj.org/PDF/anz42-free/anz42-559.pdf PDF]</ref>
==Theories of social evolution==
[[Image:Fire ants.jpg|thumb|right|Fire ants]]
In spite of the obvious advantages of common [[foraging]] and defense, eusocial animals had appeared paradoxical even to [[Charles Darwin|Darwin]]: if [[Adaptation|adaptive evolution]] unfolds by differential survival of individuals, how can individuals incapable of passing on their genes possibly evolve and persist? Since they do not breed, their fitness should be zero and any [[gene]]s causing this condition should be eliminated from the population immediately. In ''[[Origin of Species]]'' (first edition, Ch. 8), Darwin called this behavior the "one special difficulty, which at first appeared to me insuperable, and actually fatal to my theory." Darwin anticipated that the resolution to the paradox would lie in the close family relationship, but specific theories to offer such resolution (e.g., kin selection or inclusive fitness) had to wait for the discovery of the mechanisms for genetic inheritance.
Early ideas on eusociality included suggestions that [[trophallaxis]] or food sharing was a basis for sociality.<ref>Wheeler, W. M. 1918. A study of some ant larvae with a consideration of the origin and meaning of social habits among insects. Proc. Am. Phil. Soc., 57, 293-343.</ref> Other theories include [[superorganism]] theory and [[parental manipulation]] theory. The most widely accepted model to explain eusociality is based on [[W.D. Hamilton]]'s idea of [[inclusive fitness]].
According to inclusive fitness theory, eusociality may be easier for species like ants to evolve, due to their [[Haplodiploid sex-determination system|haplodiploidy]], which facilitates the operation of [[kin selection]]. Sisters are more related to each other than to their offspring. This mechanism of sex determination gives rise to what W. D. Hamilton first termed "supersisters" who share 75 per cent of their genes on average. Sterile workers are more closely related to their supersisters than to any offspring they might have, if they were to breed themselves. From the "[[selfish gene]]'s" point-of-view, it is advantageous to raise more sisters. Even though workers often do not reproduce, they are potentially passing on more of their genes by caring for sisters than they would by having their own offspring (each of which would only have 50% of their genes). This unusual situation where females may have greater fitness when they help rear siblings rather than producing offspring is often invoked to explain the multiple independent evolutions of eusociality (occurring some 11 separate times) within the haplodiploid group Hymenoptera — [[ants]], [[bees]] and [[wasps]]. However, Hymenoptera is a large group and the majority of hymenopterans are not social. Furthermore, highly developed eusociality also exists in non-hymenopterans, perhaps most prominently in [[termites]]. Certain vertebrates (such as the [[naked mole rat]]) have also been described as eusocial. Most such cases involve organisms that display high levels of [[inbreeding]], such that colony members share more than 50% of their genes, and therefore the same model is considered to apply to these species.
Reeve and Holldobler<ref>Reeve, H.K. and Hölldobler, B. 2007. The emergence of a superorganism through intergroup competition. Proceedings of the National Academy of Sciences 104: 9736-9740</ref>'s version of [[superorganism]] theory further elaborates this model by considering competition and co-operation between groups as well as within groups. In this case, an individual's inclusive fitness varies depending on how much it invests in within-group competition (e.g. hoarding a private food cache) versus between-group competition (e.g. contributing to common foraging); and on its relatedness to the other group members. In a hymenopteran colony with one breeder (queen) and many workers as described above, the [[evolutionarily stable state]] is for each individual to invest entirely in helping the group, leading to a perfect "superorganism", which implies the stability of eusociality in this case. This agrees with Hamilton's model. This is implied even without considering between-group interactions. However, they further show that any group of relatives may show high "superorganismness", provided that there are many groups competing for the same resources. This may favour eusociality, or a degree of eusociality in non-hymenopterans. Indeed, a non-zero level of inter-group co-operation is predicted, even if the group members are entirely unrelated, as long as there is competition between groups.
Theories of [[parental manipulation]] point out that the transition from solitary to eusocial appears to involve intermediate stages where dominance interactions are required to suppress the reproductive tendencies of group members; that is, females are ''manipulated'' into acting as workers, even if it is against their own self-interest.<ref>[[C. D. Michener|Michener, C.D.]], Brothers, D.J. 1974. Were workers of eusocial Hymenoptera initially altruistic or oppressed? Proceedings of the National Academy of Sciences 68: 1242-1245</ref><ref>Brian, M.V. 1983. ''Social Insects: ecology and behavioural biology'' Chapman & Hall, New York.</ref> This model does not require that individuals be highly related, though high relatedness will reduce expected levels of resistance to manipulation.
Some hypotheses about how eusociality evolved in naked mole rats include: inbreeding, ecological factors such as the dependence on large tubers that are hard to locate and reach underground, heat loss prevention, and high dispersal costs. In the mammalian cases, eusociality is believed to arise from 'reproductive suppression', where infertility in working females is only temporary, and not genetic.{{Fact|date=December 2007}}
==Other examples==
Recently, some species of gall-making [[aphid]]s (Order [[Hemiptera]]) and thrips (Order [[Thysanoptera]]) were found to be eusocial, with many separate origins of the state. These species have extremely high relatedness among individuals due to their partially [[asexual reproduction|asexual mode of reproduction]] (sterile soldier castes being of the same clone as the reproducing female), but the gall-inhabiting behavior gives these species a defensible resource that sets them apart from related species with similar genetics. In these groups, therefore, high relatedness alone does not lead to the evolution of social behavior, but requires that groups occur in a restricted, shared area.<ref>Crespi. B. J. 1992 Eusociality in Australian gall thrips. Nature 359: 724-726.</ref>
Similarly, eusociality has arisen among some [[crustacean]]s and other [[arthropod]]s. On some tropical reefs, several species of minute ''Synalpheus'' [[pistol shrimp]] that depend on certain sponges for the survival of their colony,<ref>{{cite journal|title=Multiple Origins of Eusociality among Sponge-Dwelling Shrimps (Synalpheus)|first=J. Emmett|last=Duffy|coauthors=Cheryl L. Morrison and Ruben Rios|journal=Evolution|volume=54|issue=2|year=2000|pages=503–516}}</ref> live eusocially, with a single breeding female and a preponderance of male defenders, armed with enlarged snapping claws. Again, there is a single shared domicile for the colony members, and the non-breeding members act to defend it.<ref>{{cite journal|journal=Bulletin of marine science|title=On the frequency of eusociality in snapping shrimps (Decapoda : Alpheidae), with description of a second eusocial species|author=Duffy, J. E|year=1998|volume=63|issue=2|pages=387–400}}</ref>
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Uncited trivia section
==Fictional examples==
*The idea of a human eusocial group was investigated in-depth in [[Stephen Baxter]]'s novel ''[[Coalescent]]''.
*The [[Alien (fictional creature)|Aliens]] featured in the ''[[Alien (movie)|Alien]]'' movies have a eusocial hierarchy, with the Alien Queen breeding the other types.
*The [[Nac Mac Feegle]] in [[Terry Pratchett]]'s ''[[Discworld]]'' series possess a eusocial culture, with hundreds of brothers led by a single mother, or "Kelda".
*Many of the races on [[Oddworld]], including Mudokons, Glukkons, and Sligs, are eusocial.
*The [[spiders]] in the film ''[[Arachnophobia (film)|Arachnophobia]]'' are eusocial.
*The [[Majat]] of C. J. Cherryh's ''Serpent's Reach'', set in her [[Alliance-Union universe]], are eusocial.
*The [[Formics]] or "Buggers" in [[Orson Scott Card]]'s [[Ender's Game Series]] are eusocial.
*[[Giant Black Rat]] from James Herbertson's the Rats, Lair and Domain display eusocial behavior. This may have been inspired by the [[Naked Mole Rat]] that lives a similar, more harmless lifestyle.
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==See also==
*[[Dense heterarchy]]
*[[Gyne]]
*[[Patterns of self-organization in ants]]
*[[Presociality]]
*[[Reciprocity (social psychology)]]
*[[Stigmergy]]
*[[Task allocation and partitioning of social insects]]
==References==
{{reflist|2}}
==External links==
* [http://www.iussi.org/ International Union for the Study of Social Insects]
[[Category:Behavioral ecology]]
[[Category:Superorganisms]]
[[Category:Sociobiology]]
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