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[[Image:Meat eater ant nest swarming03.jpg|thumb|250px|Meat Eater ant colony swarming]]
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[[Image:Meat eater ant nest swarming03.jpg|thumb|250px|A swarming [[Meat ant|meat-eater ant colony]]]]
'''Eusociality''' (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>
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'''Eusociality''' ([[Ancient Greek language|Greek]] ''eu'': "good/real" + "social") is a term used for the highest level of social organization in a [[hierarchy|hierarchical]] classification. Eusociality is characterized by cooperative brood care, overlapping adult generations and division of labor by reproductive and (partially) non-reproductive groups. In analogy with some human societies, groups of specialized individuals are sometimes called [[caste]]s. Several different levels of sociality have been categorized including '''[[presociality]]''' ([[Solitary but social]]), '''[[subsociality]]''',
# reproductive division of labor (with or without sterile castes)
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'''[[Biological parasocialism|parasocial]]''' (including communal, '''[[quasisocial]]''', and '''[[semisociality|semisocial]]'''), and eusocial.<ref name=costa/>
# overlapping generations
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<ref>{{cite web|last=Cromartie|first=Jamie|title=Insect Societies I|url=http://loki.stockton.edu/~cromartj/Entomology/social1.html|accessdate=12 November 2012}}</ref>
# cooperative care of young
 
   
The lower levels of social organization, [[subsociality]], were classified using different terms, including '''presocial''', '''subsocial''', '''semisocial''', '''parasocial''' and '''quasisocial'''.
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== Examples ==
==Examples==
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The most familiar examples of eusocial insects are [[ant]]s, [[bee]]s, and [[wasp]]s (order [[Hymenoptera]]), as well as [[termite]]s (order [[Isoptera]]){{spaced ndash}}all with reproductive queens and more or less [[infertility|sterile]] workers and/or soldiers.
The most familiar examples are insects such as [[ant]]s, [[bee]]s, and [[wasp]]s (the 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>
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''[[Austroplatypus incompertus]]'', a species of [[weevil]] native to Australia, is the first beetle (order [[Coleoptera]]) to be recognized as eusocial.<ref name="bee">{{cite web|url=http://www.newscientist.com/article/mg13418203.100-science-the-australian-beetle-that-behaves-like-a-bee.html |title=Science: The Australian beetle that behaves like a bee |publisher=New Scientist |date=1992-05-09 |accessdate=2010-10-31}}</ref><ref name="kent">{{cite journal |author=D. S. Kent & J. A. Simpson |year=1992 |title=Eusociality in the beetle ''Austroplatypus incompertus'' (Coleoptera: Curculionidae) |journal=[[Naturwissenschaften]] |volume=79 |pages=86–87 |doi=10.1007/BF01131810}}</ref>
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Mammalian examples include 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> however, this classification is controversial owing to disputed definitions of 'eusociality' as well as the existence of other mammals that satisfy the original definition of Wilson (1971).<ref name="O'Riain and Faulkes, (2008)">O'Riain, M.J. and Faulkes, C. G., (2008). African mole rats: eusociality, relatedness and ecological constraints. In J. Korb and J. Heinze (eds.), Ecology of Social Evolution, 207-223. http://www.springerlink.com/content/q11245457q771m3t/</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]].
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Eusociality with biologically sterile individuals represents the most extreme form of [[kin selection]]. Research about eusociality and its origins is a prolific branch of [[sociobiology]].<ref name=Herbers>{{cite journal|last=Herbers|first=Joan M|title=Darwin's ‘one special difficulty’: celebrating Darwin 200|journal=Biology Letters|date=23|year=2009|month=April|volume=5|issue=2|pages=214–217|doi=10.1098/rsbl.2009.0014|url=http://rsbl.royalsocietypublishing.org/content/5/2/214|accessdate=13 November 2012}}</ref>
   
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]]").
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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 can manifest in the appearance of individuals within a group whose behavior (and sometimes anatomy) is modified for group defense, including self-sacrificing ("[[Altruism in animals|altruism]]").
   
==Definition debates==
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== History ==
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>
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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>
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# Reproductive division of labor (with or without sterile castes)
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# Overlapping generations
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# Cooperative care of young
   
==Evolution of eusociality==
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=== Definition debates ===
[[Image:Fire ants.jpg|thumb|right|Fire ants]]
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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.zoo.ox.ac.uk/group/foster/FosterRatnieksTREE2005.pdf PDF]</ref> Others believe that the hierarchical classification may not serve much purpose.<ref name=costa>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>
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 the idea of [[inclusive fitness]].
 
   
According to [[inclusive fitness]] theory, eusociality may be easier for species like ants to evolve, due to their [[Ploidy#Haplodiploidy|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, assuming that roughly equal numbers are being produced. Even though workers often do not reproduce, they are potentially passing on more of their genes by caring for their 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.
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== Theories of social evolution ==
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{{Main|Evolution of eusociality}}
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{{Expert-subject|date=March 2011}}
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[[Image:Fire ants.jpg|thumb|right|[[Fire ant]]s]]
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Eusocial animals have appeared paradoxical to many theorists of the field of evolution, including [[W. D. Hamilton]]: 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, we ought to expect any [[gene]]s causing this condition to be highly unlikely to persist in the population. In ''[[On the Origin of Species|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 a possible resolution to the paradox might lie in the close family relationship, but specific theories (e.g. [[kin selection]] or [[inclusive fitness]]) had to wait for the discovery of the mechanisms for genetic inheritance.
   
[[Superorganism]] theory, in contrast, explains the evolutionary stability of eusociality by focusing on competition among groups of organisms, such that selection upon features of the behavior of the group as a whole outweighs selection on the individuals within each group; that is, there is a higher payoff for an individual to invest in between-group competition than to invest in within-group competition.<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> Support for this is based on the observation that multicellular life essentially started out as colonies of one-celled creatures, in which most of the one-celled creatures became specialized to other roles in the colony, losing the ability to reproduce. Thus came the transition from hordes of cooperating one-celled animals ([[algae]] are an example) to colonies of one-celled organisms acting as single, permanent units ([[slime moulds]]), to the simplest multicellular life ([[sea sponge|sponges]]), from which all higher animals evolved.
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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>
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  +
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 percent 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.<ref>{{cite journal|last=Hamilton|first=W. D.|title=The Genetical Evolution of Social Behaviour II|journal=Journal of Theoretical Biology|date=20|year=1964|month=March|volume=7|issue=1|pages=17–52|doi=10.1016/0022-5193(64)90039-6|url=http://www.sciencedirect.com/science/article/pii/0022519364900396|accessdate=13 November 2012}}</ref> 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 (arising some 11 separate times) within the haplodiploid group Hymenoptera — [[ants]], [[bees]] and [[wasps]].<ref>{{cite journal
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| author = William O. H. Hughes, Benjamin P. Oldroyd, Madeleine Beekman, Francis L. W. Ratnieks
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| title = Ancestral Monogamy Shows Kin Selection Is Key to the Evolution of Eusociality
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| journal = [[Science (journal)|Science]]
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| volume = 320
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| issue = 5880
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| pages = 1213–1216
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| publisher = [[American Association for the Advancement of Science]]
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| date = 2008-05-30
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| url = http://www.sciencemag.org/cgi/content/abstract/320/5880/1213
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| accessdate = 2008-08-04
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| doi = 10.1126/science.1156108
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| pmid = 18511689}}</ref>
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Reeve and [[Bert Holldobler|Holldobler]]'s version of [[superorganism]] theory further elaborates this model by considering competition and co-operation between groups as well as within groups.<ref>{{cite journal
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| author = Edward O. Wilson and Bert Hölldobler
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| title = Eusociality: Origin and consequences
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| journal = [[Proceedings of the National Academy of Sciences]]
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| volume = 102
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| issue = 38
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| pages = 13367–13371
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| publisher = [[United States National Academy of Sciences]]
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| date = 2005-09-20
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| url = http://www.pnas.org/content/102/38/13367.full.pdf+html
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| format = [[PDF]]
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| accessdate = 2008-08-04
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| doi = 10.1073/pnas.0505858102
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| pmid = 16157878
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| pmc = 1224642}}
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</ref><ref>{{cite journal | doi = 10.1073/pnas.0703466104 | author = Reeve H.K., Hölldobler B. | year = 2007 | title = The emergence of a superorganism through intergroup competition | url = | journal = Proceedings of the National Academy of Sciences | volume = 104 | issue = | pages = 9736–9740 | pmid=17517608 | pmc=1887545}}</ref> 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 intra-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.
 
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.
   
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 successful species, how can a species succeed in which most individuals don't breed at all? 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 alleles causing this condition should be eliminated from the population immediately. In ''[[Origin of Species]]'' (first edition, Ch. 7), 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.
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However, now many eusocial species have been discovered that are not haplodiploid (in addition to [[termites]], such as a species of platypodid [[ambrosia beetle]]s, several independent lines of ''[[Synalpheus]]'' sponge-dwelling shrimp and [[Blesmol|bathyergid mole rats]]). Conversely the [[solitary bee]]s are (as all bees) haplodiploid yet are not eusocial. The association between haplodiploidy and eusociality is below [[statistical significance]],<ref name=NTW>{{cite journal|last=Nowak|first=Martin|coauthors=Corina Tarnita, EO Wilson|title=The evolution of eusociality|journal=Nature|date=26|year=2010|month=August|volume=466|pmid=20740005|issue=7310|pages=1057–1062|doi=10.1038/nature09205|url=http://www.nature.com/nature/journal/v466/n7310/full/nature09205.html|accessdate=15-Mar-2011}}</ref> all of which suggests that haplodiploidy alone is neither [[Necessary and sufficient condition|necessary nor sufficient]] for eusociality to emerge.
   
Some hypotheses about how eusociality evolved in naked mole rats include: inbreeding, ecological factors such as the dependence on large tubers which 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}}
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== Other examples ==
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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>{{cite journal | doi = 10.1038/359724a0 | author = Crespi B. J. | year = 1992 | title = Eusociality in Australian gall thrips | url = | journal = Nature | volume = 359 | issue = 6397| pages = 724–726 }}</ref>
   
==Other examples==
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A single species of [[beetle]], ''[[Austroplatypus incompertus]]'', is known to be eusocial.<ref>{{cite journal | author = Kent D.S., Simpson J.A. | year = 1992 | title = Eusociality in the beetle ''Austroplatypus incompertus'' (Coleoptera: Curculionidae) | url = | journal = Naturwissenschaften | volume = 79 | issue = 2| pages = 86–87 | doi = 10.1007/BF01131810 }}</ref>
Another widespread insect group exhibiting eusociality is the [[termite]]s (order [[Isoptera]]), which in contrast to the Hymenoptera exhibit [[diploidy]], like most organisms. Termites are a lineage of [[cockroaches]], and are not closely related to the Hymenoptera. Eusociality arose once in an ancestral termite, whilst it arose several times in the Hymenoptera.
 
   
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>
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Among mammals, eusociality has been observed in both Naked and Damaraland mole-rats (''Heterocephalus glaber'' & ''Fukomys damarensis'').<ref>{{cite journal|last=Jennifer|first=Jarvis|title=Eusociality in a Mammal: Cooperative Breeding in Naked Mole-Rat Colonies|journal=Science|year=1981|month=May|volume=212|issue=4494|pages=571–573|url=http://www.jstor.org/stable/1686202}}</ref> Therefore, Bathyergidae phylogenies imply convergent evolution of eusociality within the African mole-rat family.<ref>{{cite journal|last=Faulkes et al.|coauthors=Verheyen, Jarvis, Bennett|title=Phylogeographical patterns of genetic divergence and speciation in African mole-rats|journal=Molecular Ecology|year=2004|month=March|volume=13|issue=3|pages=613–629|doi=10.1046/j.1365-294X.2004.02099.x|url=http://onlinelibrary.wiley.com/doi/10.1046/j.1365-294X.2004.02099.x/abstract}}</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, 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.{{Fact|date=December 2007}}
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Eusociality has also arisen among some [[crustacean]]s that, again, live in groups in a restricted, shared area. In some tropical reefs live [[Synalpheus regalis]], a shrimp that relies on fortress defense;<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|pmid=10937227|doi=10.1111/j.0014-3820.2000.tb00053.x}}</ref> they 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|url=http://www.ingentaconnect.com/content/umrsmas/bullmar/1998/00000063/00000002/art00011}}</ref>
==See also==
 
*[[Gyne]]
 
*[[Presociality]]
 
*[[Reciprocity]]
 
*[[Stigmergy]]
 
   
==References==
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== See also ==
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* [[Evolution of eusociality]]
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* [[Dense heterarchy]]
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* [[Evolutionarily stable strategy]]
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* [[Gyne]]
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* [[Patterns of self-organization in ants]]
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* [[Presociality]]
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* [[Reciprocity (social psychology)]]
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* [[Stigmergy]]
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** [[Ant colony optimization]] (ACO)
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** [[Bee colony optimization]]
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* [[Task allocation and partitioning of social insects]]
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* [[International Union for the Study of Social Insects]]
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  +
== References ==
 
{{reflist|2}}
 
{{reflist|2}}
   
==External links==
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== External links ==
 
* [http://www.iussi.org/ International Union for the Study of Social Insects]
 
* [http://www.iussi.org/ International Union for the Study of Social Insects]
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* [http://academic.reed.edu/biology/professors/srenn/pages/teaching/web_2007/molerats_lb_jg/index.html Eusociality in naked mole-rats]
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{{sociobiology}}
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{{collective animal behaviour}}
   
[[Category:Animal social behavior]]
 
 
[[Category:Behavioral ecology]]
 
[[Category:Behavioral ecology]]
[[Category:Evolutionary psychology]]
 
 
[[Category:Superorganisms]]
 
[[Category:Superorganisms]]
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[[Category:Sociobiology]]
   
 
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Meat eater ant nest swarming03

A swarming meat-eater ant colony

Eusociality (Greek eu: "good/real" + "social") is a term used for the highest level of social organization in a hierarchical classification. Eusociality is characterized by cooperative brood care, overlapping adult generations and division of labor by reproductive and (partially) non-reproductive groups. In analogy with some human societies, groups of specialized individuals are sometimes called castes. Several different levels of sociality have been categorized including presociality (Solitary but social), subsociality, parasocial (including communal, quasisocial, and semisocial), and eusocial.[1] [2]

Examples

The most familiar examples of eusocial insects are ants, bees, and wasps (order Hymenoptera), as well as termites (order Isoptera) – all with reproductive queens and more or less sterile workers and/or soldiers. Austroplatypus incompertus, a species of weevil native to Australia, is the first beetle (order Coleoptera) to be recognized as eusocial.[3][4] Mammalian examples include the naked mole rat and the Damaraland mole rat;[5] however, this classification is controversial owing to disputed definitions of 'eusociality' as well as the existence of other mammals that satisfy the original definition of Wilson (1971).[6]

Eusociality with biologically sterile individuals represents the most extreme form of kin selection. Research about eusociality and its origins is a prolific branch of sociobiology.[7]

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 can manifest in the appearance of individuals within a group whose behavior (and sometimes anatomy) is modified for group defense, including self-sacrificing ("altruism").

History

The term "eusocial" was introduced in 1966 by Suzanne Batra[8] and given a more definitive meaning by E. O. Wilson.[9] It was originally defined to include those organisms (originally, only invertebrates) that had certain features:[10][11]

  1. Reproductive division of labor (with or without sterile castes)
  2. Overlapping generations
  3. Cooperative care of young

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 vertebrates (including mole rats), none of which have irreversible castes.[12] 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.[13] Others believe that the hierarchical classification may not serve much purpose.[1]

Theories of social evolution

Main article: Evolution of eusociality
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File:Fire ants.jpg

Eusocial animals have appeared paradoxical to many theorists of the field of evolution, including W. D. Hamilton: if 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, we ought to expect any genes causing this condition to be highly unlikely to persist in the population. 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 a possible resolution to the paradox might lie in the close family relationship, but specific theories (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.[14]

According to inclusive fitness theory, eusociality may be easier for species like ants to evolve, due to their 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 percent 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.[15] 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 (arising some 11 separate times) within the haplodiploid group Hymenoptera — ants, bees and wasps.[16]

Reeve and Holldobler's version of superorganism theory further elaborates this model by considering competition and co-operation between groups as well as within groups.[17][18] 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 intra-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.[19][20] This model does not require that individuals be highly related, though high relatedness will reduce expected levels of resistance to manipulation.

However, now many eusocial species have been discovered that are not haplodiploid (in addition to termites, such as a species of platypodid ambrosia beetles, several independent lines of Synalpheus sponge-dwelling shrimp and bathyergid mole rats). Conversely the solitary bees are (as all bees) haplodiploid yet are not eusocial. The association between haplodiploidy and eusociality is below statistical significance,[21] all of which suggests that haplodiploidy alone is neither necessary nor sufficient for eusociality to emerge.

Other examples

Recently, some species of gall-making aphids (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 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.[22]

A single species of beetle, Austroplatypus incompertus, is known to be eusocial.[23]

Among mammals, eusociality has been observed in both Naked and Damaraland mole-rats (Heterocephalus glaber & Fukomys damarensis).[24] Therefore, Bathyergidae phylogenies imply convergent evolution of eusociality within the African mole-rat family.[25]

Eusociality has also arisen among some crustaceans that, again, live in groups in a restricted, shared area. In some tropical reefs live Synalpheus regalis, a shrimp that relies on fortress defense;[26] they 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.[27]

See also

References

  1. 1.0 1.1 James T. Costa & Terrence D. Fitzgerald 2005 Social terminology revisited: Where are we ten years later? Ann. Zool. Fennici 42:559-564 PDF
  2. Cromartie, Jamie Insect Societies I. URL accessed on 12 November 2012.
  3. Science: The Australian beetle that behaves like a bee. New Scientist. URL accessed on 2010-10-31.
  4. D. S. Kent & J. A. Simpson (1992). Eusociality in the beetle Austroplatypus incompertus (Coleoptera: Curculionidae). Naturwissenschaften 79: 86–87.
  5. 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 Abstract
  6. O'Riain, M.J. and Faulkes, C. G., (2008). African mole rats: eusociality, relatedness and ecological constraints. In J. Korb and J. Heinze (eds.), Ecology of Social Evolution, 207-223. http://www.springerlink.com/content/q11245457q771m3t/
  7. Herbers, Joan M (23). Darwin's ‘one special difficulty’: celebrating Darwin 200. Biology Letters 5 (2): 214–217.
  8. Batra, S. W. T. 1966: Nests and social behavior of halictine bees of India (Hymenoptera: Halictidae). — Indian J. Entomol 28 375-393.
  9. Wilson, E. O. 1971: The insect societies. — Belknap Press of Harvard University Press. Cambridge. Massachusetts.
  10. Michener, C. D., Annu. Rev. Entomol, 1969, 14, 299-342.
  11. Gadagkar, Raghavendra (1993) And now... eusocial thrips!. Current Science 64(4):pp. 215-216 PDF
  12. Crespi, B.J. and Yanega, D. (1995) The definition of eusociality. Behav. Ecol. 6, 109–115
  13. Kevin R. Foster & Francis L.W. Ratnieks 2005 A new eusocial vertebrate? TRENDS in Ecology and Evolution 20(7):363-364 PDF
  14. 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.
  15. Hamilton, W. D. (20). The Genetical Evolution of Social Behaviour II. Journal of Theoretical Biology 7 (1): 17–52.
  16. William O. H. Hughes, Benjamin P. Oldroyd, Madeleine Beekman, Francis L. W. Ratnieks (2008-05-30). Ancestral Monogamy Shows Kin Selection Is Key to the Evolution of Eusociality. Science 320 (5880): 1213–1216.
  17. Edward O. Wilson and Bert Hölldobler (2005-09-20). Eusociality: Origin and consequences. Proceedings of the National Academy of Sciences 102 (38): 13367–13371.
  18. Reeve H.K., Hölldobler B. (2007). The emergence of a superorganism through intergroup competition. Proceedings of the National Academy of Sciences 104: 9736–9740.
  19. 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
  20. Brian, M.V. 1983. Social Insects: ecology and behavioural biology Chapman & Hall, New York.
  21. Nowak, Martin, Corina Tarnita, EO Wilson (26). The evolution of eusociality. Nature 466 (7310): 1057–1062.
  22. Crespi B. J. (1992). Eusociality in Australian gall thrips. Nature 359 (6397): 724–726.
  23. Kent D.S., Simpson J.A. (1992). Eusociality in the beetle Austroplatypus incompertus (Coleoptera: Curculionidae). Naturwissenschaften 79 (2): 86–87.
  24. Jennifer, Jarvis (May 1981). Eusociality in a Mammal: Cooperative Breeding in Naked Mole-Rat Colonies. Science 212 (4494): 571–573.
  25. Faulkes et al., Verheyen, Jarvis, Bennett (March 2004). Phylogeographical patterns of genetic divergence and speciation in African mole-rats. Molecular Ecology 13 (3): 613–629.
  26. Duffy, J. Emmett, Cheryl L. Morrison and Ruben Rios (2000). Multiple origins of eusociality among sponge-dwelling shrimps (Synalpheus). Evolution 54 (2): 503–516.
  27. Duffy, J. E (1998). On the frequency of eusociality in snapping shrimps (Decapoda: Alpheidae), with description of a second eusocial species. Bulletin of Marine Science 63 (2): 387–400.

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