Eusociality

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Naked mole rat ( Heterocephalus glaber ) - they are one of the few eusocial mammals

Eusociality (from Greek εὐ 'good' and Latin socialis 'comradely') is a name for a special form of social behavior of social insects such as termites , ants and bees and other social animal species. The term was introduced in 1969 by the bee researcher Charles Michener in order to be able to more precisely describe different forms of social behavior, which until then had often not been conceptually differentiated and were mostly summarized under the term "social". Michener took up a word created by his student Suzanne Batra from 1966 and generalized it. Michener's terminology has been accepted and widely used within biology, particularly sociobiology . More recently it has been criticized by some researchers for being too schematic.

Eusoziale social organizations, mainly of social insect species , are in the German language as a state (even insects State or Hymenopterenstaat , among honeybees Bien as states forming) designated or folk, the corresponding species. In the English-speaking world, one speaks of insect societies .

definition

In Michener's terminology, “eusocial” refers to what earlier researchers of social insects usually simply called “social”. It is the most integrated form of social behavior in animal species other than humans. The following are characteristic of eusocial associations:

  • cooperative brood care by several animals
  • Joint procurement and distribution of food, for example through mutual feeding ( trophallaxis )
  • the association comprises several distinguishable subgroups that share different tasks, which are called castes . Examples would be food procurers (workers) and defenders (soldiers) in the termite states .
  • Animals of several generations living together, mostly in family groups of mothers and daughters

An association that fulfills these requirements is then called a state, the corresponding type forming a state.

Two grades can be distinguished within eusociality.

  • high, or complex (eu) social are species in which the castes not only differ in behavior, physiology and possibly body size, but also clearly include different morphologies with different characteristics. These include the honey bees ( Apini ) and the stingless bees ( Meliponini ) within the bees.
  • primitive (eu) social are species that are eusocial, but in which the castes can only be distinguished by behavior or possibly physiology, but not morphologically. In the case of bees, this includes B. many furrow bees of the genera Halictus and Lasioglossum ; the bumblebees ( bombus ) stand between primitive and complex eusocial.

Degrees of sociality

For social associations of animals that do not meet the criteria of eusociality, a number of further expressions have been introduced in the same context to describe the behavior.

  • semi- social species form colonies or social associations in which the morphologically indistinguishable individuals perform different tasks in a division of labor, often as workers and queens. This is part of the regular behavioral repertoire of the species, so it is not only facultative. The main difference to eusocial groups is that all cooperating individuals belong to the same generation (mostly sisters). The colony size is always relatively small, the individual-rich states of many eusocial species are never reached.
  • Quasi-social species include fertile, egg-laying females (which correspond to the queens of the eu- and semi-social species) who form groups when caring for their offspring, help each other if necessary, and do some of the work together, such as building a nest cover . In contrast to semi-social species, there are no castes.
  • Communal are species in which the females create and inhabit a common nest, but in which each female lives in its own part and which otherwise do not work together. Each female only cares for her own offspring here.
  • Sub-social are species in which the offspring of a female stay together for more or less long after hatching and form aggregations. The offspring is usually fed by the mother (or assisted in eating) and often defended against predators . Usually the mother dies when, or shortly before, the offspring becomes independent. Cooperation and division of labor do not occur.

Semi-social, quasi-social and communal behavior are often difficult to distinguish in nature. There are many types, e.g. B. among the furrow bees, in which a different type of cooperation occurs depending on the colony size, or in which the stages are run through one after the other. They are therefore often summarized under the term parasocial .

Species in which no social behavior of this kind is pronounced are called solitary living. This can lead to beginnings of coexistence ( i.e. gregarious behavior in the broader sense) if, for example, many bees of the same species build numerous nests close together in particularly favorable places. Such aggregations, which are sometimes also called colonies, then include animals that do not interact with one another and no longer care for their offspring after the construction has been completed, provisions have been made and the eggs have been laid. Such aggregations do not fall under the concept of sociality.

Eusociality in the animal kingdom

In addition to the social hymenoptera and termites, the parade groups for eusocial behavior, this occurs less often in other animal species. There are some cases where there is a soldier caste of non-reproductive individuals who have not developed particularly complex social behavior in other respects. This is the case, for example, with parasitic parasitic wasps that are otherwise not social . In some cases it happens that some larvae no longer develop themselves, but only defend their siblings against other parasitoid larvae in the same host ; Detected were those approaches to Eusociality even with parasitic in molluscs living flukes , of which a part of the colony can reproduce while other individuals defend the colony. These “soldiers” are only two percent the height of reproductive individuals. Similar examples exist with some Australian thrips that live in self-made, blister-like leaf galls of acacias . Some individuals develop into soldiers here, who are characterized by body size, but only have limited reproductive capabilities. Its task is to defend the bile against other species of fringed winged winged birds that use the bile without being able to produce it (kleptoparasites).

Real, highly eusocial animal species are rare in other groups. They have been detected in about living in the ocean bang crabs of the genus Synalpheus . Other representatives with eusozialem behavior of the African naked mole-rat , who also formed colonies with a single propagating female (a queen) and the less well-known in Germany fukomys the genus Fukomys and Cryptomys .

Evolution of Eusociality

Eusociality causes a high degree of altruism because some individuals forego their own offspring in favor of others, with whom they are usually closely related. The explanation of this behavior is a problem in evolutionary biology , which would actually assume that such behavior must be evolutionarily disadvantageous. Individuals who have genes for altruistic behavior end up having fewer offspring of their own than those without them, and this behavior should actually be eradicated by selection . Charles Darwin, the founder of modern evolutionary theory, was already aware of this problem . A number of theories have been developed to solve the problem, such as group selection , but most influential is the concept of kin selection developed primarily by JBS Haldane and William D. Hamilton .

literature

Individual evidence

  1. ^ SWT Batra (1966): Nests and social behavior of haliictine bees of India. Indian Journal of Entomology 28: 375-393.
  2. ↑ James T. Costa: The Other Insect Societies. Harvard University Press, 2006. ISBN 978-0-674-02163-1
  3. ↑ YP Cruz (1981): A sterile defender morph in a polyembryonic hymenopterous parasite. Nature 294: 446-447.
  4. ↑ Ryan F. Hechinger et al. a .: Social organization in a flatworm: trematode parasites form soldier and reproductive castes. In: Proceedings of the Royal Society B , online advance publication of September 17, 2010, doi: 10.1098 / rspb.2010.1753
  5. ↑ Thomas W. Chapman, Brenda D. Kranz, Kristi-Lee Bejah, David C. Morris, Michael P. Schwarz, Bernard J. Crespi: The evolution of soldier reproduction in social thrips. In: Behavioral Ecology. 13 (4) 2002, pp. 519-525. doi: 10.1093 / beheco / 13.4.519
  6. ↑ Tamsin M Burland, Nigel C Bennett, Jennifer UM Jarvis, Christopher G Faulkes: Eusociality in African mole-rats: new insights from patterns of genetic relatedness in the Damaraland mole-rat (Cryptomys damarensis). In: Proceedings of the Royal Society B: Biological Sciences . tape 269 , no. 1495 , May 22, 2002, ISSN  0962-8452 , p. 1025-1030 , doi : 10.1098 / rspb.2002.1978 , PMID 12028759 , PMC 1690998 (free full text).
  7. ↑ cf. about Bert Hölldobler & Edward O. Wilson: The superorganism. Springer Verlag, 2010. ISBN 978-3-540-93766-1 , therein chap. 2: genetic foundations of social evolution.