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Eusociality (Greek ''eu'': "good/real" + "social"), the highest level of organization of animal sociality, is defined by the following characteristics: cooperative brood care (including brood care of offspring from other individuals), overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups. The division of labor creates specialized behavioral groups within an animal society which are sometimes called castes. Eusociality is distinguished from all other social systems because individuals of at least one caste usually lose the ability to perform at least one behavior characteristic of individuals in another caste. Eusociality exists in certain insects, crustaceans and possibly mammals. It is mostly observed and studied in the Hymenoptera (ants, bees, and wasps) and in the termites. For example, a colony has caste differences; queens and reproductive males take the roles as the sole reproducers while the soldiers and workers work together to create a living situation favorable for the brood. In addition to Hymenoptera and Isoptera, there are two known eusocial vertebrates from the order Rodentia, which includes the naked mole-rat and the Damaraland mole-rat. Some shrimps such as ''Synalpheus regalis'' are also eusocial. Several other levels of animal sociality have been distinguished. These include presocial (solitary but social), subsocial, and parasocial (including communal, quasisocial, and semisocial). == History == The term "eusocial" was introduced in 1966 by Suzanne Batra who used it to describe nesting behavior in Halictine bees. Batra observed the cooperative behavior of the bees, males and females alike, as they took responsibility for at least one duty (i.e. burrowing, cell construction, oviposition) within the colony. The cooperativeness was essential as the activity of one labor division greatly influenced the activity of another. For example, the size of pollen balls, a source of food, depended on when the egg-laying females oviposited. If the provisioning by pollen collectors was incomplete by the time the egg-laying female occupied a cell and oviposited, the size of the pollen balls would be small, leading to small offspring.〔 Batra applied this term to species in which a colony is started by a single individual. Batra described other species, where the founder is accompanied by numerous helpers—as in a swarm of bees or ants—as "hypersocial." In 1969, Charles D. Michener further expanded Batra’s classification with his comparative study of social behavior in bees. He observed multiple species of bees (Apoidea) in order to investigate the different levels of animal sociality, all of which are different stages that a colony may pass through. Eusociality, which is the highest level of animal sociality a species can attain, specifically had three characteristics that distinguished it from the other levels: # "Egg-layers and worker-like individuals among adult females" (division of labor) # The overlap of generations (mother and adult offspring) # Cooperative work on the cells of the bees' honeycomb〔 E. O. Wilson then extended the terminology to include other social insects; such as ants, wasps, and termites. Originally, it was defined to include organisms (only invertebrates) that had the following three features:〔 # Reproductive division of labor (with or without sterile castes) # Overlapping generations # Cooperative care of young As eusociality became a recognized widespread phenomenon, however, it was also discovered in Chordata, specifically Rodentia. Further research also distinguished another possible important criterion for eusociality, known as "the point of no return". This phenomenon is characterized by eusocial individuals that become fixed into one behavioral group, which usually occurs before reproductive maturity. This prevents them from transitioning between behavioral groups and creates an animal society that is truly dependent on each other for survival and reproductive success. For many insects, this irreversibility has changed the anatomy of the worker caste, which is sterile and provides support for the reproductive caste.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Eusociality」の詳細全文を読む スポンサード リンク
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