Eusociality

Eusociality (Greek εὖ eu 'good' and social) is the highest level of organization of sociality. It is defined by the following characteristics: cooperative brood care (including 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, sometimes called castes. Eusociality is distinguished from all other social systems because individuals of at least one caste usually lose the ability to perform behaviors characteristic of individuals in another caste. Eusocial colonies can be viewed as superorganisms.

Eusociality has evolved among the insects, crustaceans, trematoda and mammals. It is most widespread in the Hymenoptera (ants, bees, and wasps) and in Isoptera (termites). A colony has caste differences: queens and reproductive males take the roles of the sole reproducers, while soldiers and workers work together to create and maintain a living situation favorable for the brood. Queens produce multiple queen pheromones to create and maintain the eusocial state in their colonies; they may also eat eggs laid by other females or exert dominance by fighting. There are two eusocial rodents: the naked mole-rat and the Damaraland mole-rat. Some shrimps, such as Synalpheus regalis, are eusocial. E. O. Wilson and others have claimed that humans have evolved a weak form of eusociality. It has been suggested that the colonial and epiphytic staghorn fern, too, may make use of a primitively eusocial division of labor.

The term "eusocial" was introduced in 1966 by Suzanne Batra, who used it to describe nesting behavior in Halictid bees, on a scale of subsocial/solitary, colonial/communal, semisocial, and eusocial, where a colony is started by a single individual. Batra observed the cooperative behavior of the bees, males and females alike, as they took responsibility for at least one duty (e.g., burrowing, cell construction, oviposition) within the colony. The cooperativeness was essential as the activity of one labor division greatly influenced the activity of another. Eusocial colonies can be viewed as superorganisms, with individual castes being analogous to different tissue or cell types in a multicellular organism; castes fulfill a specific role that contributes to the functioning and survival of the whole colony, while being incapable of independent survival outside the colony.

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, many 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:

E. O. Wilson extended the concept to include other social insects, such as ants, wasps, and termites. Originally, it was defined to include organisms (only invertebrates) that fulfilled the same three criteria defined by Michener.

Eusociality was then discovered in a group of chordates, the mole-rats. Some researchers have argued that another possibly important criterion for eusociality is "the point of no return". This is characterized by having individuals fixed into one behavioral group, usually before reproductive maturity. This prevents them from transitioning between behavioral groups, and creates a society with individuals 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. Other researchers have suggested that cooperative breeding and eusociality are not discrete phenomena, but rather form a continuum of fundamentally similar social systems whose main differences lie in the distribution of lifetime reproductive success among group members. Vertebrate and invertebrate cooperative breeders can be arrayed along a common axis, that represents a standardized measure of reproductive variance. In this view, loaded terms like "primitive" and "advanced" eusociality should be dropped. An advantage of this approach is that it unites all occurrences of alloparental helping of kin under a single theoretical umbrella (e.g., Hamilton's rule).

Most eusocial societies exist in arthropods, while a few are found in mammals. Some ferns may exhibit a form of eusocial behavior.

Eusociality has evolved multiple times in different insect orders, including hymenopterans, termites, thrips, aphids, wasps, and beetles.